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1994 Pest Identification and Management in Tomatillo in Louisiana. Can Huy Ngo Louisiana State University and Agricultural & Mechanical College

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PEST IDENTIFICATION AND MANAGEMENT IN TOMATILLO IN LOUISIANA

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

in

The Department of Plant Pathology and Crop Physiology

by Can Huy Ngo B.S., University of Saigon, 1966 M.S., University of Florida, 1974 December 1994 UMI Number: 9524472

UMI Microform Edition 9524472 Copyright 1995, by UMI Company. All rights reserved.

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI 300 North Zeeb Road Ann Arbor, MI 48103 ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation and gratitude to the Co­ chairs of his committee, Drs. Milton C. Rush and Rodrigo A. Valverde, whose outstanding support, guidance, and encouragement helped make this learning project both illuminating and enjoyable; their suggestions and instruction were a source of inspiration.

Heartfelt thanks also go to his graduate committee members, Dr. James L.

Griffin, Dr. Richard Story, and Dr. William Blackmon, for their constant advice, enthusiastic support, and generous logistic assistance throughout the course of the research project.

Special thanks are extended to Drs. William Young and W.A. Meadows and to the staff of the LSU Agricultural Center Horticultural Farm and the Burden

Research Plantation. Their support in land preparation and crop maintenance is greatly valued. A special note of sincere appreciation is due to Dr. Paul Wilson for his invaluable assistance concerning tomatillo processing.

The author is also indebted to Dr. Rouse Caffey for being one of the main forces behind this research and degree program. Without his continued assistance and moral support, this study would not have been possible.

Special appreciation is again extended to Dr. Marieanne Hollay for her kindness and generous assistance concerning this project, and to Drs. J.B. Baker,

Raymond Schneider, Jeff Hoy, Gordon Holcomb, and Lowell Black, and Mr. Bill

Salzer for their valuable assistance in the greenhouses. Finally, gratitude and love are extended to his wife Jeanine Linh, whose

understanding, patience, encouragement, and constant support have helped him to keep things in perspective. Again, special thanks and appreciation also go to his two sons, Ignatius and Charlie, for their unending field assistance and contribution to the completion of this research project. TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

LIST OF TABLES ...... vi

LIST OF FIGURES...... viii

ABSTRACT ...... x

INTRODUCTION...... 1 Literature Cited in Introduction ...... 15

CHAPTERS 1. VIRAL DISEASES OF TOMATILLO ...... 19 Introduction ...... 20 Materials and M e th o d s ...... 21 R esults...... 28 Discussion ...... 38 Literature Cited in Chapter 1 ...... 48

2. INSECTICIDAL CONTROL OF TOMATO FRUITWORM (HELICOVERPA ZEA) IN TOM ATILLO...... 53 Introduction ...... 54 Materials and M e th o d s ...... 55 R esults...... 56 Discussion ...... 61 Literature Cited in Chapter 2 ...... 68

3. WEED CONTROL IN LOUISIANA GROWN TOMATILLO .... 72 Introduction ...... 73 Materials and M eth o d s ...... 74 R esults...... 82 Discussion ...... 89 Literature Cited in Chapter 3 ...... 98 4. EFFECTS OF PLANTING DATES, NITROGEN FERTILIZATION, AND MULCHING ON YIELD IN TOMATILLO ...... 103 Introduction ...... 104 Materials and M eth o d s ...... 105 R esults...... 108 Discussion ...... 115 Literature Cited in Chapter 4 ...... 121

CONCLUSION: PEST AND DISEASE MANAGEMENT PRACTICES FOR TOMATILLOS GROWN IN LOUISIANA ...... 125 Literature Cited in Conclusion ...... 138

VITA ...... 141

v LIST OF TABLES

2.1 Efficacy of insecticides for the control of tomato fruitworm on tomatillo - 1991 ...... 57

2.2 Efficacy of insecticides and aluminum mulch for the control of tomato fruitworm on tomatillo - 1992 ...... 58

2.3 Efficacy of insecticides and aluminum mulch for the control of tomato fruitworm on tomatillo - 1993 ...... 59

3.1 Herbicides tested for use on tomatillo and labeled for use in other Louisiana crops ...... 75

3.2 Efficacy of herbicides for weed control in tomatillo ...... 76

3.3 Effect of preemergence herbicides on seed germination in direct-seeded tomatillo ...... 83

3.4 Tolerance of direct-seeded tomatillo to preemergence herbicides in greenhouse studies ...... 84

3.5 Tolerance of transplanted tomatillo to preemergence herbicides in greenhouse studies ...... 85

3.6 Tolerance of transplanted tomatillo to postemergence herbicides in greenhouse studies ...... 87

3.7 Effects of preemergence and postemergence herbicides on phytotoxicity of transplanted tomatillo and weed control in field studies ...... 88

3.8 Effects of preemergence and postemergence herbicide treatments on yield of tomatillo in field studies ...... 90

4.1 Effect of planting date on tomatillo yield ...... 109

4.2 Influence of planting date on the incidence of fruitworm (Helicoverpa zea) damage in tomatillo ...... 110

4.3 Yield response of tomatillo cultivars to nitrogen fertilizer - 1991 tests ...... I ll

vi Yield response of tomatillo cultivars to nitrogen fertilizer - 1992 tests ...... LIST OF FIGURES

1.1 Polyacrylamide gel electrophoresis of dsRNA extracted from Brassica campestris plants infected with turnip yellow mosaic virus (lane 1), and tomatillo plants infected with eggplant mosaic virus (lane 2), Physalis mosaic virus-Kansas (lane 3), Physalis mosaic virus-Louisiana (lane 4), and tomato spotted wilt virus (lane 5) ...... 29

1.2 Electron micrograph showing physalis mosaic virus particles...... 31

1.3 Tomatillo symptoms induced by mixed infection with CMV, PhyMV, and TSWV ...... 32

1.4 Tomatillo symptoms induced by Physalis mosaic virus (1), cucumber mosaic virus (2), and tomato spotted wilt virus (3) ...... 33

1.5 Seasonal development of insect vector populations at the LSU Burden Research Plantation (1993) ...... 36

1.6 Thrips and aphids in present in tomatillo fields: (1) Frankliniella tritici, (2) F. occidentalis, (3) Thrips tabaci, (4) Sericothrips variabilis, (5) F. fuscu, (6) Myzus persicae ...... 37

1.7 Average monthly maximum and minimum temperatures in Baton Rouge (1991-1993) ...... 42

1.8 Average monthly precipitation in Baton Rouge (1991-1993) ...... 43

2.1 Key pests of tomatillo: (1) potato flea beetle, (2) tomato fruitworm, (3) potato aphid, (4) banded cucumber beetle, (5) Western flower thrips, (6) broad m ite ...... 64

2.2 Secondary pests of tomatillo: (7) Southern green stinkbug, (8) fall army worm, (9) leafhopper, (10) sharp shooter, (11) leafhopper, (12) leaffooted b u g ...... 66

3.1 Common weeds in tomatillo fields: (1) broadleaf signalgrass, (2) large crabgrass, (3) barnyardgrass, (4) foxtail, (5) yellow nutsedge, (6) goosegrass...... 80

4.1 Influence of mulching on the seasonal yield of tomatillo in 1993 ...... 114

viii Tomatillo planting schedule for optimizing yields in Louisiana ABSTRACT

In anticipation of the regional demand for tomatillo (Physalis ixocarpa Brot.) for the fresh market and sauce industry, four years of research trials (1990-1993) showed a significant adaptation of tomatillo to Louisiana planting conditions where, like tomato, it performed best in the cooler temperatures of spring and early fall.

Field surveys indicated that virus diseases were major constraints on production. A foliar mosaic and yellow mottle found commonly affecting plants was caused by

Physalis mosaic virus (PhyMV), identified by host reaction, electron microscopy, serology, and dsRNA analysis. Cucumber mosaic virus (CMV) and tomato spotted wilt virus (TSWV) were also found. Potato flea beetles, aphids, and thrips transmitted PhyMV, CMV, and TSWV, respectively. A high incidence of aphids and thrips occurred during the flowering periods of tomatillo (May and October) while flea beetle populations began to appear in May and peaked in late September to early

October.

Field evaluations of insecticides for the control of tomato fruitworm

(Helicoverpa zea Boddie) indicated that pyrethroid treatments (permethrin, cypermethrin, esfenvalerate, cyfluthrin) were significantly more effective in controlling this pest than the organophosphates (azinphos-methyl, methomyl), a carbamate (carbaryl), or an organochlorine (endosulfan).

Greenhouse and field weed control studies showed that tomatillo was tolerant to pendimethalin, napropamide, trifluralin, metolachlor, sethoxydim, quizalofop, and fluazifop-butyl. Tomatillo was more sensitive to alachlor and clomazone, and showed no tolerance to metribuzin, acifluorfen, imazethapyr, and foinesafen. Full season control of many grass and broadleaf weeds was obtained without reducing tomatillo yields with sequential treatments of metolachlor, trifluralin, napropamide, or pendimethalin preemergence followed by sethoxydim, fluazifop-butyl, or quizalofop postemergence.

In the spring and fall tomatillo transplantings, the aluminum mulch + insecticide treatment provided a high level of fruitworm and insect vector control and gave higher yields. Beneficial effects of mulching, such as insect repellency, weed control, adjustment of soil temperature, reduction of water percolation, and prevention of fruit rot caused by Rhizocronia solani and other soilborne pathogens accounted for increased yields.

Pest management techniques for tomatillo in Louisiana were outlined in detail along with production practices. INTRODUCTION

1 CULTIVATION AND DESCRIPTION

Tomatillo (Physalis ixocarpa Brot.), commonly known as the husk tomato, and by the Spanish names of tomate de cascara, tomate verde, tomate de fresadilla, and miltomate, is a vegetable that has been widely cultivated for its fruit in Mexico and

Central America from the time of the Aztec empire (Saray-Meza et al., 1978;

Cartujano-Escobar et ah, 1985; Mulato-Brito et ah, 1985). This crop bears globular fruits enclosed in an inflated bladder-like calyx which becomes papery at maturity and is used with hot peppers to prepare green, savory sauces for popular dishes such as tacos and enchiladas. Its consumption in Central Mexico amounts to about 10% of that of the red tomato (Cartujano-Escobar et ah, 1985).

Tomatillo is widely cultivated in central Mexico with over 15,000 ha in cultivation and with yields averaging about 10,600 kg/ha (SARH/DGEA, 1984). It is often produced at elevations over 800 m where the average weekly temperature is

62-72 F° (Mosino-Aleman, 1974; Brito et ah, 1985). This corresponds with the cool weather conditions (Spring and Fall) in Louisiana. The "Rendidora" cultivar is commonly cultivated in the state of Morelos in Mexico due to its fertility and to the larger size of its fruit (Saray & Loya, 1978; Cardenas, 1981). Tomatillo was also introduced to the USSR by the Vavilov expeditions (Medvedev, 1958). Trials of its cultivation were also conducted in Spain (Cuartero et ah, 1983). In Poland, it was introduced in 1983 and was found to grow satisfactory in home gardens in the climate of central Poland (Borkowski, 1984; Jankievicz et ah, 1987). The crop was transplanted to the field on May 15 through 20 and the main fruiting occurred in August and September. The yield reached 15,000-39,000 kg/ha in Poland (Garzon-

Tiznado & Garay-Alvarez, 1986).

Tomatillo is now gaining significant importance in United States markets as a major component of Mexican-style sauces. Commercial cropping has been successful along the central and south coasts of California (Riverside, San Bernardino, San

Diego, San Luis Obispo, and Santa Barbara counties), as well as in the low deserts of the Central Valley. However, tomatillo yields vary considerably (1,600 to 11,000 kg/ha) as there has been little research on the breeding and cultural practices of this crop (Myers, 1991). According to Melhus and Smith (1953), husk tomato transplanted outside on May 5 at Ames, Iowa, began to blossom June 15 and fruit started to ripen in late July. The vines grew 0.45 to 0.60 m tall, with a canopy of diameter 0.75 to 1.20 m, and produced an average yield of 1.14 kg per plant or about

22,000 kg/ha.

CULTIVAR DEVELOPMENT

A characteristic feature of the tomatillo (ex. Rendidora) is that it can have two types of growth, that is, prostate or erect (Cartujano et al., 1985). Plants of the prostrate type generally show more vigorous growth, reach maximum yields earlier and tend to produce higher yields than erect plants. From the economic perspective, early fruit setting is an advantage because the plants may avoid the attack of insects and diseases, which are usually intensified towards the end of plant life. Mulato-Brito et al., (1985) described the phenomenon of different length of internodes in different zones of apparent main branches in the vegetative growth of tomatillo. They found the internodes of maximum length in the middle part of the stem. It was also found that the internodes of the apparent lateral branches were shorter and were also growing more slowly. In addition, the apparent lateral and sublateral branches do not produce fruit suitable for harvesting. Although the main and lateral branches show continuous growth up to the death of the plant, the nodes of main branches are more fertile than those of the lateral and sublateral branches, due to their greater competitiveness, thus producing better conditions for fruit development.

The development of the reproductive parts of tomatillo plants is characterized by an increasing number of flower buds as plants are more ramified (Cartujano et al.,

1985). The number of flower buds and flowers existing on a plant was highest at the fifth week after emergence and showed a second lower peak at the eleventh week after emergence. Probably, due to the great number of fruits present at the same time on the plant, they compete strongly with each other for the limited supply of nutrients and in consequence, a great proportion of them are shed before reaching commercial size. As shown by Mulato-Brito et al., (1985), abscision of small fruits mainly occurs from the apparent lateral and sublateral branches and much less from the main branches. In general, the phenology of tomatillo reproduction consists of the following stages: differentiation of flower buds 17-20 days after seeding, the first flowers forming after 28-30 days, fruit set at 35 days, and maturation at 55-57 days after seeding (Saray-Meza, 1974). 5

SELF-INCOMPATIBILITY AND GENETIC VARIATION

Cultivation of tomatillo can be difficult because of its wide range of genetic variability in plant size, leaf shape, fruit size, and yield potential, due to its system of reproduction which assures outcrossing via self-incompatibility (Patil, 1967;

Quiros, 1984). The spectrum of self-incompatible phenotypes observed after self- pollination included plants totally self-incompatible, plants with parthenogenic fruit, or plants with various degrees of self-incompatibility that were subject to low seed production and germination. Pandey (1957) studied the progenies of reciprocal crosses between two related plants of P. ixocarpa. A one-way cross gave two intra- sterile, intra fertile groups in the progeny. The reciprocal cross, however, produced five intra-sterile groups. Some of the inter-group crosses were compatible and others incompatible. So, he postulated that the genetic control of incompatibility in P. ixocarpa is due to two independently segregating loci, each with a series of multiple alleles.

Quiros (1984) found a wide range of self-incompatibility phenotypes obtained from self-pollination of tomatillo. He concluded that factors other than the two genes reported by Pandey (1957) were involved in the determination of self-incompatibility in P. ixocarpa. The fact that the proportion of self-incompatible plants, germination of self-compatible plants, and the low number of self-compatible plants observed in the progenies of self-compatible parents might be due to severe inbreeding depression.

It might affect the process of fertilization by weakening the male gametes to a point where they are unable to germinate or reach the ovules. Thus, it seems that the self­ 6 incompatibility of P. ixocarpa is a multigenic trait of low heritability, possibly affected by inbreeding depression and environment. Furthermore, the progenies from inbred plants produced weak individuals and several morphological variants, including plants with leaf variegation, and stem and floral abnormalities. Quiros (1985) also found that doubling of the tomatillo chromosome number did not eliminate incompatibility of this species as found by Pandey (1957).

Patil (1967) observed the presence of an accessory chromosome in P. ixocarpa in addition to the normal complement of 211=24, and postulated that this accessory chromosome or a gene situated on it caused the self-incompatibility. However, this assumption could not explain the self-incompatibility observed by Pandey (1957) and

Quiros (1984).

SELECTION AND CONTROL OF VARIATION

Tomatillo exhibits gametophytic self-incompatibility which makes self- pollination very difficult and leads to great variability in the population. This characteristic is undesirable when the goal is to obtain commercial varieties of uniform quality, high productivity, and with a significant degree of disease and pest resistance or plants with a particular growth habit. One way of fixing agriculturally important traits may be through the asexual propagation of selected individuals. This approach could be useful for the vegetative multiplication of tomatillo. Stem cuttings of P. ixocarpa root very quickly in wet sand, a possible help for the breeder, who could perform selections in the field and obtain cuttings from the best plants to regrow them under isolation for controlled pollination (Quiros, 1984). To date, research on in vitro 7 plant regeneration from tissues of P. ixocarpa (Moriconi, 1988), was successful in regenerating plants from leaf discs or hypocotyl and epicotyl explants grown on medium consisting of Murashige-Skoog (MS), B5 Vitamins, 3% sucrose, 0.25%

Gelrite or 0.7% phytoagar, supplemented with 4 mg/1 2,4-dichlorophenoxyacetic acid

(2,4-D) to induce callus formation. Shoots and embryoids were formed on callus transferred to basal MS medium supplemented with 1.0 mg/1 benzylamino-purine (BA) and 0.5 mg/1 indol-3-acetic acid (1AA) (Moriconi, 1988). Recently, Ramirez-Malagon

& Ochoa-Alejo (1991) have also succeeded in regenerating plants from hypocotyl explants of tomatillo. The highest frequency of shoot formation was observed on MS medium containing 12.5-25 /jM BA and 5 pM NAA. Likewise, a high frequency of root formation was observed on MS medium supplemented with 1 pM BA and 1 pM

NAA. In 1992, Assad-Garcia and his coworkers described an efficient method using the Agrobacterium tunufaciens Ti-plasmid based system for the generation of transgenic tomatillo plants. Up to 40 transgenic plants could be obtained in experiments using 60 cotyledon explants. The transformed nature of the regenerated plants was confirmed by NPP II and Southern blot hybridization analysis. Using the b-glucuronidase system the tissue specific and developmental patterns of expression of the Cauliflower Mosaic Virus 35S promoter were determined in transgenic tomatillo plants. It was found that this promoter is developmentally regulated during fruit and seed formation. 8

VIRUSES AND INSECT VECTORS

Tomatillo has been evaluated as a food crop in Louisiana (Can et al., 1992).

Field studies indicated that virus diseases were the major factor limiting production.

Physalis mosaic virus (PhyMV), causing a foliar mosaic and yellow mottle disease, was commonly found affecting plants in experimental plots (Valverde et al., 1993).

The virus was transmitted by the potato flea beetle (Epitrix cucumeris Harris), the banded cucumber beetle (Diabrotica balteata Lee.), and the palestriped flea beetle

{Sy.srena blamla Melsh) (Can et al., 1994). Other viruses found less frequently included cucumber mosaic virus (CMV) and tomato spotted wilt virus (TSWV)

(Valverde et al., 1993). In addition, tomatillo has also been found to be locally and systemically susceptible to other viruses, such as alfalfa mosaic virus, potato virus X, potato virus Y, tobacco etch virus, tobacco rattle virus, potato acuba mosaic virus, tobacco mosaic virus, tobacco necrosis virus, and tobacco ringspot virus (Horvath,

1988). PhyMV was first reported by Paul et al., (1968) under the name Belladonna mottle virus (BeMV) causing chlorotic mottle on Atropa belladonna. The European isolate was readily transmitted mechanically and systemic infections were obtained in

28 species of Solanaceae. The virus consists of isometric particles 25-30 nm in diameter and preparations contained infective nucleoprotein particles with a sedimentation coefficient of 113 S and an RNA-content of 35-38%. Virus preparations frequently contained non-infective empty protein shells (53 S).

Serological tests showed that PhyMV was immunologically related to the Andean potato latent virus, the dulcamara mottle virus, and the onorius yellow mosaic virus (Gibbs et al., 1966). PhyMV can therefore be classified in the "Andean potato latent

virus" group proposed by these authors.

Moline and Fries (1974) recovered PhyMV in cental Iowa from P.

heterophylla L., a common perennial weed. This PhyMV strain had a limited host

range that included members of the Solanaceae and C. quinoa. The virus was

serologically related to European belladonna mottle virus (BeMV). Peters & Derks

(1974) isolated PhyMV from P. subglabrata L. in Illinois. The virus was readily

transmitted by sap inoculation. Purified preparations of PhyMV contained isometric

particle of 27 nm in diameter, which sedimented as two components: 112 S infectious

component and 52 S non-infectious component. The virus contained 38% RNA with

a molar base content of G 14.4%, A 22.9%, C 37.2%, and U 25.5%. Purified

preparations were highly infectious with a concentration of about 6,000 particles per

ml being infectious on plants.

Lee et al., (1979) reported the isolation of a Kansas strain of PhyMV from

garden-grown pepper (Capsicum frutescens L. var. ‘Hybrid Tokyo Bell’). Isometric

virus particles approximately 27 nm in diameter sedimented in sucrose density

gradients with a non-infectious 53 S top component and an infectious 109 S bottom

component which contained RNA 26 S. Both PhyMV-Iowaand PhyMV-Kansas strains

are unique among plant viruses in that they have two cathodic electrophone forms

between pH 5 and 10. No seed transmission was found for either strain in C. frutescens L., P. alkekenyi L., and Datura stramonium L. 10

Valverde et al. (1993) reported that, based on the reaction, the PhyMV strain from Louisiana (PhyMV-L) appeared more closely related to PhyMV-Iowa than to

PhyMV-Kansas. Moreover, PhyMV-L and PhyMV-K had minor differences in their dsRNA profiles although the dsRNA profiles of all four tymoviruses were similar.

Ding et al. (1990) reported that there was only 51.6% sequence homology of coat proteins between BeMV and the North American isolates. For this reason they have suggested that these two virus groups should be regarded as distinct tymoviruses.

Based on nucleotide sequence comparisons of the coat protein gene of several tymoviruses, Jacob et al. (1992) used the name Physalis mosaic virus (PhyMV) for the North American isolates of BeMV.

INSECT PESTS OF TOMATILLO

Tomatillos can be damaged by flea beetles (Myers, 1991). Flea beetle symptoms on tomatillo were similar to those on tomatoes, except that the insect often eats through the leaves, which are thinner than tomato leaves. Despite heavy damage, infested plants can still produce a prolific crop. Potato flea beetles, banded cucumber beetles, and palestriped flea beetles are known to be vectors of PhyMV, CMV, and

TSWV, respectively (Delgado-Sanchez, 1986; Can etal., 1992). Fruit damage caused by tomato fruitworm (Helicoverpa zea Boddie) can reduce yield by 20-30% (Saray-

Meza & Loya-Ramizez, 1976). After hatching, the larvae penetrate the husk and develop inside the berry. The larvae are very difficult to reach with insecticide and have time to grow and destroy the fruit. CULTURAL PRACTICES

Very little work has been done in the United States to improve cultural practices on tomatillo. Tomatillo culture is very similar to that for tomatoes or peppers. Plantings are generally direct-seeded, although tomatillos will transplant well when necessary to fill stands. Plant spacing and population density vary considerable among growers. Row spacings of 1.0 m (single row) to 2.1 m (double rows) have been observed. In-row spacings vary from 0.30 to 0.90 m (Grazon-Tiznado & Garay-

Avarez, 1978; Villanueva & Loya-Ramizez, 1976; Myers, 1991).

Harvest of tomatillo fruit generally begins 70 to 80 days after seeding, and the harvest period can exceed 60 days. Fruits are harvested by hand as they attain market size. They are not considered ripe until the fruit begins to break through the husks.

USDA storage recommendations are 55 to 60°F at 85 to 90% relative humidity. This gives an approximate storage life of 3 weeks (Cantwell, 1987).

USES OF TOMATILLO

In Mexico, tomatillo fruit is used in the making of chili sauce and dressing for meats, which have a pre-Columbian origin (Yamaguchi, 1983). In Poland, it has been introduced in the form of a jam, relish, with zucchini and hot pepper or in other processed forms. It is not very tasty in the fresh form (Ostrzycka et al., 1988). The husk tomato can be used as food in many forms. In Iowa, Melhus and Smith (1953) published recipes for preparing husk tomatoes in the following forms: fried, baked, stewed, cooked with meat, made into soup, dessert sauce, marmalade, and salad. With the exception of iron content, data on the nutritive value of this vegetable were obtained by Sonza-Novelo (1950) and Ostrztcka et al. (1988).

The chemical characteristics of tomatillo and tomato fruit differ in several cases. Tomatillo contains a relatively high percentage of dry matter (7-10%) and solid extract (6.6-7.4%) compared with the cultivated tomato cultivars. The high content of dry matter in tomatillo predisposes it to be good material for processing. Although tomatillo contains less potassium than tomato, this is compensated for by a tendency towards higher magnesium and iron contents. The content of Mg, Cu, Fe, and Zn depended greatly, however, on the edaphic conditions and fertilization in a given year.

The unripe fruit of tomatillo contains a large percentage of glucose and fructose that ranges from 2.8 to 5.7%. As the fruit ripens, the sugar content decreases but the content of saccharose increases markedly. This process does not occur on a significant scale in tomato. An increase of total sugars as fruit ripens is a common phenomenon among plants where fresh fruit is harvested (Snock and

Neubert, 1950). However, in tomatillo the reverse process was often observed. This may explain why, in Mexico, consumers prefer the unripe fruit in spite of its lower pH.

Tomatillo contains more acids than tomato, and shows an especially high citric and malic acid content. The latter decreases drastically during ripening. The content of oxalic acid is 11-18 mg/100 g in ripe fruit and up to 54 mg/100 g in unripe. The vitamin C content in tomatillo is similar to that in popular tomato cultivars and 13 amounts to 8-21 mg/100 g of tissue with dehydroascorbic acid prevailing. Vitamin

PP is somewhat higher in tomatillo, higher than in many other plant species and comparable to that of parsley cabbage or savoy cabbage, and potatoes.

Of approximately two hundred species in the genus Physalis, family

Solanaceae, only a few are known to have medicinal use. Tomatillo leaves and fruits have been used for the treatment of headache and stomachache while the tomatillo husk has been used for the treatment of diabetes (Hernandez, 1946). Tomatillo juice also alleviates stomachache and diarrhea. Tomatillo roots were recommended as a remedy for spleen disorders and as a tonic diuretic (Martinez, 1954). P. philadelphica Law. is used for treatment of tonsillitis while P. angulata L. was used for the treatment of pustules, tiredness, testicle inflammation, venereal diseases, and bleeding (Del Amo, 1979). In Puerto Rico, P. pubescens L. is cultivated as a medicinal herb for the treatment of indigestion (William, 1971). P. alkekengi L. has been reported to have anti-cancer properties (Hartwell, 1971). All parts of P. minima

L. which is a common weed in several countries, have been widely used in folk medicine. In Java, the roots of this species are used as a vermifuge, febrifuge, and for diabetes (Watt & Breyer-Brandwijk, 1962). In India, the leaves were used in the treatment of infectious hepatitis and gonorrhea while the fruits were used against dropsy, urinary diseases and gout (Watt & Breyer-Brandwijk, 1962).

RESEARCH OBJECTIVES

In an attempt to investigate the adaptability of tomatillo to Louisiana growing conditions, the following research objectives were developed: 14

1. to identify the major constraints (climatic factors, pest, diseases,

weeds) to tomatillo production,

2. to evaluate insecticides and herbicides for the insect pest, vector, and

weed control on tomatillo,

3. to elaborate appropriate pest and disease management practices for

increasing the yield of tomatillo.

The results of these objectives are presented in five chapters, as follows:

Chapter 1 identifies the most important fungal and virus diseases transmitted by insect vectors, and describes their development during the growing season.

Chapter 2 describes the most economically important pests of tomatillo and also evaluates the efficacy of a number of insecticides for the control of tomato fruitworm.

Chapter 3 outlines studies of the tolerance of tomatillo to pre- and postemergence herbicides and gives the results of herbicide screening tests for the weed control in tomatillo.

Chapter 4 reports the yield response of tomatillo cultivars to different planting dates and to nitrogen fertilizer rates. Also reported are studies of the impact of reflective mulch and insecticide applications on virus vector abundance, virus disease incidence, and effects on plant yield in tomatillo.

Conclusion describes appropriate crop management methods for increasing yield of tomatillo through improved cultural practices, as well as, appropriate insect, disease, and weed management techniques. 15

LITERATURE CITED IN INTRODUCTION

Assad-Garcia, N., Ochoa-Alejo, N., Garcia-Hernandez, E., Herrera-Estrella, L., and Simpson, J. 1992. Agrobacterium-mediated transformation of tomatillo (P. ixocarpa) and tissue specific and developmental expression of the CaMV 35 S promoter in transgenic tomatillo plants. Plant Cell Rep. 11: 558-562.

Borkowski, J. 1984. Pomidor skorzasty-nowe warzyws. Owoce, Warzywa, Kwiaty 8: 5-14.

Can, F., Rush, M.C., Valverde, R.A., Griffin, J.L., Story, R.N., Blackmon, W.J., Wilson, P.W. 1992. Tomatillo: a potential new vegetable in Louisiana. Louisiana Agriculture 35: 21-24.

Cantwell, H. 1987. "Postharvest Handling of Specialty Crops: Tomatillo". Perishables Handling, University of California Cooperative Extension. April 1987. 1 p.

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VIRAL DISEASES OF TOMATILLO

19 20

INTRODUCTION

Tomatillo, an important vegetable crop in the diets of Mexican and Central

American people, was evaluated as a food crop in Louisiana (Can et al., 1992). Due to the popularity of Mexican foods in California, commercial croppings have been successful in Riverside, San Bernadino, San Diego, San Louis Obispo, and Santa

Barbara counties (Johnson, 1985). Between 1980 and 1988, despite the general average yield increase in California, tomatillo production varied from 1.5 to 11 tons/ha. This variation probably occurred because very little work has been done in the United States to refine cultural practices for this crop.

In Mexico, elevations over 800 m are recommended for tomatillo production with low yields generally being obtained from plants grown at lower altitudes

(Mosino-Aleman and Garcia, 1974; Dremann, 1985). In Louisiana, a limited amount of tomatillo imported from Mexico is sold fresh in some grocery stores. In response to a projected regional demand in the southern Gulf States for the tomatillo fresh market and sauce industry, a four-year research study (1990-1993) was conducted at

Louisiana State University Agricultural Center (LSU-AC) on the adaptation of tomatillo to Louisiana planting conditions. Despite the low altitude, tomatillo performed well in the cooler temperatures of spring and early fall that are similar to the climate suitable for tomatillo production in the higher elevations of Mexico.

However, favorable cool weather conditions were incidentally associated with a high incidence of virus vectors that peaked during the flowering periods (May and 21

October), thereby favoring the transmission of virus diseases (Valverde et al., 1993;

Can et al., 1994).

The immediate objective of this study was to identify the major virus diseases transmitted by insect vectors, to determine the host reservoirs of these viruses, and to determine the seasonal abundance of the insect vectors throughout the growing season. A thorough understanding of the interrelationships between the crop, the viral pathogens, and their insect vectors is necessary for developing pest management strategies to reduce crop losses in tomatillo.

MATERIALS AND METHODS

Identification of economically important viral diseases.Leaves from 170 tomatillo plants showing virus-like symptoms were collected during the early fall of

1990 from LSU-AC horticultural experimental plots in Baton Rouge, Louisiana.

Similarly, leaves of 15 Physalis puhescens L. plants showing virus-like symptoms were also collected near the experimental plots. Samples were stored at -20°C prior to double-stranded RNA (dsRNA) and serological analyses.

Double-stranded RNA analysis. The microcentrifuge tube method was used for the dsRNA extraction (Morris et al., 1983). Double-stranded RNA was extracted from 1.2-1.5 g of plant tissue in 2.0 ml extraction buffer (50 ml extraction buffer = 10 ml of 10X STE, 15 ml of 10% SDS, 6 ml Bentonite [25 m g/m l], 250 /d of 2-mercaptoethanol, and 19 ml of distilled water) in a leaf roller and collected in a

15 ml centrifuge tube. Next, 1 ml of 2X STE-saturated phenol and 0.35 ml of

CHCl,: isopentonyl alcohol (24:1) was added to the homogenate. The tubes were 22 thoroughly mixed in a shaker for 20 min. and centrifuged in a clinical centrifuge at

2,200 g for 15 min. About 1.1 ml of the upper phase of the homogenate was pipetted into the 1.5 ml microcentrifuge tube containing 50 mg of CF-11, and 21 fil of 95% ethanol per 100 ^1 of aqueous phase was added. These materials were thoroughly mixed in a shaker for 20 minutes. The tubes were centrifuged at 8000 g for 5 min. and the supernatant was discarded. The CF-11 was resuspended by three washings with 1.0 ml of IX STE-ethanol (16.5%) to remove unbound nucleic acids. Each washing consisted of a 3-minute mixing in the shaker and centrifugation at 6,000 g for 3 min. Following the final wash, 100 /d of IX STE were added to each microcentrifuge tube and again thoroughly mixed in the shaker for 3 min., followed by centrifugation for 3 min. at 6,000 g. Four hundred and fifty /d of supernatant were withdrawn and transferred into a clean 1.5 ml microcentrifuge tube with 1/20 volume (22.5 ml) of 3 M sodium acetate (pH 5.5). The tube was filled with cold ethanol (at least more than twice the supernatant volume) and was centrifuged 10-15 minutes later. Next, the samples were centrifuged at 10,000 g for 20 min. and the ethanol was poured off. The clean virus dsRNA samples were obtained by adding 200 ml of DNA buffer and 10 /d of DNAse enzyme (1 mg/ml) in a 15-minute reaction.

Seven hundred ml of cold ethanol were added, and the tubes were centrifuged at

14,000 g for 10 min. Fifty /d of IX EG were added to each tube and the contents were thoroughly mixed to resuspend the virus dsRNA. Thirty to 50/ j. 1 of each sample were electrophoresed on a 6% polyacrylamide mini slab gel at 100 V for 3 hours at room temperature. The gel was stained with ethidium bromide (25-50 mg/ml) for 10- 23

15 min. and destained three times with distilled water. Since the stained dsRNA in the gel fluoresces when exposed to UV light and it was photographed with Polaroid

(types 55, 57, or 667) black and white film.

Physalis mosaic virus purification. Fifty grams of fresh tomatillo tissue infected with PhyMV was ground in a blender using 0.5 M sodium citrate buffer containing 5 mM EDTA and thioglycolic acid (pH 6.5) for about 2-3 minutes. The extract was transferred to a 500-ml beaker and emulsified with one volume of chloroform by mixing for a few minutes in the hood, then transferred into a large centrifuge plastic bottle for a low speed centrifugation at 8000 g for 10 min. Next,

10% PEG (MW = 6,000) was added to the supernatant and stirred for 30 min. at 40°C and then centrifuged at 10,000 g for 10 min. The resulting pellet was resuspended in 20-30 ml of the extraction buffer (diluted to 0.05 M) with 2% Triton 10X and stirred for 30 min.

Following a 10-min. centrifugation at 8,000 g, the supernatant was saved at

4°C for high speed centrifugation. Two cycles of low and high speed centrifugation were conducted. The pellet was resuspended in 1 ml of 0.05 M extraction buffer and saved for sucrose density gradient centrifugation and electron microscopy.

A sucrose gradient was prepared in 0.05 M borate buffer (pH = 8.0) in centrifuge tubes. Solutions of 40%, 30%, 20%, and 10% sucrose, respectively, were carefully pipetted into the centrifuge tubes and left overnight at 4°C in order to build a smooth gradient. About 2 ml of the virus sample saved from the previous step was carefully layered on the top of the sucrose gradient. Sample tubes were weighed and 24 balanced by adjusting with borate buffer, then centrifuged in swinging bucket rotor

(SW 8) at 27,000 g for 3 hrs. The gradients in the centrifuge tubes were separated by the density gradient fractionator.

Serological analyses. To confirm virus identity and relationships, Ouchterlony double diffusion tests were performed using 1% agarose in 0.01 M potassium phosphate buffer, pH 7.0 (Ouchterlony, 1968). Samples consisted of extracted sap diluted 1:5 in phosphate buffer. Viruses included PhyMV-L, PhyMV-K (supplied by

R. F. Lee, University of Florida), eggplant mosaic virus (EMV; supplied by K. S.

Kim, University of Arkansas), local isolates of turnip mosaic virus (TYMV), tomato spotted wilt virus (TSWV), and cucumber mosaic virus (CMV). Antiserum to

PhyMV-K (titre 1/320) was provided by R. F. Lee while antiserum to PhyMV-L was prepared in our laboratory by injection of a rabbit with PhyMV-L antigen. The rabbit was given three intramuscular injections of 2 ml purified virus, emulsified with

Freund’s incomplete adjuvant, on three occasions at intervals of two weeks. One week after the last injection, the rabbit was bled by cutting a marginal vein on the ear.

Blood samples were centrifuged after coagulation and serum collected. Samples from tomatillo suspected to be infected with CMV and TSWV were tested for these viruses with double-antibody sandwich ELISA (Clark and Adams, 1977) using antisera to

TSWV-L and CMV-L prepared in our laboratory.

Electron microscopy. Sap extracts from PhyMV-infected tomatillo plants were placed on carbon-coated Formvar grids and negatively stained with 2 % uranyl 25 acetate, pH 6.8 (Horne, 1965). The grids were observed on a Jeol 100 CX transmission electron microscope.

Synergistic effects of three tomatillo viruses. Studies of the synergistic effects of three viruses CMV, PhyMV-L, and TSWV occurring in tomatillo on symptom expression were conducted in the greenhouse in the fall of 1991. Two- week-old plants of the Puebla Verde cultivar were mechanically inoculated with each of the following viruses or virus combinations: CMV, PhyMV-L, TSWV,

CMV+TSWV, CMV + PhyMV-L, PhyMV-L+TSWV, and TSWV + PhyMV-

L+CMV. Each treatment was replicated five times as single plants. Symptom expression was recorded 2-3 weeks after inoculation and virus identity was confirmed by dsRNA analysis or serological tests.

Host range. PhyMV was maintained on tomatillo cv. Rendidora, Nicotinia rustica L., and Datura stramonium L. in an insect-free greenhouse. PhyMV was used to inoculate the following test plants: Chenopodium quinoa L., D. stramonium, N. rustica, Lycopersicon esculentum M ill., Solatium melongena L., Phaseolus vulgaris

L., Vigna unguiculata L., Pisum sativum L., Cucumis sativus L., Cucurbita maxima

Duch., and Gomphrena globosa L. Ten pepper cultivars were also tested for their susceptibility to PhyMV: Jalapeho, Yolo Wonder, Hungarian Wax, Cayenne Cajun,

Havanero, Sweet Cherry, Olympic hybrid, Pepperoncini, Keystone Giant, and

Pimenta Malagueta. Ten test plants were used for each pepper cultivar.

Insect transmission of Physalis mosaic virus. In the summer of 1993, the potato flea beetle (Epitrix cucumeris Harris), the banded cucumber beetle ( Diabrotica 26 balteata LeC.), the spotted cucumber beetle (D. undecimpunctata Mann.), and the pale-striped flea beetle (Systena blanda Melsh.) were collected from tomatillo, basil, and cowpea experimental fields on the LSU-AC Burden Research Plantation (Burden

Research Plantation) in Baton Rouge, Louisiana. Virus transmission tests were conducted under greenhouse conditions. Beetles were starved for two hours and placed individually in test tubes containing tomatillo leaves infected with PhyMV.

After a 24-hour feeding period, each beetle was transferred to a three-week-old healthy tomatillo plant and covered with a plastic cup. After a 24-hour transmission feeding, characterized by the appearance of holes on the leaves, the beetles were removed and killed. Leaf samples from all test plants were collected after two weeks and tested serologically with PhyMV antiserum using the Ouchterlony double diffusion test (Ouchterlony, 1968). A set of 100 tomatillo plants were used for the transmission study with D. balteata, while 50 plants each were used for studies with D. undecimpunctata, S. blanda, and E. cucumeris.

Seasonal occurrence of tomatillo insect vectors. The 1993 tomatillo field trials consisted of a 1-hectare plot located at the Burden Research Plantation where various crops, such as cotton, sweet potato, Irish potato, pepper, tomato, corn, and cowpea were growing. There was an abundance of wild ground cherry growing near the tomatillo plots. These crops served as a reservoir for insects transmitting the virus diseases and feeding on tomatillo plants. In the study of the seasonal abundance of these pests, nontreated control plots (0.60 x 4.50 m) of the spring and fall plantings were used along with an additional summer planting, thus facilitating the monitoring 27 of the population of tomatillo insect vectors (potato flea beetle, banded cucumber beetle, aphids, thrips) throughout the entire growing season. Due to the different ecology and dynamics of the populations of insect vectors, three sampling procedures were developed to better examine their occurrence and density. During the 1993 growing season, eight nontreated tomatillo plots (0.60 x 4.50 m) were sampled every week beginning April 3 (i.e., about two weeks after the spring tomatillo transplanting on March 15) and continuing until the second week of November.

Green peach aphid populations were counted weekly on two fully expanded leaves on the upper half of each of 10 plants selected randomly per plot. Aphid counting was repeated in four similar control plots. Sweep net sampling was used to estimate the adult banded cucumber beetle and potato flea beetle populations when they were actively feeding in the late afternoon (5:00-7:00 pm). Each week, five complete (double) sweeps were made across the tops of tomatillo plants in four control plots using a 37-cm diameter net with a 1 m handle. The net was forced about 20-30 cm into the foliage. The insects caught were transferred to a plastic bag and taken to the laboratory for counting.

Four water pan traps, evenly dispersed among the control plots, were used to determine seasonal populations of winged thrips that migrated into and around the tomatillo trial plots. Traps were made of 5 mm thick Plexi-glass strips cemented to form a 14.2 x 14.2 x 6.2 cm container. The interior of each trap was painted yellow while the exterior was painted with white semi-gloss spray paint. The traps were mounted on iron ring clamps supported by 1.2 cm diameter iron poles driven into the 28 ground. Throughout the trapping period, the tops of the traps were kept even with the top of the crop canopy. Ethylene glycol was used in the collection pan trap as a preservative. To determine the time of infection and the extent of virus incidence, tomatillo plants growing in designated control plots were examined on a weekly or biweekly basis. Visual estimation of the percentage of CMV, PhyMV, and TSWV symptoms was confirmed by serological tests (Ouchterlony double diffusion and

ELISA).

RESULTS

Virus diseases of tomatillo. Results from dsRNA analysis and serological tests indicated the presence of three major viruses. These were PhyMV (previously called belladonna mottle virus and Physalis mottle virus), CMV, and TSWV which were present in 55, 20, and 8 %, respectively, of the 170 samples tested. None of these three viruses could be detected in 17% of the samples. Double-stranded RNA profiles of PhyMV-L, PhyMV-K, eggplant mosaic virus (EMV), turnip yellow mosaic virus (TYMV), and TSWV are shown in Figure 1.1. Results (not shown) from dsRNA tests of healthy plants were negative. PhyMV and CMV dsRNAs were obtained readily from field-collected samples of tomatillo that also showed from time to time the presence of the cryptic viruses. Yields of TSWV dsRNA were higher from mechanically inoculated D. stramonium than from field-collected or mechanically inoculated tomatillo plants. The dsRNA analysis technique was reliable for the screening of PhyMV-L and CMV, but inconsistent for TSWV. 29

1 2 3 4 5

Figure 1.1. Polyacrylamide gel electrophoresis of dsRNA extracted fromBrassica campestris plants infected with turnip yellow mosaic virus (lane 1), and tomatillo plants infected with eggplant mosaic virus (lane 2), Physalis mosaic virus-Kansas (lane 3), Physalis mosaic virus-Louisiana (lane 4), and tomato spotted wilt virus (lane 5). 30

Results of CMV and TSWV dsRNA analyses were confirmed with ELISA (all

170 samples were tested). Ouchterlony tests to compare PhyMV-L, PhyMV-K,

EMV, and TYMV using antiserum to PhyMV-K resulted in confluent precipitin bands between PhyMV-L and PhyMV-K. Reciprocal tests with PhyMV-L antiserum were not conducted. No reactions were obtained with other tymoviruses. Spherical

PhyMV particles about 28-30 nm in diameter were seen readily with the electron microscope (Figure 1.2).

Synergistic effects of three toinatillo viruses. Greenhouse studies of the synergistic effects of these three viruses on tomatillo cv. Puebla Verde indicated that severe mosaic symptoms were caused by a mixed infection of PhyMV and CMV, and that severe leaf deformation occurred when TSWV was present in mixed infections with either PhyMV or CMV, or with both (Figures 1.3 and 1.4). In multiple infections there tended to be more leaf distortion than that caused by the viruses alone.

Under field conditions, mixed infections of PhyMV and CMV were more frequent than PhyMV and TSWV. Plants with the PhyMV/CMV mixture showed both mosaic and leaf distortion followed by stunting and reduced branching with early infection of the young tomatillo transplants. PhyMV infections also differed among tomatillo cultivars with uniform yellowing in the cultivar Guanajuato and mosaic/mottling and leaf banding in the cultivars Puebla Verde and Rendidora.

Host reaction. Both PhyMV-L and PhyMV-K induced mottle on P. ixocarpa,

P. Jloridiana L., and D. stramonium. PhyMV-K did not infect L. esculentum cv.

Rutgers and N. rustica whereas PhyMV-L induced a mild mosaic on these two plant Figure 1.2. Electron micrograph showing physalis mosaic virus particles. 32

Figure 1.3. Tomatillo symptoms induced by mixed infection with CMV, PhyMV, and TSWV. Figure 1.4. Tomatillo symptoms induced by physalis mosaic virus (1), cucumber mosaic virus (2), and tomato spotted wilt virus (3). 34 species. All pepper cultivars showed a positive reaction with PhyMV-L, ranging from mild (Jalapeho, Sweet Cherry, Yolo Wonder, Olympic Hybrid, Keystone Giant,

Pimenta Malagueta, Pepperoncini), moderate mosaic/mottling (Cayenne Cajun,

Havanero), to severe mottling (Hungarian Wax). Negative reactions were observed among the leguminous (pinto bean, cowpea, Tvu 621, garden pea, black-eye cowpea) and cucurbitaceous (Poinsett cucumber, Ambassador squash) test plants. C. quinoa showed only local necrotic lesions whereas no symptoms were observed on G. globosa. DsRNA analysis and Ouchterlony tests for PhyMV in a number of common weeds in tomatillo trial plots also indicated that the virus was absent in both broadleaved and graminaceous weeds including common purslane, carpetweed, mayweed, pigweed, curly dock, white clover, barnyardgrass, crabgrass, crowfootgrass, goosegrass, dallisgrass, bermudagrass, and nutsedges. Only the wild ground cherry, Physalis subglabrata (Mackenzie & Bush), was considered a host reservoir of PhyMV as 10 of 15 field samples were found to be infected with the virus.

Insect transmission of PhyMV. Leaf samples from test plants analyzed with

PhyMV antiserum in an Ouchterlony double diffusion test indicated that PhyMV was transmitted by 4 of 100 D. balteata, 2 o f 50 S. blanda, and 0 of 50 D. undecimpunctata. Similar experiments conducted with E. cucumeris resulted in a higher percentage of virus transmission (11 of 50 beetles). Plants that showed a positive serological test also had typical symptoms of PhyMV. However, no virus 35 transmission occurred on 50 tomatillo plants fed on by E. cucumeris freshly collected from the field.

Seasonal abundance of insect vectors of tomatillo viruses.Visual sampling of aphids throughout the 1993 growing season indicated that the green peach aphid,

Myzus persicae (Sulzer), was more predominant than the potato aphid, Macrosiphum euphorbia (Thomas) that was found from time to time in isolated colonies. Both adults and nymphs of potato aphids were solid pink and had long, slender cornicles about twice as long as the cauda while the green peach aphid wingless adults were pale yellow to green and their nymphs yellow-green with 3 dark lines on the abdomen.

Green peach aphids were common between late April and early June, with the greatest densities in early May (Figure 1.5). Their populations then declined rapidly during the hot portion of the summer, and began to build up again in mid-September, peaking in mid-October.

The thrips species identified from traps during the investigation were

Frankliniella occidentalis Pergande, F. fusca Hinds, F. tritici Fitch, Thrips tabaci

Lindeman, Microcephalothrips spp., and Sericothrips spp. (Figure 1.6). However,

F. fusca and F. occidentalis were the most abundant species in the samples collected.

Thrips populations started to appear by early April, increased sharply in late May, and remained high until early June. A similar seasonal abundance pattern occurred in the fall, that is the thrips reappeared between mid-September and early November, with greatest densities in October. The first tomatillo plants exhibiting tomato spotted wilt symptoms were detected on May 10, about 7 weeks after being transplanted into the O F INSECTS 80

thrips (number/water pan trap) 70 aphids (number/20 leaves)

flea beetles (number/sweep) 60

50

40

30

20

10

0 3 10 17 24 1 8 15 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25 2 9 16 30 6 13 April May June July Aug. Sept. Oct. Nov. DATE •e 1.5. Seasonal development of insect vector populations at the LSU Burden Research Plantation (1993). Figure 1.6. Thrips and aphids in tomatillo fields: (1) Frankliniella tritici, (2) F. occidentalis, (3) Thrips tabaci, (4) Sericorhrips variabilis, (5) F. fitsca, (6) Myzus persicae. 38 field, and October 6, 6 weeks after transplanting. Combined data for the spring and fall plantings show that a sharp rise in tomato spotted wilt incidence occurred about

2 weeks after a large migration of thrips into the plots. Following their decline in early June, thrips populations were nearly absent during the entire summer.

The low incidence of PhyMV in the spring was a reflection of the late occurrence of potato flea beetles in the field. They began to appear by mid-May, and the population rapidly increased during the summer, peaking in late September to early October. This led to a high incidence of PhyMV in the fall tomatillo planting.

Uniform foliar yellowing was often observed on the PhyMV-infected tomatillos grown during the summer.

Banded cucumber beetles and palestriped flea beetles were recently shown to be vectors of PhyMV; however, their transmission efficiency was very low when compared to that of the potato flea beetle (Can et al., 1994). Their seasonal abundance pattern was similar to that of flea beetles. The highest captures occurred in August and September with trap captures declining in the cooler season (beginning in the fall). Unlike flea beetles, banded cucumber beetles were very active in the sunny periods of the day.

DISCUSSION

PhyMV has not been commonly found in cultivated solanaceous crops in the

United States, but two distinct strains have been reported. These are PhyMV-I, found naturally infecting P. heterophylla and P. subglabrata in Iowa and Illinois, respectively (Moline and Fries, 1974; Peters and Derks, 1974), and PhyMV-K, found 39 in Capsicum anrnatm in Kansas (Lee et al., 1979). These two strains can be differentiated by host reaction, but are serologically identical (Lee et al., 1979).

Results from serological tests, host reactions, electron microscopy, dsRNA analysis, and beetle transmission tests supported the conclusion that the virus inducing the mosaic and yellow mottle on tomatillo was PhyMV (Valverde et al., 1993). Based on host reaction, the PhyMV-L isolate appeared to be more closely related to PhyMV-

I than to PhyMV-K. Moreover, PhyMV-L and PhyMV-K had minor differences in their dsRNA profiles, although the dsRNA profiles of all four tymoviruses were similar (Valverde et al., 1993).

Ding et al. (1990) reported that the coat proteins of the European belladonna mottle virus (BeMV) and the North American isolates had only 51.6% sequence homology (Paul, 1971). They suggested that the two viruses should be regarded as distinct tymoviruses. Based on nucleotide sequence comparisons of the coat protein gene of several tymoviruses, Jacobs et al. (1992) used the name Physalis mottle virus

(PhyMV) for the North American isolates of BeMV. Based on these reports, the

CMI/AAB Committee decided to refer to this tymovirus isolate from tomatillo, and to others previously designated as BeMV in North America as physalis mosaic virus.

The high frequency of PhyMV in experimental plots of tomatillo, coupled with a weed host (ground cherry) and an insect vector (potato flea beetle) indicate the potential for an epidemic to develop in tomatoes, peppers, or other solanaceous crops in Louisiana. Despite the mild symptoms induced by PhyMV in pepper cultivars tested in the greenhouse, dsRNA profiles showed that the plants had a high PhyMV 40 titre. So, in addition to the ground cherry, peppers, tomatoes, and some other solanaceous crops (eggplant) might be considered potential hosts of PhyMV.

The presence of CMV and TSWV in tomatillo was not surprising since these viruses have a wide host range (Gibbs, 1970; Ie, 1970, 1982; Bond et al., 1983; Cho et al., 1986; Yudin et al., 1988). However, the low incidence of these two viruses might be due to low infection levels, low insect vector populations, and unfavorable weather conditions during the tomatillo seasons. The infrequency of TSWV in tomatillo was surprising, as it was common in surrounding plots of peppers and tomatoes, which had up to 60% of the plants infected. This could be due to factors such as planting date or vector preference.

The severity of the diseases on tomatillo caused by a mixed infection of 2 or

3 viruses (PhyMV, CMV, TSWV) indicated that improved cultural practices such as proper planting date and transplanting instead of direct seeding are needed to avoid the high incidence of vector populations, as well as, to allow tomatillo plants to mature before heavy insect feeding (Can et al., 1994).

Seventeen percent of the tomatillo plants showing virus-like symptoms in

Louisiana field plots were not infected with PhyMV, CMV, or TSWV. Extracts from these plants did not react with antisera of these viruses and did not yield dsRNAs.

It is possible that these plants were infected with luteoviruses, potyviruses, or geminiviruses, which yield very low quantities of dsRNA or DNA, thereby making the dsRNA analysis method impractical for routine screening (Valverde et al., 1990). 41

In addition to the potato flea beetle, banded cucumber beetles and palestriped flea beetles were recently demonstrated as new vectors of PhyMV (Can et al., 1994).

Despite the low transmission efficiency under experimental conditions, high populations of these beetles during the summer and the presence of other host crops

(sweet potato, bean, cowpea, basil) could explain the high incidence of PhyMV in tomatillo. Although the virus is stable and occurs in high concentration in the sap of infected plants, the low percent of virus transmission is probably due to the enzymes or pH of the regurgitated fluid of the beetles which may inactivate the virus (Walters,

1969).

It would be difficult to manage a virus disease that involves an insect transmission cycle without some understanding of the nature of the endemic and epidemic patterns of maintenance and spread of the insect vector. Basic to understanding the transmission cycle is knowledge of the hosts, both target and natural, the vectors, the viral pathogens, and the environmental factors affecting transmission (Sylvester, 1989).

The 4 years of field studies (1990-1993) on the adaptation of tomatillo to

Louisiana planting conditions indicated that, like tomato, tomatillo performed best in the cooler temperatures of spring and early fall. High yield increases corresponded with the presence of cool weather during the flowering and fruiting periods (April-

May, October-November). During this period, day and night temperatures were most suitable for fruit set and development of immature fruits (Figures 1.7 and 1.8) (Can et al., 1992). However, such adjustments of transplanting dates had an important 100-n

Range of optimumA temperature for Fruit fruit setting " setting

maximum temperature minimum temperature

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec MONTH Figure 1.7. Average monthly maximum and minimum temperatures in Baton Rouge (1991-1993) (Source: Southern Regional Climate Center, Louisiana Office of State Climatology). - i ^ to PRECIPITATION (CM)

1991 3 5 - 1992 1993

3 0 -

2 5 -

2 0 -

1 5 -

10-

5 -

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec

MONTH Figure 1.8. Average monthly precipitation in Baton Rouge (1991-1993) (Source: Southern Regional Climate Center, Louisiana Office of State Climatology). 44 effect on the abundance of aphids and thrips, as well as, the incidence of CMV and

TSWV (Swenson, 1968; Shands et al., 1972). Early transplanting in the spring increased yields of tomatillo, but may be inappropriate for the management of aphid populations. Aphid numbers steadily increased from late April to mid-May due to favorable, cool weather conditions (Semtner, 1984; Gray and Lampert, 1986). Aphids are much more environmentally responsive, being critically sensitive to the nutritional status and growth stages of their host plants, to extreme heat and radiation, to unanticipated cold, to low humidity, to heavy precipitation, and to uncontrollable crowding (Carver, 1989). High temperatures and heavy rains in the summer are the probable cause of the rapid decrease of green peach aphid populations in tomatillo.

Chamberlin (1958, 1992) reported that the green peach aphid population declined rapidly on tobacco after several consecutive days with temperature highs above 35°C.

In addition, DeLoach (1974) has demonstrated that the maximum infinite rate of increase for the green peach aphid occurred at 25°C, whereas a decrease occurred at a constant temperature of 30°C. A similar seasonal pattern of aphid abundance on tomatillo occurred in the month of October, but the aphid population steadily declined after that due to the rapid drop of temperature in November. In his study on the role of aphids in the ecology of plant viruses, Swenson (1968) showed that winged aphids are present in the air above crop lands during the entire growing season. These aphids carry viruses into crops and may also be responsible for secondary virus spread. The number of migrating aphids reaches definite peaks at certain times of the season. The greatest virus spread is likely to occur at these times. Knowledge of the 45 occurrence of aphid populations can be used to time planting and harvesting dates, and insecticide application (Tomlinson et al., 1970; Kring, 1972; Black, 1973; Joshi and

Dubey, 1977).

In our study, the seasonal pattern of thrips abundance in May and October, followed by a rapid decline in June and November, respectively, is similar to that of aphid populations on tomatillo (Can et al., 1994). Results of similar studies in tomato fields further substantiate these conclusions (Greenough et al., 1985; Navas et al.,

1991; Johnson, 1994). In their studies, the increase in tomato spotted wilt incidence in tomato plants trailed by 1 to 2 weeks the increase in the number of thrips trapped.

This time period corresponded well with the expected delay in symptom expression in plants following infection with TSWV (Best, 1968; Francki and Hatti, 1981;

Wijkamp et al., 1993). Wild plants and cultivated crops may play an integral part in the occurrence and spread of TSWV (Bond et al., 1983; Kobatake et al., 1984;

DaGraca et al., 1985; Yudin et al., 1988). On the Burden Research Plantation, tomato and peppers were transplanted in the field in March and April, and the cropping season was over by mid-July. The ability of thrips to colonize these crops suggests the possibility that tomato and pepper plants could serve as a virus acquisition source for the infection of tomatillo in the fall planting (Black et al., 1986; Hobbs et al., 1993). Thrips movement was reduced by heavy rain, and low and high temperatures (Newsom et al., 1953; Harding, 1961; Reddy et al., 1988). Similarly, cyclical changes that coincided with changes in temperature and rainfall have also be observed in thrips population and tomato spotted wilt incidence in tomatillo. 46

Transmission efficiency of beetles is usually thought of in terms of the relative

number of a population of beetles which transmit virus following an acquisition

feeding (Fulton et al., 1980). However, efficiency may vary greatly among species of beetles. Cartin and Gamez (1973) found that Cerotoma ruficornis (Oliver) transmitted bean rugose mosaic virus at a level of near 80% while D. balteata and D. adelpha (Harold) were much less efficient giving levels near 20% and 10%, respectively. Although vectors of the bromovirus group (e.g.D. balteata) exhibited a wide host range, the efficiency of transmission in all cases was low (Fulton et al.,

1975).

Effects of environmental variables such as temperature, humidity, and light on virus acquisition and transmission have not been thoroughly evaluated. These variables obviously affect the activities of beetles and, therefore, transmission. Field observations indicated that beetles were most active in the summer, thus increasing the chances of virus transmission. However, lower temperatures in the early spring and fall adversely affected both feeding activity and PhyMV transmission.

Virus transmission is correlated with feeding activity during which the beetle regurgitates virus-contaminated fluid (Gergerich et al., 1983, 1986). However, the biochemical nature of the regurgitant fluid differs among the beetle species, thus resulting in different transmission efficiencies such as that found among PhyMV vectors.

When producing tomatillos under Louisiana planting conditions, knowledge about the seasonal abundance of vectors is essential to understand the epidemiology 47 of plant virus diseases (Sakimura, 1963). However, a thorough understanding of the interrelationships between the crop, viruses, insect vectors, and weed hosts is also necessary for development of feasible management procedures. 48

LITERATURE CITED IN CHAPTER 1

Best, R.J. 1968. Tomato spotted wilt virus. Adv. Virus Res. 13:65-146.

Black, L.L., Bond, W .P., Story, R.N., and Gatti Jr., J.M. 1986. Tomato spotted wilt virus in Louisiana: Epidemiology aspects. In: Proceedings of the Third International Workshop on Plant Virus Epidemiology. International Society of Plant Pathology, pp. 4-5.

Black, L.L. 1973. Research on viral diseases of peppers in Louisiana. In: Proc. 1st Natl. Pepper Conf. (B. Villialon, ed.) pp. 12-13. Pickle Packers Intl., St. Charles, IL.

Bond, W.P., Whitarn, H.K., and Black, L.L. 1983. Indigenous weeds as reservoirs of tomato spotted wilt virus in Louisiana. Phytopathology 73:499.

Can, F., Rush, M.C., Valverde, R.A., Griffin, J.L., Story, R.N., Blackmon, W.J., and Wilson, P.W. 1992. Tomatillo: a potential new vegetable in Louisiana. Louisiana Agriculture 35:21-24.

Can, F., Valverde, R.A., and Rush, M.C. 1994. Two new vectors of Physalis mottle virus. Plant Disease 78:432.

Can, F., Story, R.N., Rush, M.C., and Blackmon, W.J. 1994. Impact of reflective mulch and insecticide application on insect vector abundance, virus incidence, and plant yield in tomatillo. Louisiana State University Vegetable Research Report 1993. Series 88:62-64.

Cartin, F. and Gamez, R. 1973. Transmission of two new strains of bean rugose mosaic virus by Chrysomelidae beetles. Abstracts XIII Annual Meeting, Caribbean Division, APS, San Jose, C.R. Dec. 9-14, p. 12.

Carver, M. 1989. Biological control of aphids. In: World Crop Pests: Aphids. (Minks, A.K., and Harrevijn P., eds.) PP. 141-165. Elsevier Sci. Pub. Co., NY.

Chamberlin, F.S. 1958. History and status of green peach aphid as a pest of tobacco in the United States. USDA Tech. Bull. No. 1175.

Chamberlin, J.R., Todd, J.W., Beshear, R.J., Culbreath, A.K., and Demski, J.W. 1992. Overwintering hosts and wingform of thrips, Frankliniella spp. in Georgia (Thysanoptera: Thripidae): Implications for management of spotted wilt disease. Environmental Entomology 21:121-128. 49

Cho, J.J., Mau, R.F.L., Gonsalves, D., and Mitchell, W.C. 1986. Reservoir weed hosts of tomato spotted wilt virus. Plant Disease 70:1014-1017.

Clark, M.F. and Adams, A.N. 1977. Characteristics of the microplate method of enzyme-linked immunosorbent assay for the detection of plant viruses. J. Gen. Virol. 34:475-483.

DaGraca, J.V., Trench, T.N., and Martin, M.M. 1985. A severe strain of tomato spotted wilt virus in Cape gooseberry (Physalis peruviana) in Transkei, South Africa. Plant Pathology 34(3):451-453.

DeLoach, C.J. 1974. Rate of increase of population of cabbage, green peach, and turnip aphids at constant temperature. Ann. Entomol. Soc. Am. 67:332-340.

Ding, S.W., Howe, J., McKenzie, A., Skotnicki, M., and Gibbs, A. 1990. Nucleotide sequence of the virion protein gene of belladonna mottle tymovirus. Nucleic Acids Research 18:6138.

Dremann, C. 1985. Ground Cherries, Husk Tomatoes & Tomatillos. Redwood City Seed Co., Redwood City, California, pp. 1-2, 11-14.

Francki, R.I.B. and Hatti, T. 1981. Tomato spotted wilt virus. In: Handbook of Plant Virus Infections and Comparative Diagnosis. (E. Kurstak, ed.) pp. 491-512. Elsevier Sci. Pub. Co., NY.

Fulton, J.P., Scott, H.A., and Gamez, R. 1975. Beetle transmission of legume viruses. In: Tropical Diseases of Legumes. (Bird, J., and Maramorosch,K., eds.) pp. 123-131. Academic Press, New York.

Gergerich, R.C., Scott, H.A., and Fulton, J.P. 1983. Regurgitant as a determinant of specificity in the transmission of plant viruses by beetles. Phytopathology 73:936-936.

Gergerich, R.C., Scott, H.A., and Fulton, J.P. 1986. Evaluation of Diabrotica beetles as vectors of plant viruses. In: Methods for the study of pest Diabrotica (J.L. Krysan and T.A. Miller, eds.) pp. 227-249. Springer-Verlag, NY.

Gibbs, A.J. and Harrison, B.D. 1970. Cucumber Mosaic Virus. CMI/AAB. Descriptions of Plant Viruses No. 1.

Gray, S.M. and Lampert, E.P. 1986. Seasonal abundance of aphid-borne virus vectors (Homoptera: Aphididae) in flue-cured tobacco as determined by alighting and aerial interception traps. J. Econ. Entomol. 79:981-987. 50

Greenough, D.R., Black, L.L., Story, R.N., Newsome, L.D., and Bond, W.P. 1985. Occurrence of Frankliniella occidentalis in Louisiana: a possible cause for the increased incidence of tomato spotted wilt virus. Phytopathology 75:1362.

Harding, J.A. 1961. Effect of migration, temperature, and precipitation on thrips infestations in South Texas. J. Econ. Entomol. 54:77-79.

Hobbs, H.A., Black, L.L., Story, R.N., Valverde, R.A., Bond, W.P., Gatti Jr., J.M ., and Johnson, R.R. 1993. Transmission of tomato spotted wilt virus from pepper and three weed hosts by Frankliniella fusca. Plant Disease 77:797-799.

Horne, R.W. 1965. Negative staining methods. In: Techniques for electron microscopy. (D. Kay, ed.) pp. 328-355. Blackwell Sci. Pub., Oxford.

Horvath, J. 1988. Reaction of Physalis species to plant viruses. XL Additional data on the virus susceptibility of Physalis ixocarpa Brot., Physalis philadelphica Lam ., and Physalis pruinosa L. Acta Phytopath. 23:137-141.

Ie, T.S. 1970. Tomato Spotted Wilt Virus. CMI/AAB Descriptions of Plant Viruses No. 39.

Ie, T.S. 1982. A sap-transmissible, defective form of tomato spotted wilt virus. J. Gen. Virol. 59:387-391.

Jacobs, A.N.K., Murthy, M.R.N., and Savithri, H.S. 1992. Nucleotide sequence of the 3’-terininal region of belladonna mottle virus-Iowa (renamed Physalis mosaic virus) RNA and an analysis of tyinoviral coat proteins. Archives of Virology 123:367-378.

Johnson, H. 1985. "Tomatillo." Vegetable Brief #244. University of California Cooperative Extension, April, 1985. 1 p.

Johnson, R.R. 1994. Epidemiology of tomato spotted wilt virus in Louisiana. M.S. Thesis. Louisiana State University, Baton Rouge, LA. 77 pp.

Joshi, P.D. and Dubey, L.N. 1977. Some weed reservoirs of cucumber mosaic virus in Gorakhpur and adjacent areas. Indian Phytopathol. 28:568.

Kobatake, H., Osaki, T., and Inouye, T. 1984. The vector and reservoirs of tomato spotted wilt virus in Nara Prefecture. Ann. Phytopathol. Soc. Jap. 50:541-544.

Kring, J.B. 1972. Flight behaviour of aphids. Ann. Rev. Phytopathol. 17:461-492. 51

Lee, R.F., Niblett, C.L., Hubbard, J.D., and Johnson, L.B. 1979. Characterization of belladonna mottle virus isolates from Kansas and Iowa. Phytopathology 69:985-989.

Moline, H.E. and Fries, R.E. 1974. A strain of belladonna mottle virus isolated from Physalis heterophylla in Iowa. Phytopathology 64:44-48.

Morris, T.J., Dodds, J.A., Hillman, B., Jordan, R.L., Lommel, S.A., and Tamaki, S.J. 1983. Viral specific dsRNA: Diagnostic value for plant disease identification. Plant Molecular Biology Reporter 1:27-30.

Mosiho-Aleman, P.A. and Garcia, F. 1974. The Climate of Mexico. In: World Survey of Climatology. Vol. 11. Climates of North America. (R.A. Bryson and F.K. Hare, eds.) pp. 345-404. Elsevier Sci. Pub. Co., NY.

Navas, V.E.S., Funderburk, J.E., Beshear, R.J., Olson, S.M., and Mack, T.P. 1991 .Seasonal pattern of Frankliniella spp. (Thysanoptera: Thripidae) in tomato flowers. J. Econ. Entomol. 84(6): 1818-1822.

Newsom, L.D., Roussel, J.S., and Smith, C.E. 1953. The tobacco thrips, its seasonal history and status as a cotton pest. Louisiana Agric. Exp. Stn. Tech. Bull. 474.

Ouchterlony, O. 1968. Handbook of immunodiffusion and immunoelectrophoresis. Ann Arbor Sci. Publ. Inc., Ann Arbor, ML 215 pp.

Paul, H.L. 1971. Belladonna Mottle Virus. CMI/AAB Description of Plant Viruses No. 52.

Peters, D. and Derks, A.F.L. 1974. Host range and some properties of Physalis mosaic virus, a new virus of the turnip yellow mosaic virus group. Neth. J. of Plant Pathol. 80:124-132.

Reddy, D.V.R. and Wightman, J.A. 1988. Tomato spotted wilt virus: Thrips transmission and control. In: Advances in Disease Vector Research, vol. 5. (K.F. Harris, ed.) pp. 203-220. Springer-Verlag, New York.

Sakimura, K. 1963. Frankliniella fusca, an additional vector for the tomato spotted wilt virus, with notes on Thrips tabaci, another vector. Phytopathology 53:412-415. 52

Semtner, P.J. 1984. Effect of transplantation date on the seasonal abundance of the green peach aphid (Homoptera: Aphididae) and two aphid predators on flue- cured tobacco. J. Econ. Entomol. 77(2):324-330.

Shands, W.A., Simpson, W., and Murphy, H.J. 1972. Effects of cultural methods for controlling aphids on potato on potato in Northeastern Maine. Life Sci. Agric. Exp. Stn. University of Maine Tech. Bull. No. 57.

Swenson, K.G. and Nelson, R.L. 1959. Relation of aphids to the spread of cucumber mosaic virus in gladiolus. J. Econ. Entomol. 52:421-425.

Swenson, K.G. 1968. Role of aphids in the ecology of plant viruses. Ann. Rev. Phytopathol. 6:351-374.

Sylvester, E.S. 1989. Viruses transmitted by aphids. In: World Crop Pests: Aphids. (Minks, A.K. and Harrevijn, P., eds.) pp. 65-87. Elsevier Sci. Pub., New York.

Tomlinson, J.A., Carter, A.L., and Simpson, C.J. 1970. Weed plants as sources of cucumber mosaic virus. Ann. Appl. Biol. 66:11-16.

Valverde, R.A., Nameth, S.T., and Jordan, R.L. 1990. Analysis of double stranded RNA for plant virus diagnosis. Plant Disease 74:255-258.

Valverde, R.A., Can, F., and Rush, M.C. 1993. Yellow mottle of tomatillo (Physalis ixocarpa Brot.) caused by Physalis mottle virus. Plant Pathology 42:657-660.

Walters, H.J. 1969. Beetle transmission of plant viruses. Adv. Virus Res. 15:339-363.

Wijkamp I., Lent, J.V., Kormelink, R., Goldbach, R., and Peters, D. 1993. Multiplication of tomato spotted wilt virus in its insect vector, Frankliniella occidentalis. J. Gen. Virol. 74:341-349.

Yudin, L.S., Tabashink, B.E., Cho, J.J., Mitchelle, W.C. 1988. Colonization of weeds and lettuce by thrips (Thysanoptera: Thripidae). Environmental Ecology 17:522-526. CHAPTER 2

INSECTICIDAL CONTROL OF TOMATO FRUITWORM

(HELICO VERPA ZEA) IN TOMATILLO

53 54

INTRODUCTION

The tomato fruitworm, the primary lepidopterous pest of tomatillo, usually infests tomatillo beginning in late May and remains a threat throughout the growing season which ends with the first killing frost, usually in mid-November. Because of the potential value of this processing and fresh market crop and the absence of practical sampling methods for H. zea, a series of preventive insecticide applications is normally used to protect the crop. However, none of the insecticides used are currently labeled for use on tomatillo, a potential new crop in Louisiana. Many of the insecticides evaluated in this study are those registered on a closely related plant, tomato (Creighton et al., 1971, 1973; Chalfant et al., 1979; Middlekauff et al., 1963).

The ideal characteristics of an insecticide for tomato fruitworm control would be (1) low phytotoxicity, (2) low cost, (3) low toxicity to the insect pollinators necessary for this outcrossing crop, (4) long residual effectiveness against the tomato fruitworm, and

(5) effectiveness against other pests, such as aphids, thrips, and flea beetles, which simultaneously occur during the flowering period and require treatments at the same time as tomato fruitworm (Yamaguchi, 1983; Wilcox, 1963; Cantu & Wolfenbarger,

1970; Walgenbach et al., 1991). At present, very little information is available regarding the effectiveness of insecticides on field-grown tomatillo (Myers, 1991).

Due to the potentially high economic return associated with commercial tomatillo production, pest control will be an important aspect of the crop management program (Lange & Kishiyama, 1978; Lange, 1978). 55

Current management of H. zea populations is dependent on insecticide applications; however, the economic value of control of H. zea populations has not been determined. The following study is a summary of the effectiveness of eight insecticides that have been tested during the past three years.

MATERIALS AND METHODS

Experiments were conducted on the Horticultural Farm in the summer of 1991 and spring and fall of 1992, and on the Burden Research Plantation in the spring and fall of 1993 in Baton Rouge, LA. Three-to-4-week-old tomatillo cv. Puebla Verde transplants were used for all plantings. Each experimental plot (0.6x4.50 m) consisted of 10 plants in a single row on 0.45-m centers. Fertilizer at the rate of 450 kg of 58-

58-58 (N-P205-K30) per hectare was broadcasted and incorporated before transplanting. Ammonium nitrate fertilizer was added at the rate of 225 kg/ha 2 weeks after transplanting. Methomyl 8EC was applied at 1.68 kg ai/ha for preemergence weed control while Gramoxone Super at 1.32 1/ha was used as a postdirected spray between rows for postemergence control of weeds.

Each treatment was replicated four times and arranged in a randomized complete block design. Treatments consisted of the following insecticides: permethrin

2EC (0.73 1/ha), esfenvalerate XL (0.58 1/ha), cypermethrin 3EC (0.23 1/ha), cyfluthrin 2E (0.02 1/ha), endosulfan 2EC (2.33 1/ha), azinphos-methyl 2EC (1.75

1/ha), methomyl 1.8EC (3.5 1/ha), and carbaryl 1.9L (1.40 1/ha). Two more treatments (aluminum mulch alone and aluminum mulch-1-permethrin 2EC at 0.73

1/ha) were added to the 1992 and 1993 experiments as part of a cultural management 56

practice to increase tomatillo yield. Insecticides were applied weekly with a 7.6 1

hand-held Chapin sprayer beginning 2 weeks after transplanting until harvest for the

control of both tomato fruitworm and the early infestations of virus-transmitting insect

vectors that might confound the data interpretation of the insecticide effectiveness on the reduction of the fruit damage caused by H. zea. Insecticides were applied to both sides of each plant row to ensure that tomatillo plants received complete insecticide coverage. Treatment applications in a 7-day fixed schedule were made in the late evening or early morning to minimize hazards to honey bee pollinators. Methomyl at 1.68 kg ai/ha was directly applied to the soil prior to transplanting and Gramoxone

Super at 1.32 1/ha was applied as a postdirected spray between rows for postemergence control of weeds.

Effectiveness of the treatments was determined by inspecting and recording the amount of fruitworm injury in the treated and nontreated test plots. Unripe tomatillos that were injured by caterpillar feeding were picked at regular intervals beginning shortly after fruit set and recorded. The remaining fruit was allowed to ripen. Upon ripening, fruit was harvested and scored for fruitworm injury (presence or absence).

Data were subject to analysis of variance (ANOVA) and means were separated using

Duncan’s multiple range test (/><0.05).

RESULTS

Data collected in the summer of 1991 and the spring and fall of 1992 and 1993 are shown in Tables 2.1, 2.2, and 2.3. The 1991 summer data were analyzed separately because the year and season were unique and the aluminum mulch 57

Table 2.1. Efficacy of insecticides for the control of tomato fruitworm on tomatillo - 1991.

June 26, 1991 (Planting Date)

Rate Damaged Fruit1 Total Yield2 Insecticide (form/ha (kg/ha) (kg/ha) ) 1. Permethrin 2EC 0.73 1 17.36 a3 6975 a 2. Esfenvalerate XL 0.58 1 28.17 a 6776 a 3. Cypermethrin 3EC 0.23 1 34.25 a 6637 a 4. Cyfluthrin 2E 0.02 1 45.92 a 6588 a 5. Endosulfan 2 EC 2.33 1 124.53 b 5944 b 6. Azinphos-methyl 2EC 1.75 1 144.00 b 5797 be

7. Methomyl 1.8EC 3.50 1 151.92 b 5565 be 8. Carbaryl 1.9L 1.40 1 170.91 b 5702 be 9. Control 1,043.00 c 5359 c (nontreated)

Fruit classified as damaged if one or more holes were chewed into the surface. Total yield includes all fruit, damaged and not damaged. Means followed by the same letter do not significantly differ at P=0.05, Duncan’multiple range test. Table 2.2. Efficacy of insecticides or aluminum mulch for the control of tomato fruitworm on tomatillo - 1992.

March 27, 1992 (Planting Date) August 29, 1992 (Planting Date)

Rate Damaged Fruit1 Total Yield2 Damaged Fruit Total Yield Insecticide (form/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha)

1. Aluminum mulch — 134.05 a3 14,035.50 a 120.85 a 14,795.80 a 2. Permethrin 2 EC 0.73 1 139.06 a 12,166.73 abc 168.11 abc 12,155.40 b

3. Cyfluthrin 2E 0.02 1 144.77 a 12,674.13 ab 152.83 ab 13,172.46 ab

4. Esfenvalerate XL 0.58 1 161.54 ab 11,659.80 be 155.84 ab 12,255.15 b

5. Cypermethrin 3EC 0.23 1 178.52 ab 11,374.48 be 154.32 ab 12,804.15 ab 6. Carbaryl 1.9L 1.40 1 231.84 ab 10,817.74 be 221.70 be 11,059.00 b

7. Azinphos-methyl 2EC 1.75 1 237.72 ab 11,006.37 be 176.77 abc 11,703.32 b 8. Endosulfan 2EC 2.33 1 271.04 b 10,646.00 be 178.17 abc 11,809.43 b

9. Methomyl 1.8EC 3.50 1 272.41 b 10,214.20 c 234.72 c 10,810.04 b

10. Control — 1,237.55 c 8,060.37 d 1,119.28 d 8,346.24 c (nontreated)

Fruit classified as damaged if one or more holes were chewed into the surface. Total yield includes all fruit, damaged and not damaged. Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test.

LA 00 Table 2.3. Efficacy of insecticides and aluminum mulch for the control of tomato fruitworm on tomatillo - 1993. March 20, 1993 August 15, 1993 Damaged Total Damaged Total Rate Fruit1 Yield2 Fruit Yield Insecticide (form/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha) Aluminum Mulch + Permethrin 2 EC 0.73 1 42.01 a3 16,442.7 a 34.13 a 16,923.0 a

Aluminum Mulch — 106.47 a 12,192.1 be 108.18 b 12,247.6 b Cypermethrin 3EC 0.23 1 83.42 a 13,610.8 b 106.24 b 12,008.6 b Permethrin 2EC 0.73 1 104.43 a 11,925.4 be 101.04 b 12,705.0 b Cyfluthrin 2E 0.02 1 85.39 a 13,137.8 b 122.86 b 11,312.6 be Esfenvalerate XL 0.58 1 84.50 a 12,844.9 b 106.35 b 11,485.4 be Azinphos-methyl 2EC 1.75 1 220.42 b 10,003.2 c 264.24 c 9,922.0 c Methomyl 1.8EC 3.501 234.45 b 9,771.2 c 212.53 c 10,164.0 c Endosulfan 2EC 2.33 1 253.32 b 9,743.2 c 251.38 c 9,899.7 c Carbaryl 1.9L 1.40 1 253.57 b 9,609.1 c 229.90 c 10,227.5 c

Control ------1,195.95 c 7,039.9 d 1,076.90 d 7,478.7 d (nontreated)

Means of damaged fruits are arranged by order of insecticide efficacy. Fruit classified as damaged if one or more holes were chewed into the surface. Total yield includes all fruit, damaged and not damaged. Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test.

L/l VO 60

treatment was not present. Examination of residual plots and Levene’s test indicated

that variances were homogeneous. Yields were assumed to have a normal distribution

on the Shapiro-Wilke’s test (P< W =0.4843). ANOVA showed a significant chemical

effect (P>F=0.0001) (SAS). Duncan’s multiple range test post-ANOVA procedure

indicated that pyrethroid treatments (permethrin, cypermethrin, esfenvalerate,

cyfluthrin) were significantly more effective in controlling tomato fruitworm and

treated plants gave higher yields than those treated with the organophosphates

(azinphos-methyl, methomyl), carbamate (carbaryl), and organochlorine (endosulfan)

insecticides. However, yields were still low and similar to those obtained from the

planting date-yield response study from the mid-summer planting. No phytotoxicity

was observed on any plot.

Similar results were obtained when the same treatments were applied to

tomatillo grown in the spring and fall of 1992 and 1993, with the exception of a yield

increase due to the cool weather conditions favorable to the fruit setting. The main

effects, year(P> F=0.0003) and chemical (P> F=0.0001) as well as their interaction

(P>F=0.0406) were significant at least at Pr(Type 1)=0.05. Pairwise contrasts comparing year (1992 versus 1993) for a particular treatment indicated that, in addition to the efficacy of the pyrethroid treatments over carbaryl, azinphos-methyl, and endosulfan, the aluminum mulch + insecticide (permethrin) treatment gave

significantly higher fruitworm control than any insecticide treatment alone.

Comparisons of the mulching treatments and the control with the other insecticide treatments within a year were tested at Pr(Type I) =0.00147, so that the overall 61 experimentwise error rate would remain constant. In 1992 and 1993, the control plots had significantly more fruitworm damage (about 9-10 times higher) and less marketable yield than the other treatments within a given year. The aluminum mulch treatment in 1992 had both higher yield and fruitworm control than the insecticide- treated and nontreated plots. However, in 1993, the aluminum mulch treatment had lower yield than the aluminum mulch-binsecticide treatment.

DISCUSSION

Brazzel et al. (1953) reported that H. zea was abundant during April and May in Louisiana on lupine, crimson clover, white clover, and in the buds of early grown corn. Bishopp (1929) stated that four or five generations of this insect might develop in the South and that the third generation (July-August) was destructive. However, in this study, despite the efficacy of all tested insecticides for the control of H. zea in the summer, tomatillo yield was still low in the treatment plots. It is possible that high day and night temperature during the flowering period impaired fruit set and was the main limiting factor affecting tomatillo yield in the summer (Can et al., 1992).

High temperatures greatly affected pollen abortion, thus inhibiting fertilization and reducing the outcrossing of the self-incompatible crops (Lewis, 1953; Smith, 1935).

Boudreaux (1994) also indicated that shedding of the flower buds, flowers, and small fruit was a recurrent problem in the production of bell peppers in Louisiana, thus limiting the crop yield. Any form of stress in the growing environment can induce fruit and flower loss in bell pepper, particularly high temperatures. Other stresses, 62 such as lack of moisture, too much moisture, or low light intensity, also can aggravate the problem.

Adjustment of planting dates indicated that tomatillo production in Louisiana was highest in the spring and fall (Chapter 4). Again, the fruit damage showed a significant difference in the amount of injury in treated and nontreated plots. Under the conditions of the experiments in 1992 and 1993, the synthetic pyrethroids provided high reductions in fruit damage and significant increases in marketable yield over the organophosphate, carbamate, and organochlorine insecticides. The significant attributes of this pyrethroid group are due to the high level of efficacy against a broad range of insect pests, low mammalian toxicity, and long residual activity (Breese,

1977; Hoyt et al., 1978; Ruscoe, 1977). Both organophosphates and carbamates were much more ecologically disruptive than the pyrethroids and chlorinated hydrocarbon insecticides (Anderson & Reynolds, 1960; Adkisson & Nemee, 1967; Mistric &

Smith, 1970; Pfrimmer, 1979). The important natural enemies most commonly found affecting insects were generally more susceptible to organophosphates and carbamates than to pyrethroids and organochlorine insecticides (Ripper, 1956; Moffet et al.,

1970). Also, both organophosphates and carbamates were much less persistent and thus had to be applied at more frequent intervals (Anderson & Nakakihari, 1968;

Bottrell & Adkinson, 1977). Johansen (1977) reported that azinphos-methyl and methomyl were hazardous to honey bees at any time on blooming crops while pyrethroids only caused minimal hazards. Both use of more selective chemicals and use of lower amounts of chemicals per acre will help diminish the problem of 63 pesticide damage to pollinators (bumble bees, honey bees) that play an important role in the pollination of the self-incompatible tomatillo (Shorey & Hall, 1963). One objective of our study was to search for insecticides that would be effective on the insect pests, but low in hazard to pollinators (Gaines, 1954; Johansen & Kleinschmidt,

1972).

A large and diverse group of insects attack tomatillo. Those causing direct damage by feeding on the fruit included tomato fruitworm, while aphids, thrips, and flea beetles, known vectors of cucumber mosaic virus, tomato spotted wilt virus, and

Physalis mosaic virus, respectively, are also significant pest problems (Figure 2.1).

Aphids and thrips were common pests of tomatillo. The green peach aphid ( M. persicae) and the potato aphid (M. euphorbiae) seldom occurred in the high densities that could directly impede plant growth (Semtner, 1984). However, thrips frequently built up large populations on tomatillo in May and early October. On small seedling plants thrips might destroy a large part of the photosynthesizing surface of the leaves.

E. cucumeris and D. balreata began to appear in May and their populations peaked in late September and early October, contributing to a high incidence of PhyMV

(Figure 2.1). Cool weather conditions present in the spring and fall that favored increased tomatillo yields were incidentally associated with the high incidence of virus vectors that peaked during the flowering periods (May and October), thereby favoring the transmission of virus diseases. However, in addition to the reduction of fruit damage byH, zea, it was observed that all insecticides tested provided good control of both virus vectors and other insects (armyworms, leafhoppers, leaffooted Figure 2.1. Key pests of tomatillo: (I) potato flea beetle, (2) tomato fruitworm, (3) potato aphid, (4) banded cucumber beetle, (5) Western flower thrips, (6) broad mite. 65

bug, southern green stink bug) commonly found on tomatillo (Figure 2.2) (Harding,

1971).

Establishment of an economic threshold and an economic injury level for H. zea on tomatillo would be difficult because differences in cultivars, areas of cultivation

(e.g. closeness to susceptible crops such as cotton, corn, soybean, solanaceous crops) and many other variables. Tomatillo growers might use phenological events (time of flowering, first fruit setting, etc.) to time the application of insecticides (Walgenbach et al., 1989). If this is coordinated with insect sampling, then phenological timing becomes a very useful technique and can contribute to the reduction of insecticide usage and a monetary saving to the grower, as has been reported with tomato (Lange

& Bronson, 1981). H. zea has been a primary fruit pest of tomatillo because of its wide host range, the occurrence of several generations per year, and the boring habits of the larvae -- from which developing fruit must be protected. H. zea flights were unpredictable in both time of occurrence and intensity, but were generally intense during August and September (Walgenbach et al., 1989). Early- and late-planted tomatillo often escaped heavy damage from H. zea\ but the overlapping generations of early or late insect vectors (aphids, thrips, flea beetles) may require early control to reduce the transmission of virus diseases. So, for the spring and fall tomatillo crop, an early preventive insecticide application (about 1 or 2 weeks before the flowering stage) should take place and continue at weekly intervals for the control of both virus vectors and of the first generation of H. zea. In the case of tomato,

Kennedy and coworkers (1983) also indicated that early planting followed by weekly Figure 2.2. Secondary pests of tomatillo: (7) Southern green stinkbug, (8) fall armyworm, (9) leafhopper, (10) sharp shooter, (11) leafhopper, (12) leaffooted bug. 67 insecticide application greatly reduced fruit damage by H. zea. The broad-spectrum pyrethroids will provide good insect control on tomatillo.

Due to the limited information on the cultural practices used with tomatillo and its recumbent growth habit that can prevent complete coverage of the plant with insecticides and postemergence herbicides, aluminum mulch was evaluated in 1992, then improved in 1993 with some additional insecticide applications. It appears that the aluminum-painted plastic mulch greatly reduced the amount of insects feeding on the plants. This non-insecticidal control approach also takes into consideration the additional beneficial effects of mulching on yield (e.g. soil temperature adjustment, weed control, water percolation, and prevention of fruit rot caused by R. solani and other soil-borne pathogens) (Harpaz, 1982; Greenough et al., 1990; Nawzocka et al.,

1975). However, one limitation is that with the sprawling habit of tomatillo, its developing foliage progressively shades the repellent reflection of the mulch surface.

This has proven true in regard to other recumbent plants like cucumber, squash, and watermelon (Smith & Webbs, 1969). So, results of this research indicated that two or three Permethrin applications (0.73 1/ha) on the aluminum-painted mulch plots not only significantly reduced the insect damage, but also resulted in two-fold yield increases as compared to the control plots in 1993.

Although none of the insecticides tested are labeled for tomatillo at this time, a recent release from the IR-4 program indicated that pesticide tolerances approved for tomato would be automatically approved for tomatillo, thus making it possible to label these insecticides through the IR-4 program. 68

LITERATURE CITED IN CHAPTER 2

Anderson, L.D. and Reynolds, H.T. 1960. A comparison of the toxicity of insecticides for the control of corn earworm on sweet corn. J. Econ. Entomol. 53:22-24.

Anderson, L.D. and Nakakihari. 1968. Toxicity of pesticides to corn earworm on sweet corn in Southern California 1962-1967. J. Econ. Entomol. 61:1477- 1482.

Adkisson, P.L. and Nemee, S.J. 1967. Effectiveness of certain organophosphorus insecticides against chlorinated hydrocarbon-resistant bollworm and tobacco budworm larvae. J. Econ. Entomol. 60:268-270.

Bishopp, F.C. 1929. The bollworm or corn earworm as a cotton pest. USDA Farm Bull. 1495. 14 pp.

Bottrell, D.G. and Adkinson, P.L. 1977. Cotton insect pest management. Ann. Rev. Entomol. 22:451-481.

Boudreaux, J. 1994. South-east Louisiana bell pepper crop may be hurt by fruit shedding. LSU Agriculture Center New. (6/2/94) 2 pp.

Brazzel, J.R., Newsom, L.D., Roussel, J.S., Lincoln, C., Williams, F.J., and Barnes, G. 1953. Tomato Fruitworm and Budworm in Louisiana and Arkansas. La. Agric. Exp. Sta. Tech. Bull. No. 482.

Breese, M.H. 1977. The potential for pyrethroids as agricultural, veterinary, and industrial insecticides. Pesticide Sci. 8:264-269.

Can, F., Rush, M.C., Valverde, R.A., Griffin, J.L., Story, R.N., Young, R.N., Blackmon, W.J., and Wilson, P.W. 1992. Tomatillo: a potential new crop for Louisiana. Louisiana Agric. 35:21-24.

Can, F., Valverde, R.A., and Rush, M.C. 1994. Two new vectors of Physalis mottle virus. Plant Disease 78:432.

Cantu, E. and Wolfenbarger, D.A. 1970. Toxicity of three pyrethroids to several insect pests of cotton. J. Econ. Entomol. 63:1373-1374.

Chalfant, R.B., Phatak, S.C., and Threadgill, E.D. 1979. Protection of direct-seeded tomatoes from insect injury with systemic insecticides in Georgia. J. Econ. Entomol. 72:587-589. 69

Creighton, C.S., McFadden, T.L., and Cuthbert, R.B. 1971. Control of caterpillars on tomatoes with chemicals and pathogens. J. Econ. Entomol. 64:737-739.

Creighton, C.S., McFadden, T.L., and Cuthbert, R.B. 1973. Tomato fruitworm: Control in South Carolina with chemical and microbial insecticides 1970-1971. J. Econ. Entomol. 66:473-475.

Gaines, R.C. 1954. Effects on beneficial insects of several insecticides applied for cotton insect control. J. Econ. Entomol. 47:543-544.

Greenough, D.R., Black, L.L., and Bond, W.P. 1990. Aluminum-surfaced mulch: An approach to the control of tomato spotted wilt virus in solanaceous crops. Plant Disease 74:805-808.

Harding, J.A. 1971. Field comparisons of insecticidal sprays for control of four tomato insects in South Texas. J. Econ. Entomol. 64:1302-1304.

Harpaz, I. 1982. Nonpesticidal control of vector-borne virus. In: Pathogens, Vectors, and Plant Diseases: Approaches to Control. (K.F. Harris and K. Maramorosch, eds.). pp. 1-24. Academic Press, New York.

Hoyt, S.C., Westigard, P.H., and Burts, E.C. 1978. Effects of two synthetic pyrethroids on the codling moth, pear psylla, and various mite species in northwest apple and pear orchards. J. Econ. Entomol. 71:431-434.

Johansen, C.A. and Kleinschmidt, M.G. 1972. Insecticide formulations and their toxicity to honeybees. J. Apic. Res. 11:59-62.

Johansen, C.A. 1977. Pesticides and pollinators. Ann. Rev. Entomol. 22:177-192.

Kennedy, G.G., Romanow, L.R., Jenkins, S.F., and Sanders, D.C. 1983. Insects and diseases damaging tomato fruits in the coastal plain of North Carolina. J. Econ. Entomol. 76:168-173.

Lange, W.H. 1978. Integrated control of tomato pests. Calif. Tomato Growers 21:4- 5.

Lange, W.H. and Bronson, L. 1981. Insect pests of tomatoes. Ann. Rev. Entomol. 26:345-371.

Lange, W.H. and Kishiyama, J.S. 1978. Integrated pest management on artichoke and tomato in Northern California. Calif. Agric. 32:28. 70

Lewis, D. 1953. Some factors affecting flower production in the tomato. J. Hort. Sci. 23:207-220.

Middlekauff, W.W., Gonzales, C.Q., and King, R.C. 1963. Effects of various insecticides in the control of caterpillars attacking tomato in California. J. Econ. Entomol. 56:155-158.

Mistric, W.J. Jr. and Smith, D.F. 1970. Organophosphorus and carbamate insecticides as substitutes for DDT in controlling the tobacco flea beetles in flue-cured tobacco. J. Econ. Entomol. 63:509-511.

Moffet, J.O., MacDonald, R.H., and Levin, M.D. 1970. Toxicity of carbaryl- contaminated pollen to adult honeybees. J. Econ. Entomol. 63:475-476.

Myers, C. 1991. Tomatillo. University of California Cooperative Extension Bulletin SM C-034. 2 pp.

Nawzocka, B.Z., Eukenrode, C.J., Uyemoto, J.K., and Young, D.H. 1975. Reflective mulches and foliar sprays for suppression of aphid-borne viruses in lettuce. J. Econ. Entomol. 68:694-698.

Pfrimmer, T.R. 1979. Heliothis spp.: Control on cotton with pyrethroids, carbamates, organophosphates, and biological insecticides. J. Econ. Entomol. 72:593-598.

Ripper, W.E. 1956. Effects of pesticides on balance of arthropod populations. Ann. Rev. Entomol. 1:403-438.

Ruscoe, C.N.E. 1977. The new NRDC pyrethroids as agricultural insecticides. Pesticide Sci. 8:236-242.

Semtner, P.L. 1984. Effect of transplantation date on the seasonal abundance of the green peach aphid (Homoptera: Aphididae) and two aphid predators on flue- cured tobacco. J. Econ. Entomol. 77:324-330.

Shorey, H.H. and Hall, I.M. 1963. Toxicity of chemical and microbial insecticides to pest and beneficial insects on poled tomatoes. J. Econ. Entomol. 56:813- 817.

Smith, F.F. and Webbs, R.E. 1969. Repelling aphids by reflective surfaces, a new approach to the control of insect-transmitted virus. In: Virus, Vectors, and Vegetation. (Maramorosch, K., ed.) pp. 631-639. Wiley (Interscience), New York. 71

Smith, O. 1935. Pollination and life history studies of the tomato Lycopersicon esculentum. Mill. Cornell Univ. Agric. Exp. Sta. Memo. 184. pp. 1-16.

Walgenbach, J.F., Shoemaker, P.B., and Sorenson, K.A. 1989. Timing pesticide applications for control of Heliothis zea (Boddie), Alternaria solani (Ell. and G. Martin) Sor., and Phytophthora infestans (Mont.) DeBary on tomatoes in western North Carolina. J. Agric. Entomol. 6:159-168.

Walgenbach, J.F., Leidy, R.B., and Sheets, T.J. 1991. Persistence of insecticides on tomato foliage and implications for control of tomato fruitworm (Lepidoptera: Noctuidae). J. Econ. Entomol. 84:978-986.

Wilcox, J. and Howland, A. 1963. The tomato fruitworm: How to control it? USDA Leaflet 367.

Yamaguchi, M. 1983. World Vegetables: Principles, Production, and Nutritive Values. Avi. Westprot Co. 415 pp.

Zalom, F.G., Wilson, L.T., and Hoffman, M.P. 1986. Impact of feeding by tomato fruitworm (Heliothis zea) and beet armyworm (Spodoptera exigua) on processing tomato fruit quality. J. Econ. Entomol. 79:822-826. CHAPTER 3

WEED CONTROL IN LOUISIANA GROWN TOMATILLO

72 73

INTRODUCTION

Tomatillo, commonly known as husk tomato, has generated interest as a new crop in California in the last decade and has potential for expansion in the Southern

Gulf States due to increased popularity of Mexican food. Imported tomatillo fruit are available seasonally in Louisiana supermarkets, but at higher prices (about $4.4/kg) reflecting transportation and storage costs.

In anticipation of the regional demand for tomatillo use in the Louisiana sauce industry, the fresh vegetable market, and for local garden use, recent research in

Louisiana has demonstrated that tomatillo yield was highest in the cooler temperatures of spring and fall (Can et al., 1992).

Despite potential as a new horticultural crop, little research has been conducted in the United States to address cultural practices for tomatillo. Field evaluations of planting dates and nitrogen fertilization showed that weeds, which compete for light, water, nutrients, and spaces, and harbor various pests and diseases, are often a limiting factor to maximizing yield of tomatillo (Tomlinson et al., 1970; Cho et al.,

1986; Rist & Lorbeer, 1989; Valverde et al., 1993). Yield losses of tomatillo of more than 70-80% with no weeding have been observed. The degree of yield loss can be influenced by the growth habit of the weed. Perennial, deep-rooted weeds with a spreading growth habit can cause more loss. The fast-growing weeds may cause greater losses from early season competition. At early stages of growth the annuals are troublesome, at later stages the perennial weeds are more harmful. Weeds are more competitive during the summer and early fall than in spring, thereby causing 74 greater losses (Gworggwoz, 1990). Although no herbicides are labeled for tomatillo at this time, a recent release from the IR-4 program indicated that pesticide tolerances approved for tomato (Doub et al., 1988; Fortino & Splittstoesser, 1974; Johnson &

Hopen, 1984; McGriffin & Masinuas, 1991; Usoroh, 1988) would also be approved for tomatillo (Monaco, 1977).

The purpose of this study was to evaluate the tolerance level of tomatillo to a number of herbicides presently labeled in tomatoes (Glaze, 1988; Henne, 1975; Hertz

& Anklam, 1986) and other vegetable crops (pepper Capsicum annuum L. var. annuum, potato Solatium tuberosum L., sweet corn Zea mays L. var. rugosa) as well as for those recommended for major agronomic crops (soybean, rice, cotton ) grown in Louisiana (Hilton & Minotti, 1983) (Tables 3.1 and 3.2). Information generated from this research should lead to increased potential for economic production of tomatillo under typical weed infestation levels in Louisiana.

MATERIALS AND METHODS

Preemergence and nostemergence greenhouse studies. Tolerance to preemergence and postemergence herbicide treatment was evaluated in the greenhouse during the springs of 1991 and 1994 using direct-seeded and transplanted tomatillo.

Seven preemergence herbicides, trifluralin (Treflan 4EC) at 1.12 kg ai/ha, metribuzin

(Sencor 75DF) at 0.28 kg ai/ha and 0.43 kg ai/ha, napropamide (Devrinol 2EC) at

1.12 kg ai/ha, metolachlor (Dual 8EC) at 2.24 kg ai/ha, pendimethalin (Prowl 4EC) at 1.12 kg ai/ha, alachlor (Lasso 4EC) at 2.24 kg ai/ha, and clomazone (Command

4EC) at 0.84 kg ai/ha, were applied to both field soil and a steamed soil mixture (2 Table 3.1. Herbicides tested on tomatillo and labeled for use in other Louisiana crops.

CROP

H ER B IC ID E1 Tomato Potato Pepper Cucurbits Soybean Cotton Rice Sweet Corn

Trifluralin • o 9 9

Metribuzin •• 9

Napropamide • 9

Metolachlor • 9 9 9

Alachlor 9 9

Pendimethalin •

Clomazone •

Sethoxydim •• 9 9

Fluazifop-butyl 9 9

Imazethapyr •

Fomesafen •

Bentazon 9 9

Acifluorfen 9 9

1 None of these herbicides is labeled for tomatillo. * Source: Louisiana’s Suggested Chemical Weed Control Guide for 1993. Table 3.2. Efficacy of herbicides for weed control in tomatillo.

BROADLEAVES GRASSES Common White Curly Bamyard- Crowfoot- Crab- Goose- Yellow Bermuda- HERBICIDE Carpetweed Purslane Pigweed Clover Dock Mayweed grass grass grass grass Nutsedge grass PRE Trifluralin •••• • Metribuzin ••••• •• • • Napropamide • • • •• • Pendimethalin 9 •• •••• Metolachlor • 9 • 9 • ••• Alachlor • 9 • o 9 •• • Clomazone 0 9 • • POST Sethoxydim • 9 ••• Fluazifop-butyl 9 • •• Quizalofop 9 • 9 9 •

Bentazone 9 •• 9 •

Imazethapyr • 9 9 9 • 9 9 Fomesafen • 9 • 9 9 •

Acifluorfen • 9 • e 9 9 9 0

* Sources: Louisiana’s Suggested Chemcical Weed Control Guide for 1993 1994 Crop Protection Chemicals Reference

O s 77 field soil: 1 sand: 1 peat moss) placed in 1-gallon pots. Herbicides were applied in a spray volume of 86 1/ha at 28kPa spray pressure. Trifluralin (Treflan), metribuzin

(Sencor), napropamide (Devrinol), and pendimethalin (Prowl) were incorporated to

2.5 cm deep immediately after application. Each treatment was replicated five times.

Twenty tomatillo seeds were planted in each pot containing the treated soil mixture at a depth of 1 cm. Percent seed germination was recorded three weeks after treatment. In the study of the preemergence herbicide tolerance of tomatillo grown in treated field soil and soil mixture, direct-seeded and transplanted tomatillo were thinned to one plant per pot. Six weeks after treatment, plant injury (based on a scale of 0 to 100 where 0 = no injury and 100 = plant death), plant height, and top and root growth were recorded to determine the level of tomatillo tolerance to the herbicide treatments.

Postemergence herbicides, sethoxydim (Poast 1.5 EC) at 0.20kg ai/ha, fluazifop-butyl (Fusilade 2000 1EC) at 0.22 kg ai/ha, quizalofop (Assure 0.8EC) at

0.08 kg ai/ha, bentazon (Basagran 4EC) at 0.84 kg ai/A, imazethapyr (Pursuit 2EC) at 0.07 kg ai/ha, fomesafen (Reflex 2EC) at 0.42 kg ai/ha, and acifluorfen (Blazer

2EC ) at 0.28 kg ai/ha, were applied to three-week-old tomatillo grown in soil mixture in pots in the greenhouse. A non-ionic surfactant at 0.25% (v/v) was added to all herbicide treatments except sethoxydim to which a 1 % (v/v) crop oil concentrate was added. Herbicides were applied in a spray volume of 86 1/ha. Each treatment was replicated five times. Plant height and injury (based on a scale of 0 to 100% where

0=no injury and 100=plant death) were evaluated two weeks after treatment while 78

top and root growth, and plant height were measured six weeks after treatment. Less phytotoxic herbicides were further evaluated under field conditions for both tomatillo

tolerance and weed control. The study was repeated twice and the experimental design was a randomized complete block with five replications. Data were subjected to analysis of variance. Means were separated using Duncan’s multiple range test at the P=0.05 probability level.

Pre and post-emergence herbicide studies. This experiment was conducted two times in 1992 on the LSU-AC Horticultural Farm and in 1993 at the LSU-AC

Burden Research Plantation in Baton Rouge, Louisiana. Each experimental plot was

0.60 m wide by 4.50 m long and consisted of 10 plants set in a single row spaced

0.45 m apart. Fertilizer 58-58-58 kg/ha (N-P20 5-K20) was broadcasted and incorporated before transplanting. Nitrogen fertilizer as ammonium nitrate (76 kg

N/ha) was applied three weeks after transplanting. Puebla Verde tomatillo plants started in the greenhouse were transplanted into the field March 25 and August 20,

1992 and March 20 and August 18, 1993. Preemergence herbicide treatments included: metribuzin (Sencor 0.75DF) at 0.28 kg ai/ha, trifluralin (Treflan 4EC) at

1.12 kg ai/ha, napropamide (Devrinol 2EC) at 1.12 kg ai/ha, metolachlor (Dual 8EC) at 1.68 ai/ha, pendimethalin (Prowl 4EC) at 1.12 kg ai/ha, and clomazone(Command

4EC) at 0.84 kg ai/ha. Metribuzin, trifluralin, napropamide, and pendimethalin were incorporated with hand rakes immediately after herbicide application while the other herbicides were not incorporated. All herbicides were applied 1 day before transplanting with a C02 powered backpack sprayer delivering 86 1/ha at 28 kPa 79

pressure. Postemergence herbicides sethoxydim (Poast 1.5EC) at 0.22 kg ai/ha,

fluazifop-butyl (Fusilade 2000 1EC) at 0.20 kg ai/ha, quizalofop (Assure 0.8EC) at

0.08 kg ai/ha, bentazon (Basagran 4EC) at 0.80 kg ai/ha, imazethapyr (Pursuit 2EC) at 0.07 kg ai/ha, and fomesafen (Reflex 2EC) at 0.43 kg ai/ha were applied 3 weeks after transplanting. With the exception of sethoxydim, where crop oil concentrate was applied at 1.0% (v/v), postemergence herbicide treatments contained non-ionic surfactant (X-77) at 0.25% (v/v).

Both grasses and broadleaf weeds were present in the field studies. The most predominant weeds included: barnyardgrass ( Echinochloa crus-galli L.), large crabgrass (Digitaria sanguinalis L.), crowfootgrass (Dactyloctenium aegyptium L.

Willd.), goosegrass (Eleusine indica L. Garetn.), yellow nutsedge (Cyperns escidentus

L.), carpetweed (Mollugo verricillata L.), Virginia pepperweed ( Lepidium virginicum

L.), spiny pigweed (Amaranthus crusgulli L.), common purslane ( Portulaca oleracea

L.), corn spurry (Sperguki arvensis L.), and curly dock ( Rumex crispus L.) (Figure

3.1).

Visual assessment of the crop injury and weed control were made around three weeks after transplanting, using a scale of 0 to 100 where 0 represented no crop injury or no weed control and 100 represented complete death of the crop. Yield of tomatillo plots were determined ten weeks after transplanting by harvesting fruit from all plants present in each plot. The experimental design was a randomized complete block with four replications. Figure 3.1. Common weeds in tomatillo fields: (1) broadleaf signalgrass, (2) large crabgrass, (3) barnyardgrass, (4) foxtail, (5) yellow nutsedge, (6) goosegrass (figure con’d). Figure 3.1 (Continued). Common weeds in tomatillo fields: (7) spergula, (8) Virginia pepperweed, (9) spiny pigweed, (10) groundcherry, (11) common purslane, (12) white clover. 82

RESULTS

Greenhouse studies. Tomatillo seed germination was at least 89% with napropamide, pendimethalin, and trifluralin, and greater than for alachlor, clomazone, or metribuzin (Table 3.3). For metolachlor, tomatillo germination was equivalent to that for napropamide, pendimethalin, or trifluralin, but less than for the nontreated control. Seed germination was 75 and 35% for alachlor and clomazone, respectively, and emerged seedlings were stunted. Bleaching of tomatillo leaves was noted with clomazone. With pendimethalin, seedling emergence occurred with subsequent death within 3-5 days after emergence. Significant differences in plant height, and top and root growth were not noted for tomatillo in soil mixture treated with napropamide, pendimethalin, or trifluralin (Table 3.4). Other herbicide treatments for the soil mixture showed reduction in the growth parameters compared with the nontreated control. In contrast, for the field soil, only pendimethalin resulted in tomatillo plant height and root growth comparable to the control. The variation in response is probably due to greater adsorption of herbicide in the soil mixture containing peat moss compared with the field soil. Less herbicide would be present in the field soil solution and therefore unavailable for plant uptake.

For the transplanted tomatillo, tolerance to pendimethalin, napropamide, trifluralin and metolachlor was equivalent to the nontreated control when planted in the soil mixture (Table 3.5). When tomatillo was transplanted into treated field soil, plant height and top growth was less than for the nontreated check, but root growth in soil treated with pendimethalin, trifluralin, napropamide, or metolachlor was 83

Table 3.3. Effect of preemergence herbicides on seed germination in direct-seeded tomatillo1.

Rate Seed Germination Herbicide (kg ai/ha) (%)

Nontreated — 96 a2

Napropamide 1.12 91 ab Pendimethalin 1.12 90 ab Trifluralin 1.12 89 ab

Metolachlor 1.68 83 be Alachlor 2.24 75 c Clomazone 0.84 35 d Metribuzin 0.28 0 e Metribuzin 0.43 0 e

1 Values represent an average for two greenhouse studies. 2 Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test. Table 3.4. Tolerance of direct-seeded tomatillo to preemergence herbicides in greenhouse studies.

Soil Mixture Field Soil

Plant Top Root Plant Top Root Rate Height Grow th G row th Height Grow th G row th T reatm ent (kg ai/ha) (cm) (g) (g) (cm) (g) (g) Pendimethalin 1.12 109.2 ab1 12.40a 1.16a 85.0 a 8.42 b 0.84 ab

Trifluralin 1.12 106.0 ab 12.30a 1.12a 78.6 b 7.86 be 0.78 be

Napropamide 1.12 102.2 ab 11.70 a 1.04 ab 77.8 b 7.47 c 0.77 c

Metolachlor 1.68 99.8 b 10.47 b 0.96 be 62.4 c 6.50 d 0.61 d

Alachlor 2.24 85.2 c 8.33 c 0.85 c 58.8 c 5.29 e 0.57 d

Clomazone 0.84 11.4 d 0.76 d 0.05 d 0.0 d 0.00 f 0.00 e Metribuzin 0.28 0.0 e 0.00 d 0.00 d 0.0 d 0.00 f 0.00 e

Metribuzin 0.43 0.0 e 0.00 d 0.00 d 0.0 d 0.00 f 0.00 e

Control — 111.2a 12.82a 1.11 a 89.2 a 10.30 a 0.89 a (nontreated)

1 Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test. Table 3.5. Tolerance of transplanted tomatillo to preemergence herbicides in greenhouse studies.

Soil Mixture Field Soil

Plant Plant Dry Root Dry Plant Plant Dry Root Dry Rate Height Weight Weight Height Weight Weight Treatment (kg ai/ha) (cm) (g) (g) (cm) (g) (g) Pendimethalin 1.12 121.6 ab1 14.60 ab 1.27 a 98.0 b 9.40 b 0.94 a

Trifluralin 1.12 113.8b 13.21 be 1.20 a 96.2 b 9.36 b 0.97 a

Napropamide 1.12 129.2 a 15.40 a 1.37 a 97.6 b 9.11 b 0.96 a Metolachlor 1.68 115.4b 14.20 ab 1.24 a 96.0 b 9.11 b 0.95 a Alachlor 2.24 97.8 c 11.95 cd 0.95 b 83.6c 8.10c 0.85 b

Clomazone 0.84 84.4 d 10.64 d 0.86 b 60.4 d 5.16 d 0.50 c

Metribuzin 0.28 0.0 e 0.00 e 0.00 c 0.0 e 0.00 e 0.00 d

Metribuzin 0.43 0.0 e 0.00 e 0.00 c 0.0 e 0.00 e 0.00 d

Control — 122.6 ab 15.10a 1.32a 103.0 a 10.50 a 1.00 a (nontreated)

1 Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test.

00 86 equivalent to the control. Tomatillo was more sensitive to clomazone than alachlor when direct-seeded but injury was unacceptable regardless of whether direct-seeded or transplanted. Tomatillo showed no tolerance to metribuzin whether direct-seeded or transplanted.

All transplanted tomatillo plants died following postemergence application of imazethapyr, fomesafen, or acifluorfen (Table 3.6). Significant phytotoxicity was observed on plants treated with bentazon. Even though visual injury to tomatillo was not noted with sethoxydim, fluazifop-butyl, or quizalofop, stunting was observed with sethoxydim and fluazifop-butyl 6 weeks after treatment, resulting in reduced plant growth and root biomass when compared to the nontreated control. This response, however, was not noted for quizalofop. Since sethoxydim, fluazifop-butyl, and quizalofop only have activity against grasses, the reason for the response was not apparent.

Field studies. Preemergence and postemergence herbicides were compared in field studies for tomatillo phytotoxicity and weed control. Tomatillo planted in spring or fall was injured no more than 4% with metolachlor, trifluralin, napropamide, or pendimethalin applied preemergence or fluazifop-butyl, sethoxydim, or quizalofop applied postemergence (Table 3.7). Injury to tomatillo was 18 to 68% for imazethapyr, fomesafen, clomazone, alachlor, and bentazon. Metribuzin caused injury to at least 96% of treated tomatillo tissues. For a herbicide to be useful it should not only be safe to tomatillo, but must also provide acceptable weed control.

Of the herbicide treatments which provided acceptable tomatillo tolerance Table 3.6. Tolerance of transplanted tomatillo to postemergence herbicides in greenhouse studies.

Soil Mixture

Rate Plant Injury Plant Height Plant Height Plant Dry Root Dry Treatment1 (kg ai/ha) (%) 14 DAT (cm) 42 DAT (cm) Weight (g) Weight (g)

Sethoxydim 1.12 0.0 c2 52.30 b 111.0 b 13.37 c 1.08 b

Fluazifop-butyl 0.20 0.0 c 71.30 a 113.0 b 13.82 be 1.11 b

Quizalofop 0.08 0.0 c 75.40 a 121.8 a 14.60 ab 1.28 ab

Bentazon 0.80 15.0 b 56.20 b 107.8 b 13.23 c 1.12 b

Imazethapyr 0.07 100.0 a 0.00 c 0.0 c 0.00 d 0.00 c

Fomesafen 0.43 100.0 a 0.00 c 0.0 c 0.00 d 0.00 c

Acifluorfen 0.28 100.0 a 0.00 c 0.0 c 0.00 d 0.00 c

Control (nontreated) — 0.0 c 72.20 a 126.2 a 15.08 a 1.36 a

1 Non-ionic surfactant (X-77) at 0.25% (v/v) added to all treatments except sethoxydim where crop oil concentrate was added at 1.0% (v/v). 2 Means followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test. Table 3.7. Effects of preemergence and postemergence herbicides on phytotoxicity of transplanted tomatillo and weed control in field studies. Phytotoxicity (%) Weed Control (%)' Spring Fall Spring 1993 Fall 1993 Herbicide Rate Appl. 1993 1993 Grasses Broadleaves Grasses Broadleaves (kg ai/ha) Time Metribuzin 0.28 PRE 96 a2 97 a 95 ab 97 a 96 ab 97 a Imazethapyr 0.07 POST 57 b 64 b 29 e 94 a-d 21 e 96 a Fomesafen 0.43 POST 62 b 67 b 24 e 95 abc 17 e 95 ab Clomazone 0.84 PRE 39 c 39 c 96 a 97 a 97 a 95 ab Alachlor 2.24 PRE 24 d 25 d 82 d 89 d 86 d 90 b Bentazon 0.80 POST 17 e 24 d 87 cd 47 e 92 abc 44 c Metolachlor 1.68 PRE 4 f 1 e 95 ab 96 ab 94 abc 94 ab Trifluralin 1.12 PRE O f Oe 90 abc 91 bed 93 abc 95 ab Napropamide 1.12 PRE O f 0 e 86 bed 92 bed 91 bed 92 ab Pendimethalin 1.12 PRE O f 0 e 84 cd 91 bed 90 cd 90 b Fluazifop-butyl 0.20 POST O f 0 e 90 abc 13 g 92 abc 19 e Sethoxydim 1.12 POST O f Oe 85 cd 25 f 89 cd 32 d Quizalofop 0.08 POST O f 0 e 85 cd 8 g 89 cd 12 f ------Control O f Oe O f Oh O f Og (nontreated)

1 Grasses=barnyardgrass, goosegrass, crowfootgrass, large crabgrass. Broadleaves=carpetweed, spiny pigweed, Virginia pepperweed, common purslane. 2 Means within each column followed by the same letter do not significantly differ at P=0.05, using Duncan’s multiple range test.

oo 00 89

(metolachlor, trifluralin, napropamide, and pendiinethalin preemergence and fluazifop-

butyl, sethoxydim, and quizalofop postemergence) control of grasses following fall or

spring application was at least 84% (Table 3.7). Broadleaf weed control (Virginia carpetweed, spiny pigweed, common purslane) was at least 90% with the preemergence herbicides exhibiting tomatillo tolerance, but unacceptable with the postemergence treatments.

When weed control was obtained and herbicides were not injurious to tomatillo, significant yield increases were obtained when compared with the nontreated control (Table 3.8). Tomatillo yields of more than 11,200 kg/ha were obtained in both spring and fall plantings which reflected the crop safety and weed control with the selective preemergence (metolachlor, trifluralin, napropamide, pendiinethalin) and postemergence (sethoxydim, fluazifop-butyl, quizalofop) herbicides (Table 3.8).

Injury to tomatillo with clomazone and alachlor resulted in 12 to 25% yield reductions despite applications of sethoxydim, fluazifop-butyl, and quizalofop. Yield reductions of 20 to 80% were observed with bentazon, imazethapyr, and fomesafen applied postemergence, following the preemergence herbicides metolachlor, trifluralin, napropamide, or pendiinethalin. Despite its high efficacy on weed control, phytotoxicity observed with metribuzin resulted in tomatillo yields of no more than

98 kg/ha, which were lower than the nontreated control.

DISCUSSION

An integrated weed management program involves proper use of herbicides best suited to the weed problem, correct assessment of the critical period for weed Table 3.8. Effects of preemergence and postemergence herbicide treatments on yield of tomatillo in field studies. Yield (kg/ha) Rate Treatment1 (kg ai/ha) Spring ’92 Spring ’93 Fall ’92 Fall ’93 T rifluralin + sethoxydim 1.12+1.12 11,853 a-d2 12,199 abc 13,879 a 12,089 def Trifluralin + fluazifop-butyl 1.12+0.20 12,109 abc 12,011 be 13,648 ab 12,837 be Trifluralin+quizalofop 1.12+0.08 11,363 cde 11,733 cd 13,328 ab 11,843 ef Metolachlor+sethoxydim 1.68+1.12 12,279 ab 12,749 a 13,193 be 13,558 a Metolachlor+fluazifop-butyl 1.68+0.20 12,401 a 12,513 ab 13,456 ab 13,261 ab Metolachlor-(-quizalofop 1.68+0.08 11,935 abc 12,165 be 12,483 de 12,730 bed Naproamide+sethoxydim 1.12 + 1.12 11,500 b-e 11,966 be 12,515 de 12,280 c-f Naproamide-t- fluazifop-butyl 1.12+0.20 11,776 a 12,112 be 12,807 cd 12,604 bed Naproamide+quizalofop 1.12+0.08 11,451 b-e 11,782 be 12,289 de 11,623 c-f Pendimethalin +sethoxydim 1.12 + 1.12 11,343 cde 11,739 cd 12,280 def 12,572 cd Pendimethalin + fluazifop-butyl 1.12+0.20 11,829 a 11,923 be 12,109 ef 12,482 cde Pendi methalin+ quizalofop 1.12+0.08 11,297 cde 11,287 de 11,836 f 11,631 fg Alachlor+sethoxydim 2 .24+ 1.12 10,998 def 10,721 ef 10,780 g 11,083 gh Alachlor+fluazifop-butyl 2 .2 4 + 0 .2 0 10,721 efg 10,886 ef 10,503 ghi 10,591 hij Alachlor-t- quizalofop 2 .24+ 0.08 10,394 fgh 10,316 fg 10,265 g-k 10,799 hij Clomazone -1- sethoxydim 0 .8 4 + 1 .1 2 9,660 hi 10,604 f 10,159 h-k 10,898 hi Clomazone+fluazifop-butyl 0.84+0.20 9,860 hi 10,665 f 10,402 g-j 10,591 hij Clomazone+quizalofop 0.84+0.08 9,374 i 10,338 fg 10,035 ijk 10,115 jkl (table con’d) o Table 3.8. Continued.

Yield (kg/ha) Rate Treatment (kg ai/ha) Spring ’92 Spring ’93 Fall ’92 Fall ’93 T rifluralin+ bentazon 1.12+0.80 9,713 hi 9,511 hi 10,694 gh 9,800 kl Metolachlor+ bentazon 1.68+0.80 10,130 ghi 10,768 ghi 10,312 g-j 10,999 ghi Naproamide+ bentazon 1.12+0.80 9,751 hi 9,588 gh 10,041 'jk 10,360 ijk Pendimethalin+bentazon 1.12+0.80 9,654 hi 9,261 i 9,885 jk 9,625 1 Alachlor+bentazon 2.2 4 + 0 .8 0 9,771 hi 8,382 j 9,687 k 8,541 mno Clomazone+bentazon 0.8 4 + 0 .8 0 8,577 j 9,141 i 8,943 1 . 9,587 1 Trifluralin+imazethapyr 1.12+0.07 7,224 kl 7,886 ju 8,973 1 8,137 n-q Metolachlor+ i mazethapy r 1.68+0.07 6,838 1m 7,200 mno 8,316 m 7,822 pq Naproamide+imazethapyr 1.12+0.07 7,420 kl 8,187 jk 7,900 m 8,427 m-q Pendimethalin+imazethapyr 1.12+0.07 8,035 jk 8,002 jkl 7,947 m 8,828 m Clomazone+i mazethapy r 0.84+0.07 6,119 mn 6,702 op 7,069 n 7,454 qr Alachlor+imazethapyr 2.24+0.07 7,437 kl 6,453 P 7,198 n 6,770 s Trifluralin+ fomesafen 1.12+0.43 6,955 1 7,487 mn 8,311 m 7,827 pq Metolachlor+fomesafen 1.68+0.43 6,749 lm 7,084 no 7,959 m 7,566 q Naproamide+ fomesafen 1.12+0.43 7,172 kl 7,755 klm 7,822 m 7,891 opq Pendimethalin+ fomesafen 1.12+0.43 7,001 1 7,531 lmn 7,815 m 8,596 mn Alachlor+ fomesafen 2.24+ 0.43 7,084 1 6,811 op 6,926 n 6,868 rs Clomazone+ fomesafen 0.84+0.43 5,840 n 6,229 P 6,766 n 7,632 q (table con’d) Table 3.8. Continued. Yield (kg/ha) Rate Treatment (kg ai/ha) Spring ’92 Spring ’93 Fall ’92 Fall ’93

Metribuzin + sethoxydim 0.28+1.12 49 P 59 r 94 P 87 u Metribuzin+ fluazifop-butyl 0 .28+ 0.20 53 P 81 r 73 P 97 u Metribuzin+quizalofop 0.28+0.08 24 P 77 r 78 P 79 u Metribuzin+bentazon 0.28+0.80 35 P 54 r 68 P 69 u Metribuzin+ i mazethapyr 0.28+0.07 18 P 27 r 63 P 56 u Metribuzin+fomesafen 0.28+0.34 21 P 16 r 58 P 54 u Control (nontreated) 3,583 0 5,168 q 5,044 o 5,587 t

1 Non-ionic surfactant (X-77) at 0.25% (v/v) added to all treatments except sethoxydim where crop oil concentrate was added at 1.0% (v/v). 2 Means within each column followed by the same letter do not significantly differ at P=0.05, Duncan’s multiple range test.

VO K> 93 control, and use of cultural practices that favor the growth of the crop and minimize the competitiveness of weeds (Ashley, 1989). Unlike an integrated pest management program where observations relative to pest problems are made while the crop is growing, the integrated weed management begins before the crop is planted, preferably in the previous year where weeds can be identified and weed control programs can be developed. The critical period of weed interference is a specific minimum period of time during which the crop must be free of weeds in order to prevent loss in yield and represents the overlap of two separate components: (a) the length of time weeds can remain in a crop before interference begins and (b) the length of time that weed emergence must be prevented so that subsequent weed growth does not reduce crop yield. These two components clearly depend in part upon the relative growth rates of the crop and its associated weeds (Durgy & Ashley,

1993; Friesen, 1979; Weaver & Tan, 1983). In a three-year field survey it was observed that the critical period of weed interference in tomatillo was between 3 and

5 weeks after transplanting.

For direct-seeded tomatillo, a selected herbicide should be toxic to weeds but not detrimental to germination or growth of tomatillo seedlings. In transplanted tomatillo, the basis of selectivity could be related to the difference in crop and weed growth stages. Consequently, a herbicide may be toxic to direct-seeded tomatillo but may not injure transplanted tomatillo. This type of activity has been common in tomato (Wilson et al. 1969; Henne, 1979; Gottlieb, 1982; Gorske, 1982; Glaze, 1988;

Hertz & Anklam, 1986). Tomatillo production in Louisiana was highest in the spring 94 and fall due to the cool weather conditions suitable for the fruit set and to the low infestation of tomato fruitworm. However, favorable cool weather conditions were incidentally associated with the high incidence of virus vectors that peaked during the flowering periods (May and October), thereby favoring the transmission of virus diseases (Can et al., 1992). So, tomatillo transplanting will probably be preferred to direct seeding.

Results from both greenhouse and field studies indicated that tomatillo was highly tolerant to preemergence (trifluralin, napropamide, pendimethalin) and postemergence (sethoxydim) herbicides currently labeled for use on tomatoes.

Although metribuzin provided adequate preemergence control of carpetweed, pigweed, purslane, crabgrass, barnyardgrass, goosegrass, and crowfootgrass in direct-seeded or transplanted tomatillo, it caused severe crop injury (Fortino & Splittstoesser, 1974;

Labrada & Garcia, 1978; Balinova & Konstantinov, 1982; Nelson & Ashley, 1980).

Phytotoxicity was greatly enhanced when treatments were applied during cold, wet, cloudy conditions as seen in early-season tomato planting (Phatak & Stephenson,

1973; Friesen & Hamil, 1978; Pritchard & Warren, 1980; Boudreaux et al., 1990).

Trifluralin at 1.12 kg ai/ha incorporated before transplanting of tomato gave excellent control of pigweed, carpetweed, purslane, barnyardgrass, and crabgrass

(Mullins & Coffey, 1983). Preplant incorporation of trifluralin is needed to increase its persistence in the soil since factors that govern its disappearance from soil include physical loss by volatilization and leaching, and degradation through photochemical, microbiological, and chemical processes. Such factors are influenced by methods 95 of application and incorporation, climatic conditions, and soil properties (Savage,

1973). Wright and Warren (1965) demonstrated photochemical degradation of trifluralin on soil surfaces when exposed to solar radiation. Messmersmith et al.

(1971) also found complete detoxification of trifluralin in aqueous solution after exposure to sunlight for 4 hours. Horowith (1969) reported that, if properly incorporated before transplanting, trifluralin was more persistent under field conditions than under greenhouse conditions. Preplant incorporated applications of napropamide and pendimethalin in the present studies provided good control of purslane, pigweed, carpetweed, crabgrass, goosegrass, barnyardgrass, and yellow nutsedges commonly found in tomatoes (Cruz & Saito, 1982; Sanok et al., 1980; McCarty & Talbert,

1990; Romanowski et al., 1980; Glaze, 1990). Due to their relatively broad-spectrum weed control, soil persistence, and safety to tomato, trifluralin, napropamide, and pendimethalin, either alone or in combination with fluazifop-butyl, acifluorfen, or metribuzin provide excellent weed control in tomato (Sanok et al., 1980; Skrock &

Monaco, 1980). However, the sprawling growing habit of tomatillo did not facilitate the incorporation of preemergence products in the herbicide combinations.

Postemergence application of metribuzin following preemergence application of metribuzin or preplant incorporated trifluralin or napropamide significantly improved weed control on tomatoes (Henne & Guest, 1947; Stephenson et al., 1960; Kalia &

Sani, 1980; Mohammed & Sweet, 1976). Metribuzin was more effective in controlling broadleaf weeds than grass weeds, but grass control was acceptable at higher rates. However, tomatoes were more susceptible to injury when they were less 96 than 10 cm tall (Fortino & Splittstoesser, 1974; da Silva & Warren, 1976).

Metribuzin applied after transplanting would be to phytotoxic to tomatillo, resulting in crop stand reduction.

Although preplant applications of metolachlor slightly reduced tomatillo vigor in the first ten days following transplanting in the late spring, the plant quickly recovered as the soil temperature increased. Similar occasional injury symptoms from metolachlor were also observed on transplanted tomato although yield was not affected

(Glaze, 1988; Teasdale, 1985). Due to its soil residual activity on annual grasses, it might be used after planting or late season weed control.

A significant reduction in tomatillo vigor and yield resulted with alachlor and clomazone despite their high efficacy of weed control (Swain 1980; Sweet et al.,

1980; Medrano et al. 1976). Both acifluorfen and fomesafen are diphenyl ether herbicides that disrupt cell membranes. Acifluorfen provided some adequate weed control in tomato with some crop injury (Orr et al. 1987; Teasdale, 1987). Tomato injury was also influenced by size of the plants at the time of applications.

Phytotoxicity 7 DAT was 59% for plants treated at 15 to 20 cm and only 18% when plants were 60-67 cm tall. Increased phytotoxicity occurred at the higher rates.

Temperature at time of acifluorfen application did not influence phytotoxicity

(Masiunas & Weller, 1986). Unlike tomato, tomatillo was severely injured by acifluorfen, fomesafen, and imazethapyr applications in the present studies. Plant death under field conditions occurred within 48 hours following acifluorfen applications, while severe crop injury occurred on maturing tomatillo plants treated 97 with imazethapyr or fomesafen. The addition of surfactant with these herbicides gave both increased weed control and increased crop injury in tomatillo.

Fluazifop-butyl, sethoxydim, and quizalofop were nonphytotoxic to tomatillo and provided excellent postemergence grass control. These herbicides are often applied in weed control programs with preemergence herbicides to increase the efficacy of both grasses and broadleaf weeds in tomato (Singh et al., 1984; Wells et al., 1985; Brathwaite, 1986; Johnson & Hopen, 1984; Hilton & Minotti, 1983).

Sequential treatments of trifluralin, metolachlor, napropamide, and pendimethalin preemergence and sethoxydim, fluazifop-butyl, and quizalofop postemergence provided excellent weed control with crop safety and subsequent increases in tomatillo yield.

In general, when an herbicide is applied for selective weed control in crops the objective is to find the best combination of a relatively susceptible stage of weed growth and a relatively tolerant stage of crop growth. In these weed control studies conducted over 3 years, full season control of many grass and broadleaf weeds was obtained with sequential treatments of metolachlor, trifluralin, napropamide, or pendimethalin preemergence followed by sethoxydim, fluazifop-butyl, or quizalofop postemergence without reducing tomatillo yields. Further studies should be conducted to refine rates and times of application which may provide season-long control in tomatillo. 98

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McCarty, J.T., and Talbert, R.E. 1990. Reduced Herbicide Rate in Vegetable Crops. Proc. South. Weed Sci. Soc. 43:173.

McGriffin, M.E., Jr. and Masiunas, J.B. 1991. Postemergence control of broadleaf weeds in tomato (Lycopersicon esculentum). Weed Technology 5:739-7445.

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EFFECTS OF PLANTING DATES, NITROGEN FERTILIZATION,

AND MULCHING ON YIELD IN TOMATILLO

103 104

INTRODUCTION

Tomatillo, a self-incompatible species, is an important vegetable crop in

Mexico, with over 11,000 ha planted. Small plantings are also grown in the warmer areas of California (Myers, 1991). Commercial cropping has been successful along the central and south coasts of California, as well as in the low deserts and the Central

Valley. However, due to the limited research on breeding and cultural practices with this crop, yield in California varied considerably from 1,500 to 11,000 kg/ha. Plant density and spacing also varied among growers who attempted to improve the tomatillo yield by their own cultivar selections. They also preferred direct seeding to transplanting, which was only used to fill in stands.

Cultivation of tomatillo is tricky because of its wide range of genetic variability

(plant habit, leaf shape, fruit size and shape, and yield) due to its breeding system, which insures outcrossing via self-incompatibility (Quiros, 1984). The spectrum of self-incompatible phenotypes observed upon self-pollination included plants totally self-incompatible, to plants with parthenogenic fruit, or with various degrees of self­ incompatibility subject to low seed production and germination (Pandey, 1957; Quiros,

1977, 1984).

In Mexico, tomatillo is produced at elevations over 800 m where climate corresponds to cool weather seasons in Louisiana due to its higher latitudes (Mosino-

Aleman et al., 1974). However, virus diseases (Physalis mottle virus, cucumber mosaic virus, tomato spotted wilt virus), pests ( H. zea, P. latus), variability of some important agronomic traits, and high temperature during the flowering and fruit setting 105

periods were found to be the major constraints to tomatillo production in Louisiana

(Can et al., 1992; Valverde et al., 1993).

The investigations reported here were conducted to determine planting dates leading to adequate tomatillo production under Louisiana planting conditions, to determine yield response to nitrogen fertilization, and to increase tomatillo yield through improved cultural practices by using reflective mulch to reduce both insect vector populations and the incidence of virus diseases.

MATERIALS AND METHODS

Influence of planting dates on tomatillo yields. Seeds of the tomato cultivar

Celebrity and the tomatillo cultivars Puebla Verde, Rendidora, and Guanajuato were provided by Peto Seed Inc. of Saticoy, California, for use in field trials. Seeds of these cultivars were sown directly into Jiffy Mix® pots in flats and thinned to one seedling/pot about 10 days after emergence. Three- or 4-week old seedlings were transplanted to the horticulture plots of the LSU Horticultural Farm in Baton Rouge on the following dates: 1990 (May 26, Aug. 1, Sept. 1), 1991 (May 29, June 23,

July 15, Aug. 18, Sept. 1) and 1992 (March 20, Aug. 20).

Plots of 0.60 m x 9 m were arranged in a randomized complete block (RCB) design with four replicates of each cultivar on each planting date. Plots consisted of

20 plants set in a single row on 0.45-m centers with a 1.20-m spacing between rows.

Fertilization was a preplant application of 448 kg/ha of 58-58-58 (N-P20 5-K20) with an additional 224 kg/ha of Ammonium nitrate. Trifluralin 4 EC at 1.75 1/ha was incorporated thoroughly into the top two inches of the soil prior to transplanting for 106

weed control in 1990, while metolachlor 8 EC was applied at 2.34 1/ha in the 1991 and 1992 growing seasons. Sethoxydim 1.5 EC (1.75 1/ha) and Gramoxone Super

(2.92 1/ha) applications and additional hoeing were used to ensure efficient postemergence weed control. Permethrin 2 EC (0.73 1/ha) was applied to 1991 and

1992 plots for the control of tomato fruitworm. The 1990 plots were left untreated to study the level of damage caused by this pest.

Yield response of tomatillo cultivars to nitrogen fertilizer.Field studies were conducted on the same location with transplanting on May 26 and June 16 of

1990, and March 21 and August 20 of 1992. A split plot design was employed with nitrogen treatments ranging from 58 to 260 kg of nitrogen per hectare as main plots.

Subplots 0.60x9 m were composed of 20 plants each of 3 cultivars: Puebla Verde,

Rendidora, and Guanajuato. Four hundred forty-eight kg of 58-58-58 (N-P20 5-K20) per hectare were applied after plowing as preplant fertilizer and disced in before rows were prepared. Ammonium nitrate was used as the nitrogen source for the side application in all experiments reported here. The nitrogen treatments employed were as follows: 58, 108, 159, 209, and 260 kg of ammonium nitrate per hectare. Weed and pest control measures were the same as in the previous study. Mature fruits were harvested weekly starting about 6-7 weeks after transplanting and recorded for the total weight of fruits. Fruits damaged by tomato fruitworm feeding were only recorded from the 1990 trial plots.

Effects of mulching on tomatillo yield.Tomatillo cv. Puebla Verde was planted in seedling trays containing a commercial potting mixture (Jiffy-Mix®) in the 107 glasshouse. Four-week-old seedlings were used for the spring transplanting (March

18, 1993) while 3-week-old seedlings were transplanted on August 15, 1993 for the fall crop, into the horticultural plots on the LSU-AC Burden Research Plantation Farm in Baton Rouge.

Plots 0.60x9 m were arranged in a randomized complete block (RCB) design with four replications for each of the following six treatments on each planting date: aluminum painted plastic mulch, aluminum painted plastic mulch + insecticide, black plastic mulch, black plastic mulch-Finsecticide, bare ground, bare ground + insecticide.

Plots consisted of 20 plants transplanted in a single row on 0.45-m centers. A buffer row was used to separate the plots with treatments to facilitate the insecticide application and fruit harvest.

Fertilization consisted of 448 kg of 58-58-58 (N-P20,-K20) per hectare that was broadcast and incorporated to a depth of about 30 cm before planting. Additional ammonium nitrate fertilizer was applied at the rate of 1 tablespoon/plant about 3 weeks after transplanting. Metolachlor 8 EC was applied at the rate of 2.34 1/ha on the bare ground plots for preemergence weed control. Preplant treatment with Terr-

O-gas (67% methyl bromide + 33% chloropicrin) at 120 kg/ha was used only in the mulched plots for cutworm control in the spring. Gramoxone Super at 2.92 1/ha was applied as a postdirected spray between rows for postemergence weed control.

Permethrin 2 EC (0.73 1/ha) was applied weekly on the insecticide treated plots starting with the initiation of flower buds. Kelthane 35 WP was also applied at the rate of 1 tsp/3.8 1 for the control of broad mite (P. latus). This mite has become a 108 serious problem in solanaceous crops (tomato, pepper) in the hot dry weather conditions in Louisiana since 1992.

RESULTS

Effects of planting dates on tomatillo yield. The yield responses of both tomatillo and tomato cultivars to planting dates are shown in Table 4.1. Like its close relative, tomato, all three tomatillo cultivars had low yields when they were transplanted in May, June, July, and early August. The lowest yield occurred with the Guanajuato cultivar. In addition, tomato fruitworm incidence increased as the season progressed, with the peak occurring from July to September (Table 4.2).

Without appropriate insect control, the mean weight of damaged fruits reached 23% in the summer of 1990. Damage from virus diseases, which are insect-transmitted, was also most severe during this period, resulting in uniform yellowing of the plants, which was often seen in the Guanajuato cultivar.

With the exception of the early frost on November 3, 1991, causing both fruit abortion and damage of the immature fruits of late planted tomatillo, the highest yields were observed in both tomato and tomatillo planted in late March or late August.

Transplanting in early September increased the risk of damage from unexpected early frosts in November.

Yield response of tomatillo cultivars to additional nitrogen fertilization.

Yield responses of the Puebla Verde, Rendidora, and Guanajuato cultivars to different nitrogen fertilizer rates are shown in Tables 4.3 and 4.4. There was a significant 3- way interaction between cultivar x planting date x nitrogen rate. The higher yield 109 Table 4.1. Effect of planting date on tomatillo yield.

Yield (kg/ha)

Tomatillo Tomato Planting ------Date Rendidora Puebla Verde Guanajuato Celebrity

3/15/92 13,279 a1 11,525 b 11,060 b 47,754 a

5/26/90 5,647 ef 7,635 d 1,617 j 30,718 d 5/29/91 5,415 f 6,788 d 2,302 i 29,067 d 6/23/91 5,825 ef 6,693 d 2,621 h 30,477 d

7/15/91 6,494 e 6,883 d 4,054 g 29,843 d 8/01/90 5,806 ef 5,337 e 4,449 f 31,315 d

8/18/91 9,626 c 11,048 b 7,262 d 44,431 b 8/20/92 13,535 a 12,631 a 11,518 a 48,874 a

9/01/90 12,165 b 12,824 a 10,466 c 47,060 ab

9/01/912 8,154 d 8,697 c 5,583 e 41,320 c

Means followed by the same letter do not signficantly differ at P=0.05, using Duncan’s multiple range test. Early frost on November 3, 1991. Table 4.2. Influence of planting date on the incidence of fruitworm (Helicoverpa zed) damage in tomatillo.

Rendidora Puebla Verde Guanajuato Celebrity

Date Total Damaged Total Damaged Total Damaged Total Damaged Weight Fruit Weight Fruit Weight Fruit Weight Fruit (kg/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha) (kg/ha)

5/26/90 5,647 1,289 b 1 7,635 1,677 b 1,617 370 c 30,718 7,035 ab

8/01/90 5,806 1,336 b 5,337 1,253 c 4,445 965 b 31,315 6,591 b

9/01/90 12,165 1,996 a 12,824 2,399 a 10,466 1,855 a 47,060 7,757 a

’ Means followed by the same letter do not significantly differ at P=0.05 using Duncan’s multiple range test. Table 4.3. Yield response of tomatillo cultivars to nitrogen fertilizer - 1991 tests.

Yield (kg/ha), March 20, 1991 Yield (kg/ha), June 20 , 1991 Yield (kg/ha), August 26, 1991

Nitrogen (kg/ha) Guanajuato Puebla Verde Rendidora Guanajuato Puebla Verde Rendidora Guanajuato Puebla Verde Rendidora

58 10,806b1 1 l,839ab 12,126b 2,883a 6,119b 6,525 a 6,778a 8,782cd 8,362a

108 ll,169ab 11,608b 11,919b 2,781a 6,612a 6,774a 6,453 a 8,634d 8,514a

158 11,520a 12,119ab 12,813a 3,093 a 6,307 ab 6,088b 6,887a 9,167bc 8,238a

209 10,937 ab 1 l,676ab 13,177a 2,642 a 6,767 ab 6,657 a 6,299 a 9,733a 8,240a

260 10,780b 12,244a 12,911a 2,543 a 6,412a 6,413ab 6,386a 9,249 b 8,491a

1 Means followed by the same letter do not significantly differ at P=0.05 using Duncan's multiple range test. Table 4.4. Yield response of tomatillo cultivars to nitrogen fertilizer - 1992 tests.

Yield (kg/ha), March 21, 1992 Yield (kg/ha), August 20, 1992

Nitrogen (kg/ha) Guanajuato Puebla Verde Rendidora Guanajuato Puebla Verde Rendidora

58 10,604 b1 11,594 be 11,968 c 10,955 be 11,854 b 12,231 c

108 10,870 b 11,341 c 12,961 ab 10,643 c 12,180 ab 12,641 be

158 10,732 b 12,401 a 12,649 b 11,372 ab 11,706 b 13,409 a

209 11,387 a 11,934 b 13,429 a 11,661 a 12,853 a 13,055 ab

260 10,836 b 11,675 be 12,863 ab 11,059 be 12,350 ab 13,624 a

1 Means followed by the same letter do not significantly differ at P=0.05 using Duncan’s multiple range test. 113

patterns were observed among all three cultivars grown in the spring (1991, 1992) and

fall (1992) when fruit setting was enhanced by cool weather conditions. In each cultivar, the highest yield increases corresponded with the 159, 209, and 260 kg/ha topdressings of nitrogen over the base 58 kg/ha. However, there was less difference in yield among the three rates as well as between the control and 108 kg/ha of nitrogen. At all five planting dates, cultivars were ranked by yield as

Rendidora > Puebla Verde > Guanajuato. A significant yield reduction occurred in all three cultivars when grown in the summer. The Guanajuato cultivar was more heat sensitive than the other cultivars. Transplanting in late August (1991) or early

September increased the risk of early frost (November 3, 1991), causing both fruit abortion and damage to immature fruit.

Effects of mulching on tomatillo yield. Yield response of tomatillo to mulching is shown in Figure 4.1. In the spring planting, tomatillo plants grown with aluminum painted plastic mulch yielded higher than those grown with black plastic mulch or no mulch. Highest yields resulted from the insecticide application to the aluminum mulch plots during the flowering period, when the effects of reflective mulch were reduced by the dense canopy of tomatillo. The same trend was observed in the fall planting. Field observations indicated that the insect vector populations were lower in the aluminum mulched plots than in nontreated black plastic mulched and bare ground plots.

In black plastic mulched plots with or without insecticide treatment, tomatillo yield in the fall planting was significantly lower than in the spring. Like its close YIELD (kg/ha)

18000 —i Spring 1993 16000 Fall 1993 1 4 0 0 0 -

12000 -

10000 -

8 0 0 0 -

6 0 0 0 -

4 0 0 0 -

2000 -

Aluminum mulch Aluminum Black plastic Black plastic Insecticide Nontreated + insecticide1 mulch 1 mulch mulch3 only3 control3 + insecticide2 TREATMENT Figure 4.1. Influence of mulching on the seasonal yield of tomatillo in 1993.1 Significant contrast at P<0.05,2 significant contrast at P<0.01,3 nonsignificant contrast at P<0.05. 115 relative, tomato, fruit set in tomatillo was highly influenced by temperature. Yield data from tomatillo experimental plots in three consecutive growing seasons (1991-

1993) indicated that both tomato and tomatillo had poorer fruit set due to the high daytime and nighttime temperatures during the flowering period. Heat absorption by the black plastic mulch may cause other adverse effects on the physiological development of tomatillo plants, thereby resulting in poor fruit set.

DISCUSSION

Vegetable production in Louisiana has been traditionally oriented toward spring and fall cropping sequences because of climatic conditions and wholesale market

"windows". Much of the vegetable research has been carried out in that reference time. Young (1961, 1962) reported that for warm season vegetables, spring plantings were made as soon as the danger of frost was over and land could be prepared. Fall crops were planted in late summer so that harvest would occur before the first killing frost in November. Similar studies using the spring-fall cropping sequences have been reported in tomato by Bryan (1966), Halsey (1975), and Hanna & Hernandez (1982).

The data presented here indicated that tomatillos appear to respond to planting dates in a manner similar to tomato and pepper (Halsey, 1975). The high tomatillo fruit set under cool weather conditions suggested that maximum yields of the three cultivars tested were obtained when tomatillo was transplanted from mid-March through April or late in August (Can et al., 1992). With the exception of a late frost on December 10, 1990, transplanting in early September increased the risk of damage 116 from early frosts in November. Yield increases corresponded with cool weather during the flowering and fruiting periods (May and October).

Yield reduction with both tomato and tomatillo cultivars in the summer appeared to be due to high day and night temperatures that impaired the fruit set

(Lewis, 1953; Levy et al. 1978). In addition, this was the period when a high incidence of tomato fruitworm and virus-transmitting flea beetles also occurred along with other common pests, such as banded cucumber beetles, leaffooted bugs, and southern green stinkbugs. The effect of high temperature on flower drop has been reported in tomato when grown during hot humid summers (Raspinner, 1932; Smith,

1932, Leopold & Scott, 1952; Saito & Ito, 1967). Flower drop in tomato is essentially the result of a lack of fertilization which, in turn, is affected by a number of factors. Under high temperatures, gametogenesis is disturbed in tomato, gamete viability is reduced (Iwahori, 1965, 1966) and less pollen is produced in the flower

(Abdalla & Verkerk, 1968). High temperatures can also affect the germination and elongation of the tomato pollen tube in the style and thus inhibit fertilization (Smith,

1935; Lewis, 1953; Iwahori, 1967). The commonly accepted explanation for reduced yields with tomatillo was that it was due to pollen abortion at high day and night temperatures that enhanced self-incompatibility and also reduced the success of cross­ pollination (Lipton, 1970; Quiros, 1977). In the summer, high fruit setting was observed in certain single tomatillo plants. This suggests that attempts should be made to select a genotype in which there is normal endothecium development at high temperatures. However, the production of potential heat-tolerant cultivars is still 117 hampered by the high infestation of both tomato fruitworm and potato flea beetles that transmit PhyMV. Field evaluation of insecticides for the control of major insects during summer plantings indicated that, despite the adequate insect control, yields were still low. High temperature during the flowering period has been considered the main limiting factor for high yields in tomatillo (Abdalla et al., 1968).

To date, there is no work published dealing with the relationship between rates of nitrogen application and flower formation and fruit set of field-grown tomatillos.

The results reported here clearly show that the effects of different nitrogen rates on tomatillo yield were not consistent among the three cultivars and at the different planting dates. Doss and his coworkers (1981) also reported that the effect of nitrogen rate on marketable tomato yield was not consistent among years. Average yields from the lowest nitrogen rate were greater than from the highest nitrogen rate in the driest years and were similar or higher than the highest nitrogen rate in the year with a higher average rainfall. It appeared that the lowest nitrogen rate used (58 kg/ha) was adequate, especially when moisture was limited. Broadbent et al. (1980) also demonstrated that processing tomatoes in California typically did not require heavy application of nitrogen to attain maximum yields, presumably because they were able to efficiently use available soil nitrogen. Fisher (1969) also pointed out that tomato plants grew faster initially and were harvested earlier in response to high nitrogen applied prior to the initiation of the first truss (= inflorescence); but, there were no differences in yields of either the earlier harvest or the final harvest.

Investigations conducted by Garrison et al. (1967) have indicated that high rates of 118 nitrogen fertilizers did not contribute to decreasing flowering, fruit set, or yield of field-grown tomatoes when the other environment conditions were favorable for fruit set (Raspinner, 1932; Wittwer, 1957; Saito & Ito, 1967). The reduction of fruit set of the three tomatillo cultivars during the summer of 1991, when they received high nitrogen fertilizer rates, may have been due to high temperatures rather than excess nitrogen.

Despite the favorable weather conditions in spring and fall, tomatillo production in Louisiana was still impaired by the virus infection resulting from vector migration in mid-May and late September to early October. The use of aluminum- painted plastic mulch greatly increased tomatillo yield by about 70-90% as compared to the nontreated bare ground plantings. This treatment apparently repelled insects, thereby reducing the incidence of virus transmission. The yield was more than doubled if additional insecticidal applications were used during the fruit setting period for controlling fruitworms and virus vectors since the dense canopy of mature tomatillo greatly reduced the repellent aluminum mulch effects according to research with other solanaceous crops (Nawrocka et al., 1975; Schalk et al., 1979; Wien et al.,

1988; Greenough et al., 1990). Soil temperature was also found lower under the reflective aluminum mulch than under black plastic, thus enhancing more uniform plant development and fruit setting in tomato and pepper (Black & Rolston, 1972;

Rudich, 1979). However, tomatillo fruit set was reduced when tomatillo was grown with black plastic mulch in the spring and fall seasons. Heat absorption by black plastic might create a significant fluctuation of soil temperature causing more flower 119 drop and adverse effects on root development. This was not studied. Phatak et al.

(1965) demonstrated that root temperatures influenced the number of tomato flowers in the first inflorescence. There were significantly more flower on plants with roots exposed to 50-55°F as compared to those at 60-70°F. Experiments conducted by

Iwahori (1965, 1966) have also indicated that tomato fruit set was reduced when the flower buds were exposed to high temperatures 5-9 days before anthesis and 1-3 days after anthesis. Such situations often occurred in the hot dry days in April and

September for field-grown tomatillos.

Water percolation through the root zone was also reduced by the plastic mulch, thereby reducing fertilizer loss though leaching and the damaging effects of heavy rain or overhead irrigation as observed in tomato (Rudich, 1979). Due to the sprawling tomatillo habit, the plastic mulch also aided in weed control as weeds were important virus and vector reservoirs for of a number of diseases of economically important crops (Tomlinson et al., 1970; Cho et al., 1986; Hobbs et al., 1993). Association between vector and virus source plants and subsequent movement to other susceptible plants is essential for transmission of plant virus diseases by insects (Wolfenbarger &

Mooree, 1968; Watson & Plumb, 1971; Can et al., 1994). On the reflective mulch, tomatillo fruit could properly develop and were cleaner and less damaged by fruit rotting than those on bare ground, thus facilitating the potential use of a tomato harvester (Can et al., 1992).

In South Louisiana, farmers are used to growing strawberries and bell peppers on plastic mulch. Strawberry harvest is usually completed between February and 120

March, and bell pepper harvest is completed in the month of July (Black & Rolston,

1972). As the black plastic already in place is usually still in good condition after the strawberry and bell pepper crops, it might be aluminum-painted and maintained in place for a second tomatillo crop, thus facilitating continued management of the land.

Tomatillo is attacked by a diversity of insects and disease pests which are capable of causing devastating crop losses. A better understanding of the complex of disease and insect pests affecting the crop throughout the production season will help to reduce the potential intensive use of pesticides and develop a meaningful pest management program. 121

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Black, L.L. and Rolston, R.L. 1972. Aluminum foil mulch reduces infection of pepper. Louisiana Agriculture 15:6-7.

Broadbent, F.E., Tyler, K.B., and May, D.M. 1980. Tomatoes make efficient use of applied nitrogen. Calif. Agric. 34:24.

Bryan, H.H. 1966. Effect of plastic mulch on the yield of several vegetable crops in North Florida. Proc. Fla. State Hort. Sci. 79:139-146.

Calvert, A. 1957. Effect of the early environment on development of flowering in the tomato. 1. Temperature. J. Hort. Sci. 32:9-17.

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Iwahori, S. 1967. Auxin of tomato fruit at different stages of its development with special reference to high temperature injuries. PI. Cell Physiol. 8:15-22.

Leopold, A.C. and Scoot, F.I. 1952. Physiological factors in tomato fruit set. Ann. J. Bot. 30:310-317.

Levy, A., Rabinowitch, H.D., and Kedar, N. 1978. Morphological and physiological characteristics affecting flower drop and fruit set of tomatoes at high temperatures. Euphytica. 27:211-218.

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Nawrocka, B.Z., Eckenrode, C.J., Uyemoto, J.K., and Young, D.H. 1975. Reflective mulches and foliar sprays for suppression of aphid-borne viruses in lettuce. J. Econ. Entomol. 68:694-698.

Pandey, K.K. 1957. Genetics of self-incompatibility in Physalis ixocarpa Brot. Agr new system. Am. J. Bot. 44:879-887.

Phatak, S.C., Wittwer, S.H., and Teubner, F.G. 1965. Top and root temperature effects on tomato flowering. Proc. Amer. Soc. Hort. Sci. 88:527-531.

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Valverde, R.A., Can, F., and Rush, M.C. 1993. Yellow mottle of tomatillo (Physalis ixocarpa Brot.) caused by Physalis mottle virus. Plant Pathology 42:657-660.

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FOR TOMATILLOS GROWN IN LOUISIANA

125 126

Tomatillo pest management is still in its infancy; it is a "living document" that changes as we acquire experience and information (Flint & van den Bosch, 1981;

Flint et al., 1988). The following pest management techniques need to be developed for managing tomatillo diseases and pests in Louisiana: identification of major pests and pathogens, optimizing pest sampling and prediction methods, development of economic thresholds and injury levels, determining the role of natural enemies and understanding their manipulation for suppression of pest populations, determining the impact of pesticides on natural enemies, developing techniques for manipulation of insect vector populations through cultural practices, and development of ecologically selective pesticide controls that are compatible with the limitations of the agricultural system in which tomatillo is grown (Aliniazee & Oatman, 1979; Apple, 1977; Bishop et al., 1979; Gibbs et al., 1986; Heathcote et al., 1969).

Identification of major pathogens and pests. Results from dsRNA analysis and serological tests on samples from diseased tomatillos indicated the presence of three major viruses: Physalis mosaic virus (PhyMV), cucumber mosaic virus (CMV) and tomato spotted wilt virus (TSWV). Fruit rotting caused by R. solani and other soil borne pathogens often occurred when the fruit remained on the soil for a long period of time. The 3-year field survey of seasonal development of insect populations indicated the key or primary insect pests of tomatillo were flea beetles, aphids, and thrips; which were found as vectors of PhyMV, CMV, and TSWV, respectively, and tomato fruitworm which caused direct loss during the flowering and fruit setting stages of tomatillo development. Secondary pests included the banded cucumber beetle, 127 leafhoppers, armyworms, cutworms, leaffooted bugs, the southern green stink bug, and broad mites.

Study of the population dynamics of the insect vectors.M anagement o f the tomatillo crop is somewhat challenging due to the pathogen and pest complex in which the severity of each virus disease depends on the population dynamic of each insect vector throughout the growing season (Plumb & Thresh, 1983). Tomatillo production in Louisiana was highest in the spring and fall (Can et al., 1992). However, the favorable weather conditions associated with high yields were incidentally associated with the high incidence of virus vectors during the flowering periods (May and

October), therefore favoring the transmission of virus diseases. Field observations of the virus symptoms on tomatillo showed some correlation between the severity of virus disease and the occurrence of insect vectors. Serological tests (Ouchterlony and

ELISA) and dsRNA analysis indicated that the high incidence periods of CMV and

TSWV were significantly correlated with high populations of aphids and thrips occurring in May and October. The western flower thrip ( F. occidentalis) was perhaps the newest pest to production in Louisiana. Flower feeding and damage occurred in May and October, with thrip populations reaching 10-15 larvae per flower. The low incidence of PhyMV in the spring was a reflection of the late occurrence of flea beetles in the field. They began to appear in May and the population peaked in late September-early October, thus causing a high incidence of

PhyMV (Figure 5.1). (eF) • Mean Max.Temperature 100

70 -- 4 - - Range of Optimal 60 •*-- . Temp.for Fruit ------S e t 50 40 30 20 10 0 MARCH APRIL MAY JUNE JULY • AUGUST SEPT OCTOBER NOVEMBER-

Broad Mltej B road M ite

'T h rip s (TSWV) J T h r ip s (TSWV)

A phids (CMV!) 1 AphldB (CMV1)

jF le a B e e tle s (PhMV)

i . * Baijded Cucumber Beetle,PajLestrlped Flea Beetle,(Jtinbug

High Weed Infestation

i > , Higt{ Day & Night Temperature

Figure 5.1. Tomatillo planting schedule for optimizing yields in Louisiana. oo 129

Tomato fruitworm (H. zed) damaged all stages of developing tomatillo fruit.

After larvae penetrated the husk and were inside the berry, they were very difficult to reach with insecticides and had time to grow and destroy the fruit. Severe infestations of tomato fruitworm often occurred from late spring to late summer.

However, the insect population varied from year to year, depending on the climatic conditions and on the presence of other susceptible host crops grown nearby (Lange

& Bronson, 1981).

Broad mite (P. latus) was an occasional pest that could become a major problem for tomatillo transplants. Its outbreaks occurred during periods of dry weather. Two miticide applications (Dicofol, Vendex) were required for control.

Economic injury thresholds for key pests.In spite of considerable research effort over the 3-year period, reliable economic thresholds were not established for the major pests of tomatillo. Nevertheless, much progress was made based on monitoring the seasonal development of insect vectors of important viruses through water pan traps and direct sampling of vectors and through field scouting for tomato fruitworm. Additional information on effects of climate and weather on development of vector populations would be very helpful in forecasting outbreaks of the vectors and insect pests.

The interactions among tomatillo, viruses, and insect vectors, as in other crops, are complex and greatly affected by environmental conditions (Harrison, 1981).

Weather is an important modifying factor affecting the prevalence of viruses whose distribution is mainly determined by climate and type of plant community (Watson et 130

al. 1975; Zitter, 1977). It has many effects, including the amount of virus replication

in source plants, susceptibility of healthy plants to infection, effects on vector

reproduction, and effects on the prevalence of parasites and predators of insect

vectors. However, the most important climatic and weather effects are on vector

activity. For flea beetles, warm weather conditions were most favorable for

population increases, whereas, for aphids and thrips, the crucial factors in other crops

are known to be temperature, wind speed and direction, and the degree of air

turbulence. So, the success of attempts to forecast the amount of virus spread depends

in large measure on the extent to which such factors can be assessed in advance,

especially for migrating aphids and thrips. Weather forecasting is helpful in

assessing, during April or September, the risk of CMV or TSWV outbreaks and the

probable need for preventive insecticide treatments; but it does not define when

spraying is needed.

Host plant resistance. At present, the best means of controlling virus diseases

is by varietal resistance (Hill et al., 1969; Adkisson & Dyck, 1980). Unfortunately,

resistance to the major tomatillo viruses is not available at this time. However, single plant selections were made among the Guanajuato, Puebla Verde, and Rendidora cultivars during the 1991 and 1992 growing seasons. Seeds from individual plants that appeared to be virus-resistant, high-yielding, and heat-tolerant were collected and saved for future breeding programs. Breeding for multiple resistance in plants is often a difficult task for the plant breeder to achieve (Pirone & Harrison, 1977). It would appear that one of the best approaches to controlling virus diseases in tomatillo would 131

be to develop methods for preventing viruliferous insect vectors from entering the

crop or for preventing those that do enter the crop from transmitting the viruses that

they carry.

Control of virus vectors in tomatillo. The use of insecticides is an important

component of the pest management program in tomatillo. Insecticides are important

for the quick effect they have in reducing the pest population, the broad range of their

impact (one insecticide may kill several pest species), and because they are relatively

cheap and easy to buy, store, and apply. Field evaluation of insecticides for the

control of major tomatillo insects indicated that all eight insecticides tested

significantly decreased insect damage. However, none of the insecticides tested,

permethrin 2EC, esfenvalerate XL, cypermethrin 3EC, cyfluthrin 2E, endosufan 2EC,

azinphos-methyl 2EC, methomyl 1.8EC, and carbaryl 1.9L, is labeled for use on

tomatillo at this time. These insecticides are labeled for use on tomato and this will

make them easier to register through IR-4.

The most effective way to reduce virus spread in tomatillo is through

preventive measures. As in other crops, maintaining low vector populations is

possible through good cultural practices and through the judicious use of insecticides

(Moyers & Larren, 1991; Broadbent, 1969; Maelzer, 1986; Maramorosch & Harris,

1981).

Eggs or small larvae of tomato fruitworm near the top of the tomatillo canopy are the most vulnerable to insecticides. Conversely, larger larvae lower in the canopy are more difficult to control unless the entire plant is thoroughly sprayed and 132

surfactants are added to the insecticides. Pyrethroid insecticides are likely to be more

effective than the organophosphates and carbamates; however, the alternate use of

these insecticides should be used to avoid developing pesticide resistance among the

tomatillo pests. Proper timing during late evening or early morning provides relative

safety to pollinators (bumble bees, honey bees) against short-residual insecticides

(Johansen, 1977).

The weed control component of pest management in tomatillo.For

controlling plant virus diseases, there is perhaps no phase of virology more important

than epidemiology. The role of weeds in the occurrence and spread of plant virus

diseases is an integral part of the ecological aspect of virus transmission (Duffus,

1971; Tomlinson et al., 1970; Thresh, 1981; Knott, 1990). With the wide host ranges

of CMV and TSWV, and the annual occurrence of Physalis weed species along with

the production of solanaceous vegetables susceptible to PhyMV, weeds may serve as

reservoirs of both viruses and virus vectors. Under these conditions, virus spread

may be very severe. The importance of weeds in the disease cycle of tomatillo virus

diseases transmitted by insects indicates that, as with other vegetable crops, weed

control would probably be at least as effective as chemical control of vectors. Weeds

are also important primary causes of yield loss in tomatillo. An effective weed

control program in tomatillo must consider the species of weeds that may become a

problem. It is important to know what the target weeds are before planting the crop.

This information can be obtained from weed surveys conducted during the previous

year(s). One of the best ways of improving the cost-effectiveness of herbicides in 133 tomatillo is to maintain a steady flow of safer and more precisely targeted herbicide recommendations (Cussans, 1988; Lawson, 1988).

Results from greenhouse and field studies on the tolerance of tomatillo to pre- and post emergence herbicides showed that tomatillo was very tolerant to trifluralin, napropamide, pendimethalin, and metolachlor. Fluazifop-butyl, sethoxydim, and quizalofop were the most satisfactory postemergence herbicides for transplanted tomatillo. However, no single herbicide was found that would selectively control all of the weeds that infest tomatillo fields. So, the choice of herbicides will depend largely on the weed species to be controlled, and also soil type, irrigation methods, crop rotation, environmental conditions, and the equipment available for herbicide applications (Kavanagh, 1974; Bouchet, 1988). Two basic strategies are available for using herbicides in tomatillo. The first strategy is the use of a herbicide that will selectively kill weeds without harming the crop. The second strategy is the use of non-selective herbicides (gramoxone, glyphosate) where the crop is protected by adjusting the timing and methods of application. Selective herbicides allow more flexibility in application, but the prolonged use of specific herbicides in the same field may allow the build up of tolerant weeds.

The use of herbicide combinations and sequential application of herbicides can result in long-season weed control in tomatillo. A weed management program might include a preplant herbicide, a postemergence herbicide treatment, and cultivation combined with hand hoeing. An alternative program would be plastic mulching followed by a nonselective herbicide application between rows. However, no single 134 weed control practice will provide perfect weed control in tomatillo crops. So, weed control programs that integrate several methods are essential. It is necessary to know weed biology, herbicide application techniques and equipment, cultural practices, and crop management techniques (Jonkers, 1988; Williams, 1981; Way & Cammel, 1981).

In field selection, one must also consider the residual properties of herbicides that may have been used in previous crops other than tomatillo. Small quantities of certain herbicides (metribuzin, clomazone, acifluorfen, imazethapyr, fomesafen) used in another crop, such as soybean, cotton, or tomatoes may remain in the soil and adversely affect the growth of tomatillo seedlings. Also, fields infested with perennial weeds and annual weeds tolerant to available selective herbicides should be avoided.

Cultural practices. Tomatillo does not thrive in cold weather and will not set fruit at temperatures below 58°F. They are best started as transplants and planted after the last frost in the spring. The hot, dry summer in Louisiana is not favorable for planting tomatillo since high day and night temperatures during the flowering period greatly impairs fruit set. The fall-planting of tomatillo should be started by mid-August in order to avoid the early frost in November. Tomatillos grown in the field from transplants require 14 or 20 days less from planting to harvest than tomatillos that were direct-seeded, thus avoiding heavy infestations of aphids, thrips, and tomato fruitworm in May and the peak infestations of flea beetles in late

September and early October (Figure 1.6). Maintenance of vigorous tomatillo plants through regular fertilization and irrigation will help the crop to maintain a dense canopy, thus competing better with late-season weeds. 135

Plants showing unusual growth habits such as stunting, leaf distortion, or vein banding, should be rogued because these plants may be infected with a virus which can be transmitted to healthy tomatillo plants. Removal of perennial weed hosts, with special attention to those belonging to the Solanaceae, should help to reduce overwintering virus. In addition, removal and destruction of plant material remaining after harvest should be a common cultural practice (Palti, 1981; Zitter & Simon,

1980).

Crop rotation can reduce some weed problems in tomatillo. Crops useful for rotation with tomatillo include rice, cotton, cereal crops, onions, cucurbits, and sugarbeets. Crops closely related to tomatillo, such as tomato, potato, and peppers, do not provide an effective rotation, as the cultural practices and herbicides used for these crops are similar to those used in tomatillo.

The preparation of a good seedbed free of clods and excessive residues from the previous crop also facilitates several weed control procedures and promote rapid growth of tomatillo. Vigorous tomatillo seedlings or transplants compete more favorably with weeds than slow-growing plants. Moreover, the performance of preemergence and preplant soil incorporated herbicides is enhanced by good seedbed preparation. Cultivation performed for weed control after emergence of the tomatillo crop is also more effective and less detrimental to tomatillo with good seedbeds.

Cultivation for weed control in tomatillo (30 cm high) can be used alone, but is more widely used in conjunction with herbicides. Herbicides may be band applied in plant rows and cultivation used between rows. The crop can be directly 136 transplanted to good seedbeds that are then cultivated between rows 2 weeks later.

Cultivation is more effective for the control of annual weeds than for perennial weeds.

Cultivation is most effective when the weed seedlings are small, in the three to four- leaf stage. At this time, they have had minimal time to compete with the crop, and they can be removed with little crop damage. A postemergence herbicide treatment should be applied 2 weeks following cultivation. Experience with field-grown tomatillo indicates that the crop could be directly transplanted into good seedbeds.

Cultivation and side-dressing of additional nitrogen can be done 14-18 days following transplanting. Then a single application of metolachlor or a combination of metolachlor+sethoxydim (fluazifop-butyl, or quizalofop) can be made 2 weeks later, thereby reducing the number of applications, operation costs, and ensuring good control of late-season grasses and broadleaf weeds. Future use of these and other pesticides studied is dependent on their registration for use in tomatillo.

Despite the use of planting dates favorable for high yields in the spring and fall, tomatillo production in Louisiana is still impaired by the vagaries of rainfall and temperature and by virus vector immigration into the crop. The use of aluminum- painted plastic mulch increased tomatillo yields by about 70-90%, when compared to nontreated plots. This treatment repelled insects, thereby reducing the incidence of insect-borne viruses, and it conserved moisture. However, the yield was more than double if additional insecticide applications were used during fruit setting for the control of tomato fruitworm and other incoming virus vectors. 137

Due to the pest and disease complex and the specific climatic requirements, the management of tomatillo production needs to utilize many resources including improved cultural practices, insect sampling, and nonpesticidal and chemical control.

Such an integrated approach resulted in tomatillo yields (17,900 kg/ha) that were greater than those produced in Mexico and California (4,500-11,200 kg/ha) (Rude,

1982; Giesenberg & Stewart, 1986). 138

LITERATURE CITED IN CONCLUSION

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Aliniazee, M .T., and Oatman, E.R. 1979. Pest Management Programs. In: Biological Control and Insect Pest Management. Univ. Calif. Div. Agric. Sci. Publ. 4096.

Apple, J.A. 1977. The theory of disease management. In: Plant Disease: An advanced Treatise. Vol I. How Disease is managed. (J.G. Horsfall and E.B. Cowling eds.) pp. 791-820. Academic Press, NY.

Bishop, G.W., Davis, D.W., and Watson, T.F. 1979. Cultural practices in pest management. In: Biological Control and Insect Pest Management. Univ. Calif. Div. Agric. Sci. Publ. 4906.

Bouchet, C. 1988. Climatic conditions and field behavior of herbicides. In: Weed Control in vegetable production (R. Cavalloro and A.E. Titi, eds.) pp. 125- 136. A.A. Balkema Publ., Rotterdam, Netherlands.

Broadbent, L. 1969. Disease control through vector control. In: Virus, Vectors and Vegetation (K. Maramorosch ed.) pp. 593-630. Academic Press, N.Y.

Can, F., Rush, M.C., Valverde, R.A., Griffin, J.L., Story, R.N., Blackmon, W.J., and Wilson, P.W. 1991. Tomatillo: a potential new vegetable crop in Louisiana. Louisiana Agriculture 35:21-24.

Cussans, G.W. 1988. The economics of weed control in vegetable crops. In: Weed Control in vegetable production (R. Cavalloro and A.E. Titi, eds.) pp. 99-106. A.A. Balkema Publ., Rotterdam, Netherlands.

Duffus, J.E. 1971. Role of weeds in the incidence of virus diseases. Ann. Rev. Phytopathol. 9:319-340.

Flint, M.L. and van den Bosch, R. 1981. Introduction to integrated pest management. Plenum Press, N.Y. 240 pp.

Flint, M.L., Daar, S., and Molinar, R. 1988. Establishing integrated pest management policies and programs: A guide for public Policies. Univ. Calif. IPM Publication 12. 139

Gibbs, A., Skotnicki, A., and Skotnicki, M. 1986. Plant Virus Control Strategies: Future Prospects. In: Plant Virus Epidemics. (G.G. McLean, R.G. Garrett, and W.G. Ruesinik, eds.). pp. 513-524. Academic Press, NY.

Giesenberg, C. and Stewart. 1986. Field crop management. In: The tomato crop (J.G. Atherton and J. Rudich, eds.). pp. 511-557. Chapman and Hall Ltd. N.Y.

Harrison, B.D. 1981. Plant virus ecology: Ingredients, interactions and environmental influences. Ann. Appl. Biol. 99:195-209.

Heathcote, G.D., Palmer, J.M.P., and Taylor, L.R. 1969. Sampling for aphid by traps and by crop inspection. Ann. Appl. Biol. 63:133-166.

Hill, F.J., Lange, W.H., and Kishiyama, J. 1969. Varietal resistance to yellows, vector control, and planting date as factors in the suppression of yellows and mosaic of sugarbeet. Phytopath. 59:1728-1731.

Johansen, C.A. 1977. Pesticides and Pollinators. Ann. Rev. Entomol 22:177-192.

Jonkers, J. 1988. Integrated weed control in some vegetable crops. In: Weed control in vegetable production. (R. Cavalloro and A.E. Titi eds.) pp. 259-264. A.A. Balkema Publ. Rotterdam, Netherlands.

Kavanagh, T. 1974. The influence of herbicides on plant tissue. II. Vegetable root crops and potatoes. Scient. Proc. R. Dubl. Soc. Ser. B3:251-265.

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Maelzer, D. A. 1986. Integrated Control of Insect Vectors of Plant Virus Diseases. In: Plant Virus Epidemics. (G.G. McLean, R.G. Garrett, and W.G. Reresink, eds.). pp. 483-512. Academic Press, NY.

Maramorosch, K. and Harris, K.F. (eds.) 1981. Plant Diseases and Vectors: Ecology and Epidemiology. Academic Press, NY. 368 pp. 140

Moyers, J.W., and Larren, R.C. 1991. Management of insect vectors of viruses infecting sweet potato. In: Sweet potato pest management. (R.K. Jansson and K.V. Raman eds.). pp. 341-358. Westview Press. San Francisco.

Palti, J. 1981. Cultural practices and infections crop diseases. Springer-Verle Berlin. 350 pp.

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Francis Can Huy Ngo was born on September 27, 1940 in North Vietnam.

He received his primary and secondary French education by the French Canadian missionaries in South Vietnam. He received his B.S. degree in Natural Sciences from the University of Saigon in 1966 and then worked at the College of Agronomy and

Forestry. As a USDA participant, he attended the University of Florida where he received his Master of Science degree (Major: Botany, Minor: Plant Pathology) in

1974. Following the collapse of South Vietnam in 1975, he returned to the United

States and worked as an agricultural inspector in the Louisiana Department of

Agriculture. He then joined the LSU Department of Plant Pathology and worked under Dr. Rush’s supervision from 1980 to 1981. He took a 7-year assignment in

Senegal (West Africa) where he assumed the position of Crop Protection Specialist and was then promoted as chief-of-party in LSU’s Casamance Integrated Rural

Development project funded by USAID (1981-1985). With the approval of the LSU

Agricultural Center, he joined the USAID/Senegal mission and worked as research/extension coordinator in several USAID-funded projects in close collaboration with different directorates and rural development agencies of the ministry of rural development (1986-1988). Back in the U.S., he worked at the LSU

International Programs Office (1988-1990) and was transferred again to the

Department of Plant Pathology and Crop Physiology where he enrolled in his Ph.D. program in integrated pest management. He is a member of the American

141 142

Phytopathological Society, Gamma Sigma Delta, Louisiana Plant Protection

Association, and Senegal Lions Club (in charge of the leprosy program). DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Can Huy Ngo

Major Field: Plant Health

Title of Dissertation: Pest Identification and Management in Tomatillo in Louisiana

Approved: i?A. Major Professor and Chairman

Dean of the Graduate School

EXAMINING COMMITTEE:

/

7 ^

Date of Examination:

July 26, 1994