As a preliminary demonstration Several fungicides control project, funding limited the scope of variables that could be measured and reported here. Continued research of powdery mildew in peppers this type on a larger scale will help further define the potential benefits of Richard F. Smith D Steven T. Koike o Mike Davis a mob-rearing system and should in- Krishna Subbarao a Frank Laemmlen clude additional stress and infection measurements, such as white blood cell counts, neutrophi1:lymphocyte ra- tios, and acute phase proteins. In the early 199Os, powdery mil- powdery mildews that infect most ~ ~ ~ ~~ dew became a recurring problem other plants. The fungus typically in- B. Reed is Farm Advisor, UCCE Glenn on chili peppers and bell peppers fects the older leaves first and can be County; C. Stull is Animal Welfare Spe- in all production districts in Cali- seen with the naked eye when masses cialist, Veterinary Medicine Extension, of the spore-bearing structures (conid- UC Davis; S.L. Berry is Dairy Extension fornia. Growers were initially un- prepared to deal with the disease. iophores) extend out of the leaf tissue Specialist and C. Batchelder is Graduate and appear as typical white, fuzzy Student, Department of Animal Science, Research has shown that several fungicides can control this dis- powdery mildew growth. UC Davis. The conidiophores are most often ease. Sulfur is most effective as a The authors wish to thank the cooperat- seen on the under sides of pepper preventive fungicide. In variety tri- ing dairy farm and the Western Region leaves; however, when infection is se- Sustainable Agriculture Research and als, four experimental varieties vere, they can also be observed on the Education Program for funding support were significantly less suscep- upper sides of the leaves. The spores for this project. Statistical support was tible to powdery mildew than the of pepper powdery mildew, unlike provided by Neil Willits, Ph.D., in the standard commercial variety. Ge- most other powdery mildew types, Division of Statistics, using SAS 6.12 netic resistance is likely to even- contain sufficient water for growth, (1995-96). tually be incorporated into com- and as a result can germinate at rela- mercial bell pepper varieties. Further reading tively low humidities. For instance, they can germinate at relative humidi- Kung L, Demarco S,Siebenson LN, et al. 1997. An evaluation of two management sys- Powdery mildew infects many crops, ties of 40% to 90%, with the optimum tems for rearing calves fed milk replacer. J ornamentals and weeds commonly being 90%. The additional water that Dairy Sci 80:2529-33. found in California. The pathogen is the spores contain makes them suscep- McDonough SP, Stull CL, Osburn BI. able to infect 710 host species from 59 tible to bursting in the presence of free 1994. Enteric pathogens in intensively reared veal calves. Am J Vet Res 55(11):1516-9. plant families, and has been recorded water in the environment from sources Stull CL, McDonough SP. 1994. Multi- around the world. It was noted on bell such as rain or sprinkler irrigation. disciplinary approach to evaluating welfare of peppers in Israel in the 1950s, but did Pepper powdery mildew is typi- veal calves in commercial facilities. J Anim Sci 72:2518-24. not occur on bell peppers in the cally observed at first harvest. How- Thurmond MC. 1986. Epidemiologic ap- United States until 1976, when it ap- ever, during years of severe infection, proaches used in a herd health practice to in- peared in Florida. In 1983 it appeared it may be seen earlier in the growth vestigate neonatal calf mortality. Prev Vet in California in the Santa Clara Valley. cycle. If left uncontrolled, powdery Med 4:317-28. USDA: APHIS: Veterinary Services, Na- The disease was not a production mildew can affect a majority of the tional Animal Health Monitoring System. problem in California until 1992, when leaves on the plant. Severe infection 1996. Part 11-C: Heifer Health. In Dairy '96, it infected numerous fields in the causes yellowing of the leaves and Part II: Changes in the US. Dairy Industry: coastal production districts from San subsequent defoliation of the plant, 1991-1996. Fort Collins, CO: USDA APHIS Veterinary Service. Benito County to Ventura County and exposing pepper fruit to sunburn USDA: APHIS: Veterinary Services, Na- caused losses of up to 50% to 60%. Af- damage. Losses from sunburned fruit tional Animal Health Monitoring System. ter 1992, pepper powdery mildew can range from 50% to 60%. 1994. Part II: Dairy Heifer Morbidity, Mortality, and Health Management Focusing on spread to the San Joaquin Valley and Pepper powdery mildew is an on- Preweaned Heifers. National Dairy Heifer the desert production districts, and going problem in the coastal produc- Evaluation Project. Fort Collins, CO: USDA now it can be found in all major pep- tion districts because of favorable hu- Animal and Plant Inspection Service Veteri- per production areas of the state. midities and temperatures. However, nary Service. USDA: APHIS: Veterinary Services, Na- Leveillula taurica (= Oidiopsis taurica) in years with favorable weather condi- tional Animal Health Monitoring System. is unique among powdery mildews tions, the disease can cause significant 1993. Part I: Dairy Herd Management Prac- because of its internal growth habit. damage in all pepper production dis- tices Focusing on Preweaned Heifers. Na- tional Dairy Heifer Evaluation Project. Fort The fungal hyphae grow inside the tricts, including the desert and San Collins, CO: USDA APHIS Veterinary host tissue, rather than on the surface Joaquin Valley. This disease caused Service. of the leaf tissue, as is common with particularly severe losses in the early

40 CALIFORNIA AGRICULTURE, VOLUME 53,NUMBER 6 This close-up of the underside of a bell pepper leaf shows initial The undersides of these bell pepper leaves show extensive sporulation of pepper powdery mildew. sporulation of pepper powdery mildew. years of the epidemic, when growers ease. Since that time, we have contin- 25 feet long. All fungicides were ap- and crop consultants were unpre- ued to evaluate the efficacy of fungi- plied with a CO;! backpack sprayer us- pared. In 1992, we responded to the cides, as well as varietal resistance to ing four 8005 nozzles directed over the problem by initiating trials to identify the disease. top and to the sides of the plants. All fungicides that can effectively control disease ratings and yield evaluations the disease. A collaborative effort be- Fungicide control trials were taken from the middle row of tween the university, growers and the We established trials in commercial each plot. Fungicide treatments were agricultural chemical industry re- pepper fields to test the efficacy of sul- initiated at early flowering. All treat- sulted in an emergency registration for fur and other fungicide materials. ments were replicated four times in a a fungicide that controlled the dis- Each treatment was 3 beds wide and randomized complete block design. Sulfur better as preventive Sulfur applied after the onset of pepper powdery mildew in 1992 and 1993 did not provide satisfactory con- trol of this disease (table 1). Sulfur is a better preventive fungicide, and begin- ning in 1994 we conducted a series of trials to evaluate the efficacy of sulfur applications initiated at the early flow- ering stage, prior to the onset of mil- dew. Trials conducted in 1994 and 1995 showed that five or eight applica- tions, respectively, of 5 pounds of sul- fur per acre, applied on a 10-day spray application schedule, provided signifi- cantly greater control of pepper pow- dery mildew than did the untreated controls (data not shown). In 1996, we expanded the range of sulfur rates and timing intervals. Lower rates of sulfur, 1 and 3 pounds per acre, provided comparable control (table 2). At these low rates, however, six applications applied at 10-day spray intervals were necessary to obtain acceptable control over the entire season. At the higher rate of 5 pounds of sulfur per acre, four applications provided control that was comparable to six applications of the lower rates.

CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 1999 41 the furrows shortly after fruit set. As a sued with assistance from the Federal result, growers rely on aerial applica- IR-4 program. tions to apply sulfur later in the sea- We also evaluated other powdery son. However, there is uncertainty re- mildew control materials (tables 1,2 garding the efficacy of aerial applica- and 3). tions of sulfur on pepper powdery Bicarbonate materials are active on mildew because of reduced pressures several species of powdery mildew. and lower water volumes used in However, ammonium and potassium aerial applications (5 to 10 gallons of bicarbonates had limited efficacy on water/acre by air vs. 25 to 50 gallons pepper powdery mildew and also of water/acre by ground rig). To an- caused phytotoxic effects such as swer the question of whether aerial brittleness of the leaves and marginal applications could provide satisfactory leaf burn. control of pepper powdery mildew, Light plant oils coat the plant and we conducted a trial in 1997 simulat- reduce invasion of the plant tissue by ing aerial applications of sulfur. We germinating spores. We evaluated two made six applications of 5 pounds of oils, JMS Stylet Oil and two formula- sulfur in 5 gallons of water at 10 psi. tions of Trilogy (an extract of the neem Powdery mildew control was good tree seed). JMS Stylet Oil had limited This plant shows extensive defoliation. early in the season; however, control efficacy and caused necrotic spots on Only young, less severely infected leaves declined to unacceptable levels later in the foliage. Trilogy provided limited remain on the plant. the season (table 3). control of pepper powdery mildew early in the season, but control quickly In spite of the efficacy of sulfur, Other fungicides more effective declined to unacceptable levels by there are limitations to its use. Sulfur Triadimefon (Bayleton) and midseason. can be phytotoxic at temperatures myclobutanil (Rally) provided excel- AQ-10 is a fungus (Ampelomyces above 90"F, limiting its use in many lent control of powdery mildew in all quisqualis) that parasitizes and kills pepper production districts. In addi- of the fungicide trials conducted from powdery mildew. Applications of this tion, sulfur is not a systemic fungicide, 1992 to 1997 (tables 1,2 and 3).These material provided limited control and proper spray coverage is required materials controlled the disease with early in the season, but efficacy to obtain satisfactory control of pepper only one or two applications of 4 quickly declined. powdery mildew. ounces of material per acre, even after Azoxystrobin (Quadris) and triflu- Ground sprays typically provide the onset of symptoms. Triadimefon mizole (Procure)provided intermediate adequate coverage because they use was available for growers under an control of the pepper powdery mildew, high pressure and volumes of water to emergency Section 18 from 1992 to while tebuconazole (Folicur) and apply the material. Under typical 1996. Beginning in 1997, myclobutanil propiconazole (Tilt)provided excellent planting configurations, it is difficult was available to pepper growers un- control of the disease. However these to use ground rigs later in the season der a Section 18 registration, and cur- four materials are not currently regis- because the plant canopy closes over rently a full registration is being pur- tered on peppers in California.

42 CALIFORNIA AGRICULTURE, VOLUME 53,NUMBER 6 b This green bell pepper field shows sunburned fruit. The furrows are full of leaves that have fallen from the plants due to defo- liation from pepper powdery mildew.

4 Close-up of sun- burned fruit due to defoliation from pepper powdery mildew.

Varietal resistance tance to powdery mildew Resistance to pepper powdery mil- will be bred into com- dew exists in peppers. From 1992 to mercial pepper varieties, 1997, we conducted trials on 20 foot- which will offer growers long replicated strips of commercial, additional options to as well as experimental, lines of pep- combat this serious pro- pers. No commercially available vari- duction problem. eties of bell peppers were found to be resistant to powdery mildew, but Powdery mildew is there were differences in varietal sus- controllable ceptibility. For example, yellow bell Pepper powdery mil- and lamuyo peppers are extremely dew became a serious susceptible to powdery mildew and threat to the California defoliate under slight infections. Green pepper industry in 1992. bell pepper varieties are intermediate Sulfur can provide rea- in susceptibility, while most varieties sonable control of pepper of jalapeno peppers can withstand powdery mildew fit is heavy infections before defoliation oc- applied prior to the onset of the dis- curs. Our 1995 trial included two ease and at frequent intervals. Good Davis; F. Laemmlen is Farm Advisor, ”wild type” peppers - PI 152225 and coverage is required for effective con- Santa Barbara County. PI 159236 - that were shown to be re- trol with this material. Sulfur is the The authors thank the cooperating sistant to the disease. HV-12 is a long- principal method that organic growers growers and crop consultants, as well as podded sweet pepper from France that use to control pepper powdery mil- the California Pepper Commission,for is also resistant to powdery mildew. dew. However, the issues of applica- their support of this research. Variety 94-128, a hybrid cross between tion, temperature restrictions and pos- HV-12 and a commercial bell pepper sible crop injury make the use of Further reading line, was slightly susceptible to pepper sulfur a less desirable and difficult op- Blazquez, CH. 1976. A powdery mildew of powdery mildew. All four of these va- tion for many growers in California. chilli caused by Oidiopsis sp. Phytopathology 66:1155-7. rieties had significantly less powdery Careful monitoring of the disease and Correll JC, Gordon TR, Elliott VJ. 1987. mildew than the standard commercial the effective use of fungicide programs Host range, specificity, and biometical mea- variety, D-93, that was included in the can provide acceptable control of pep- surements of Leveillula taurica in California. Plant Disease 71 :248-51. trial (P < 0.05). However, the field re- per powdery mildew. Varietal resis- Hartz TK, LeStrange M, Mayberry K, sistance that we observed in this trial tance to this disease will eventually be Smith RF. 1996. Bell Pepper Production in has not been substantiated in green- incorporated into commercial cultivars California. Vegetable Research and Informa- house evaluations conducted by com- of peppers, which will provide grow- tion Center, Vegetable Production Series. DANR Publication 7217. mercial seed companies. ers with further options. Reuveni R, Rotem J. 1973. Epidemics of Seed companies are continuing to Leveillula taurica on tomatoes and peppers evaluate these varieties, as well as oth- R.F. Smith and S.T. Koike are Farm Advi- as affected by the conditions of humidity. ers, for effective sources of resistance sors, Monterey County; M. Davis and Phytopathology 76: 153-7. Smith RF, Laemmlen F, Koike ST, to pepper powdery mildew. It is K. Subbarao are Extension Plant Patholo- Subbarao K. 1998. Strategies to control pow- hoped that in the near future resis- gists, Department of Plant Pathology, LIC dery mildew, Pepper News 6:4.

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