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DODDER CONTROL IN

Jean H. Dawson*

Introduction

The common name "dodder" is applied to any species of the genus Cuscuta. All species of the genus Cuscuta are obgligate parasites. They are flower­ ing plants that usually behave as annuals. A high percentage of the seed is hard. Consequently, seeds tend to be long-lived in . After seedlings emerge, they remain in contact with the soil for only a short period before they become attached to a host plant and the connection with the soil is severed.

Dodder is very efficient in diverting the photosynthetic production of the host plant to itself. Consequently, plants parasitized by dodder can be severely suppressed or killed, and crop yields can be reduced severely or completely eliminated.

Some species of dodder parasitize only non-economic plants, and are thus simply interesting botanical curiosities. Others, like field dodder (Cuscuta campestris) and largeseed dodder (Cuscuta indecora), parasitize numerous important crop plants such as alfalfa, tomatoes, potatoes, sugar­ beets, carrots, onions, safflower, and others. In such , dodder must be controlled or prevented, or yield losses can be severe. In this paper we shall review the control of dodder in alfalfa grown for both forage and seed production.

Established alfalfa parasitized by dodder for 1 year will be severely suppressed, but usually not killed. If the same plants are parasitized a second year, many will die. Alfalfa seedlings frequently are killed during the season of emergence if parasitized by dodder.

Parasitism by dodder greatly reduces the production of forage. Dodder is especially troublesome in alfalfa grown for seed production. In addition to the general suppression of vigor, alfalfa tends to drop its blossoms if parasitized by dodder. In addition to severe losses in yield, the quality of alfalfa seed is reduced if contaminated by dodder seed.

Dodder requires both high temperature and full sunlight to grow well. The shade from a vigorous cover of alfalfa prevents twining of alfalfa seedlings, and thus prevents attachment to the host plant. Shade also greatly suppresses the growth of dodder that does become attached to the host. Management of the crop to take advantage of the benefit of shading can form an important part of an integrated management system for dodder control in alfalfa.

*Research Agronomist, Irrigated Research and Extension Center, Prosser, Washington.

149 In areas with cool climate and short growing season, where alfalfa produces 2 to 4 cuttings of hay per year, the potential problem from dodder is considerably less than it is in areas where alfalfa is cut more than four times per year. The cool temperature when the first and last cuttings are produced favor alfalfa much more than dodder. During the heat of summer, especially in warmer regions, the high temperatures favor dodder over alfalfa, and dodder can be a severe problem during the summer cuttings. In cooler climates, mowing the hay tends to remove the dodder before it has a chance to produce seed. However, in warmer climates, such as the San Joaquin Valley, there is adequate opportunity for seed production during the summer cuttings.

Dodder is especially favored by the culture of alfalfa seed production. The parasite has time to grow large and produce abundant seed before the alfalfa seed crop is harvested. The harvesting operation can then very ea~ily disseminate the seed. Considerable research has been conducted to de'elop methods for controlling dodder in alfalfa grown for seed production. So e of these methods also apply to alfalfa for forage.

Pr vention and Early Destruction

In any situation, prevention is the most effective and most economical method of controlling dodder in alfalfa. Great care should be taken to plant only alfalfa seed that is free of contamination by dodder. Of equal importance is constant vigilance to find and destroy any individual dodder patches before they have a chance to produce seed. Scattered patches of dodder can be controlled by any method that destroys the parasite along with the alfalfa foliage to which it is attached. Cutting the alfalfa below the point of dodder attachment, burning the foliage completely, or applying a contact that destroys all above ground vegetation are effective methods for destroying individual patches of dodder.

Resistant Varieties and Biological Control

Because dodder is an obligate parasite, and cannot live except when supported by a host plant, it would seem logical that alfalfa varieties resistant to attack by dodder might eliminate the dodder problem. Research to date on developing resistant varieties of alfalfa has been very limited, and has not yielded promising results. Considering that several different species of dodder will attack many different host species, the possibility of finding resistance to dodder within alfalfa seems rather unlikely.

There has been considerable interest in biological control of dodder especially in countries other than the USA. Several organisms have been reported to attack dodder selectively. Unfortunately, their destructive action tends to be slow and incomplete. Consequently, they have not been able to protect alfalfa from the severe and rapid damage that results from parasitism by dodder.

Cultural Control Methods

There are various cultural methods for controlling dodder that can form an important part of integrated weed management systems.

150 Dodder is one of the few that can be controlled completely by crop rotation. Dodder will not parasitize grasses, and is therefore no problem in corn, small grains, or pasture grasses. Dodder can parasitize numerous broadleaf weed species, and can complete its life cycle and produce seed on such hosts. Thus, in a scheme of crop rotation, complete control of broadleaf weeds is necessary to prevent re-seeding of the dodder in the grass crop.

Although dodder does not depend upon its own to live, it cannot develop normally without full sunlight. Shade from dense alfalfa foliage suppresses dodder almost completely. Such shade can extend the period of control initiated by other control methods applied during the period before shade was complete.

Tilling the surface soil will kill dodder seedlings before they become attached to a host plant. Shallow tillage with implements such as a skew treader, a tine tooth harrow, or rotary hoe can control dodder effectively during the early spring before alfalfa becomes large. Such tillage will tend to reduce the growth of alfalfa foliage, and may be undesirable in hay production. Such suppression may not be a problem in seed production, and may even be desirable as a means of delaying the alfalfa to control the time of blossom for maximum pollination.

Dodder seedlings mostly emerge from the surface half inch of soil. If the surface soil is dry, seeds cannot germinate. Under irrigated condi­ tions, dodder can be controlled for a period simply by delaying irrigation to maintain a dry soil surface. Prevention of germination by delaying irrigation and suppression of dodder by shade are very inexpensive ways to extend the period of control from a soil-applied herbicide of rather short duration.

Alfalfa normally should not be seeded in fields known to be infested with dodder. However, if alfalfa is to be seeded in soil infested with dodder, simply seeding the alfalfa in late summer or fall rather than in the spring will allow the alfalfa to escape devastation by dodder. In Washington, alfalfa seeded in late August or early September in soil heavily infested with dodder was not injured. Although dodder emerged with the alfalfa and parasitized it, the onset of cool weather of autumn inhibited the development of the dodder, while the alfalfa seedlings continue to grow into the late fall. Freezing weather eventually killed the topgrowth of alfalfa and all of the attached dodder. The alfalfa grew normally the following spring, and was then large enough to tolerate treatments for control of dodder in established alfalfa.

Soil-applied for Dodder Control

Most chemical control of dodder is based on soil-applied herbicides that kill dodder seedlings before they become attached to the alfalfa. Chlorpro­ pham (isopropyl m-chlorocarbanilate) was the first soil-applied herbicide developed for the control of dodder. Whether applied as a liquid or granular formulation, the standard rate of 6 lb/A usually controls dodder

151 for only about 1 month. Because control is needed for at least 4 months each season, a program of dodder control based on chlorpropham requires two applications per season in conjunction with other cultural control methods.

DCPA (dimethyl tetrachloroterephthalate) at 10 lb/A is also used for dodder control in alfalfa grown for seed. Control from DCPA usually persists somewhat longer than that from chlorpropham, but other methods still must be applied in conjunction with the application of DCPA for full-season dodder control.

Dichlobenil (2,6-dichlorobenzonitrile) and pronamide [3,5-dichloro (N-1,1-dimethyl-2-propynyl)benzamide] are also sometimes applied to the soil to control dodder selectively in alfalfa.

Chlorpropham does not persist long in soil because it is lost as a vapor, and soil microorganisms decompose it. Research has demonstrated that PCMC (p-chlorophenyl N-methylcarbamate) applied with chlorpropham significantly prolongs the period of dodder control from the herbicide. PCMC has not become commercially available for this use. The insecticide carbaryl (1-napthyl N-methylcarbamate) is chemically related to PCMC and also inhibits the breakdown of chlorpropham. Carbary! is less effective than PCMC in extending the period of dodder control from chlorpropham, but is registered for use in alfalfa. There is now some commercial use of mixtures of carbaryl with chlorpropham for dodder control in alfalfa.

Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] controls dodder seedlings, but-Only at unusually high rates of application. Two related herbicides, prodiamine [2,4-dinitro-N3,N3,-dipropyl-6-(trifluoro­ methyl)-1,3-benzenediamine] and [N::(l::ethylpropyl)-3-4-dimethyl- 2,6-dinitrobenzenamine] control dodder considerably more effectively than does . Research at Prosser, Washington, in 1984 and 1985 indicated that both herbicides applied at rates of 4 or 6 lb/A in March controlled dodder effectively through August. Both pendimethalin and prodiamine offer tremendous potential for improved dodder control. They control dodder effectively and persist long enough that they can be applied early in the spring well ahead of dodder germination, and persist beyond the period when control is needed. For the first time, we have achieved full season protection of the alfalfa seed crop from a single application of herbicide.

Herbicides Applied Postattachment

Certain inorganic chemicals such as sulfuric acid and lime sulfate were used for postattachment dodder control before 1900. In more recent times, contact herbicides such as (1,1'-dimethyl-4,4'-bipyridinium ), (6,7-dihydrodipyrido[l,2-&:2' ,1'-c]pyrazinediium ion), and (2-sec-butyl-4,6-dinitrophenol) have also been used. These treatments destroy the alfalfa foliage as well as the dodder, but do not kill the perennial root of the alfalfa. They are useful as spot treatments to destroy scattered dodder patches.

152 It has recently been discovered that extremely low rates of (N-(phosphonomethyl)) control dodder selectively after it is attached and growing on alfalfa. Rates of 75 to 150 kg ae/ha (1.1 to 2.2 oz. ae/a) controlled dodder with only minor effect on the alfalfa. At current prices these rates of application are in the range of $2 to $4 per acre. Thus, glyphosate would be very economical for dodder control. Glyphosate is especially attractive for controlling dodder in forage alfalfa, because current herbicide programs for seed alfalfa are usually considered too expensive for forage alfalfa.

Glyphosate also controls dodder in alfalfa grown for seed production. Unfortunately, dodder control is seldom 100%. Thus the possibility of some dodder seed maturing remains, even though the devastating damage from parasitism is eliminated.

The related herbicide sulfosate ( carboxymethylamino­ methylphosphonate) also controls dodder selectively after attachment to alfalfa in much the same way as glyphosate does.

Dodder Control from General

Dodder seedlings very commonly make their first attachment to seedlings of broadleaf weeds rather than directly to alfalfa. They then spread to alfalfa. In the spring, dodder seedlings commonly emerge first in open spaces between the alfalfa crowns, where the lack of shade allows the soil to warm up early. Many of these dodder seedlings are beyond reach of the alfalfa crowns, and would die soon after emergence, unless they become attached to weed seedlings. Several persistent herbicides such as diuron [3-(3,4-dichlorophenyl)-l,l-dimethylurea], [3-cyclohexyl-6- (dimethylamino)-l-methyl-l ,3,5--2,4-(1H,3H)-dione], [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], and [2-chloro-4,6-bis(ethylamino)-s-triazine] control a variety of annual weed selectively in alfalfa. Although these herbicides do not affect dodder directly, they can reduce the dodder problem by eliminating the weed seedlings upon which early emerging dodder seedlings would otherwise attach.

Integrated Control Systems

A single control method applied alone seldom controls dodder for the entire season in alfalfa, especially alfalfa grown for seed production. In Washington, integration of several control methods protects alfalfa grown for seed production for the necessary 4 months. A typical schedule begins with broadcast tillage from mid March to mid April. Chlorpropham at 6 lb/A is applied in mid April and again in mid May. Chemical control is usually gone by mid June, but control can be extended for an additional period simply by delaying irrigation so that dodder cannot germinate. By late June, cover of alfalfa foliage is usually dense enough to suppress any additional dodder seedlings that emerge. Control of new seedlings is not needed after mid July, because there is not time for such dodder to harm the alfalfa before the crop is chemically defoliated in late August or September in preparation for seed harvest.

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