DODDER CONTROL IN ALFALFA 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 soil. 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, crop 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 crops, 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 weed management system for dodder control in alfalfa. *Research Agronomist, Irrigated Agriculture 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 herbicide 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 weeds 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 photosynthesis 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 Herbicides 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)