48

CONTROL OF DODDER IN TOMATOES

James M. Hutchison Department of Botany University of California, Davis

Dodder is the common name for members of the family Cuscutaceae. There is only one genus, Cuscuta; 16 species have been reported to occur in California (Munz and Keck, 1959). These plants are parasitic, attaching to the stems of other plants and living on them. Over the last decade, an increase of field dodder, Cuscuta campestris Yunker, has been observed in many of the tomato growing areas of California. A research program was initiated five years ago at the University of California at Davis to study this plant and develop methods for control. Reconunendations for the control of field dodder in tomatoes as well as aspects of its biology are presented here. The support of the California Tomato Growers through their Processing Tomato Advisory Board is gratefully acknowledged.

Biology

Dodder seeds germinate in the soil and emerge as threadlike seedlings. The seedlings elongate rapidly until a host plant is encountered. Upon contact, the dodder stem tightly coils about the stem of the host and attaches by means of haustoria. Haustoria are suckerlike organs similar to adventitious roots which penetrate the host tissue and join with the vascular tissue. Food, nutrients, and water are removed, resulting in a reduction in growth of the host plant. The attached seedling has a great capacity for growth. It sends out numerous branches; each can attach to another host. The dodder plant grows along with the host plant, often flowering at the same time as the host. The flowers produced by field dodder are inconspicuous, but numerouso From one to four seeds are produced by each flower.

The seeds produced are termed 'hard seeded' and most will not germinate readily. This dormant:condition is imposed by the seed coat which excludes both water and oxygen. Germination will occur rapidly in the laboratory if the seed coat is altered chemically by immersion in concentrated sulfuric acid or by grinding away the outer two layers of the seed coat. During my research for my Master's thesis, I determined that the middle layer must be altered to allow germination. Once seeds were rendered permeable, I was able to study many parameters of germination. The effect of temperature was examined and it was found that field dodder germinates at temperatures between 18 and 36 C with an apparent maximum between 30 and 33 c. Depth of planting was also found to affect germination. Field dodder emerged well from depths approaching 2 cm but poorly from lower depthso The level of oxygen in the soil may be controlling this germination. Furthermore, unlike witchweed (Striga) or broomrape (Orobanche) which germinate under the influence of their host plants, dodder seed germination is not affected by the presence of tomato plants.

In the field, germination occurs when soil temperatures are warming and moisture is abundanto As temperatures become excessive, germination ceaseso 49

Due to the large number of seeds produced and the dormancy imposed by the seed coat, dodder is capable of remaining in the soil for many years.

Cultural Control

The most successful method of control is to prevent the introduction of seed into noninfested tomato fields. Alfalfa seed has been known to be contaminated with dodder seed, but efficient methods for removing dodder have been developed. The California State Department of Food and Agriculture examined over 200 samples of tomato seed and never found a single dodder seed. Therefore, it is unlikely that tomato seed is a source of infestation. Dodder growing on weeds in areas adjacent to tomato fields is a more likely source of seed. Elimination of dodder from the edges of irrigation ditches would limit the possibility of seed entering in irrigation water. Dodder seed can also be transported on equipment which has been used in an infested field and is being moved to a noninfested field. Cleaning of equipment is a ready solution.

Once seed has been introduced into tomato fields, control is most successful after germination and before attachment. Early irrigations to germinate dodder seed, late plantings of tomatoes, and close cultivation can reduce the number of seedlings eventually attaching. Rotation from susceptible to resistant crops has been suggested as a solution for severe infestations (Macrae, 1952; Loveridge, 1966).

Chemical Control

In an early review of dodder seed germination, Gaertner (1950) noted that common laboratory chemicals, such as, formalin, ammonium nitrite, and monochloracetic acid controlled dodder seed germination in the soil. However, these are not economically feasible. Dawson (1967) examined 100 herbicides in the greenhouse and found 32 to be 9~ to 100% effective in preventing dodder seedlings from attaching to alfalfa. Most of these compounds are not available for use in tomatoes. Of the registered materials for tomatoes, CDEC (Vegadex ®) was shown to control dodder. On the basis of four years of field experiments, Vegadex at 6 lb/A as a preemergence treatment with sprinkle irrigation or as a preplant soil incorporation treatment has been recommended. Further research has shown that pebulate (Tillam ®) gives fairly good control of dodder and that the use of these two in combination give excellent dodder control and a better spectrum of weed control than Vegadex alone. If this combination is used, a 25% reduction in the rate of each should be used to minimize tomato growth supression. This is especially true in light soil and in early plantings when temperatures are low. Either Tillarn or Vegadex can be used in combina­ tion with naproparnide (Devrinol ®) to further broaden the species of weeds controlled.

No commercially available herbicides can control dodder once attach­ ment has occurred with safety for tomatoes. Flaming, spraying with weed oils, or contact herbicides can eliminate spot infestations of dodder. The host plant is also destroyed by these treatments. An experimental herbicide H-26905 shows some promise for use as a postemergence spray. It quickly stops the 50

growth of the dodder plant with minimal damage to the tomato plant. The timing of application appears to be critical to achieve control and avoid excessive injury. Further research is presently in progress.

Biological Control

No biological methods for the control of dodder presently exist within the United States. However, an insect from West Pakistan, Melanagromyza cuscutae, has been shown to be a specific herbivore of dodder (Baloch et al., 1967), This insect may prove to be an important biological control agent where its natural enemies do not exist, In the Soviet Union, research has shown that the spores of the fungus Altenaria cuscutacidae are 90% effective for the control of dodder on alfalfa (Wilson, 1970). No further studies concerning the feasibility of this method for commercial control have been reported,

Sununary

Dodder has proven especially difficult to control in tomatoes, primarily due to its peculiar mode of growtho However, a program of good cultural practices and use of the chemicals available should lead to effective control of field dodder.

Literature Cited

Baloch, G. M., I. M, Din, and M. H, Ghani, 1967, Biological control of Cuscuta spp. I. Cuscuta spp, and insects associated with these in W. Pakistan. Conunonwealth Inst, of Biol Control Tech, Bull, 8:149-158,

Dawson, J. H. 1967, Soil-applied herbicides for dodder (Cuscuta) control; an initial greenhouse evaluation. Wash. Agr. Exp. Sta. Bull, 69,

Gaertner, E. E, 19500 Studies of seed germination, seed identification, and host relationships in dodders, Cuscuta spp. N,Y. (Cornell) Agr. Exp. Sta, Memoir 194.

Loveridge, J, A. 1966. Ridding lucerne of dodder. Power Farmer Australia New Zealo Better Farming Dig, 25:31.

Macrae, J. w. 1957, The increase of dodder; suggestions for its control. Canada NatLWeed Conf. East, Sect. Proc. 5:80-81,

Munz, P. A, and D. D, Keck, 19590 A California Flora. Univ. of California Press, Berkeley.

Wilson, c. L. 1970, Use of plant pathogens in weed control: dodder, Pest Articles and News Summaries 16:484,