Developing Healthier Vegetables

H. Rex Thomas, W. Y. Zaumeyer.

Work progresses steadily toward the important goal of breeding vegetables that can withstand the ravages of disease. How close are we to it?

The latest edition of the vegetable seed catalog of one company lists more than one-half of the varieties of pea, approximately one-third of the varieties of watermelon, and one-fourth of the varieties of tomato and cabbage as resistant to one or more diseases. The proportion would be higher if all of the new varieties released within the past few years were included. That is an excellent record for an endeavor that had barely started in this country before 1900.

What are some of the problems facing us today and how are we prepared to meet them? Emphasis currently is directed toward incorporating higher degrees of resistance into a wider range of varieties and resistance to several diseases; multiple resistance within one variety. Superior quality, resistance to insects, wide adaptation, high nutritive value, and good qualities of processing, shipping, and storage also are essential.

The need for multiple resistance is becoming more and more important. It is no longer enough to have varieties resistant to only the diseases present in the areas where the crop is grown for market or processing. They must also be resistant to the diseases in the seed-producing areas. For example, most of the seed of snap beans is produced in the drier sections of Idaho and California, where the bacterial blights and anthracnose rarely occur. In parts of Idaho, however, the curly top virus is destructive, and resistance to it is necessary for the safe production of the seed crop, even though the virus does not occur in the southern and eastern production areas of the United States where snap beans are grown most extensively for market and processing.

Progress in multiple resistance has already been demonstrated. The Southland tomato, introduced by the Department's Southeastern Vegetable Breeding Laboratory at Charleston, S. C., possesses high resistance to collar rot and fusarium wilt and moderate resistance to early blight and one race of late blight. The Hawaii Agricultural Experiment Station has developed and introduced several varieties of tomato that resist the spotted wilt, fusarium wilt, and gray leaf spot. The rapidity with which the varieties were developed was the result of using new, streamlined methods of eliminating susceptible plants in the seedling stage. The Hawaiian workers made crosses between varieties that would produce progeny with resistance to the three diseases mentioned above and would also add good horticultural characters.

When the crossing program has progressed far enough for disease testing, the seeds are planted in gallon cans filled with sterile soil. After 12 to 15 days, the seedlings are sprayed with a suspension of gray leaf spot spores and placed in a humidity chamber for 2 days to allow the spores to germinate and infect the leaf. Within 2 to 3 days after the plants are removed from the humidity chamber, the plants with infected cotyledons seed leaves are discarded. A high correlation between cotyledon infection and mature plant resistance had previously been determined. The seedlings are allowed to .harden off for 4 to 8 days. They are removed from the gallon can and the roots are dipped into fusarium inoculum previously produced in the lab-oratory. A few seedlings are then transplanted again to gallon cans, which are placed in the spotted wilt nursery, where virus-infected host plants are grown and a large population of thrips is maintained as a carrier for the virus. After 3 to 4 weeks, the plants susceptible to spotted wilt and fusarium wilt are removed. Notes on horticultural characters earliness, growth habit, vigor, type of flowering are evaluated. The most promising plants are finally transplanted to the field to mature. During this period further eliminations for poor horticultural type are made. With this procedure many thousands of plants can be tested and the few resistant and promising types selected with the minimum of effort, space, and expense.

Plant breeders are utilizing the greenhouse as a substitute for field plots for testing the disease resistance of the young hybrid seedlings because the soil, temperature, nutrition, and moisture can be controlled in a manner most favorable for the development of disease. As many as four or five generations a year can often be tested where only one could be tested in the field.

Improved pathological techniques that insure consistent, severe disease tests provide the means for separating the hybrids with high resistance from those with only low or moderate resistance. Marglobe tomato is moderately resistant to fusarium wilt and will often stand up in infested field plots. In badly infested greenhouse soil, however, it is killed in the seedling stage, but a highly resistant variety like Pan America survives. The high resistance in Pan America, which was derived from a tomato species introduced from South America, has been included in most of the new tomato varieties developed since 1947.

Two types of resistance to fusarium yellows, a serious disease of cabbage in the warmer climates, occur in cabbage. The higher type of resistance is effective at both high and low temperatures. The other type is active only at low temperatures. To obtain hybrid cabbage seedlings with the high degree of resistance, they are grown in infested soil at 75 F. for 2 to 3 weeks. All seedlings with no resistance or only a low degree of resistance are eliminated.

For many diseases no resistance has yet been found. The search continues. New, valuable sources of resistance are continuously being made available because of an extensive screening of large collections of vegetable seeds made in foreign countries by the Department of Agriculture and other organizations.

Often the plant breeder must settle for a moderate degree of resistance to a disease. If a foliage disease is involved, the addition of a spray or dust schedule may permit the successful culture of the crop that otherwise might be impossible if a susceptible variety were grown.

It is important that the degree of resistance arid the number of diseases to which the variety is resistant be fully explained when it is released. If additional chemical control is necessary this should be mentioned. Any particular susceptibility to other diseases should be noted. A plant breeder's enthusiasm should not overbalance his judgment as to the limitations of his new creation.

How difficult is it to develop an acceptable disease-resistant variety once a source of resistance has been found? Sometimes it is easy. Resistance may be found within individual plants of a commercial variety. In that way were wilt resistance in peas and yellows resistance in cabbage found.

Sometimes an off-type resistant plant may be found in a commercial variety. Resistance to common bean mosaic virus was found in an off-type Stringless Green Refugee plant. The selection, named Corbett Refugee, was of Poor commercial type. Crosses to Stringless Green Refugee were necessary to improve the quality. But even that was relatively easy, as both plants had stringless pods and similar habit of growth.

Compare this with the development of the anthracnose-resistant Congo watermelon. No commercial varieties resistant to anthracnose could be found. A poor-quality melon, supplied by a missionary in South Africa, had to be used. Crosses were made between it and the commercial variety Iowa Belle. After several generations, a promising selection was crossed to another commercial variety, Garrison. After several generations of selections of that, the Congo variety was produced.

When the plant breeder cannot find disease resistance within a species he must turn to a related species. Crosses between species; interspecific have been used successfully in potatoes to obtain resistance to late blight and in tomatoes for resistance to fusarium wilt and leaf mold.

Sometimes the crosses are easily made. More often they are difficult, as when a cross was attempted between tomato and the related species Lycopersicon peruvianum, which is resistant to tobacco mosaic virus and nematodes. Some of the newer techniques, however, made the cross possible and enough hybrid plants were obtained to give hope of controlling these two serious diseases of tomatoes.

Some breeders have used colchicine, a narcotic alkaloid chemically related to morphine and codeine, to double the number of the chromosomes, the small bodies in the cell that contain the genes that control the development of inherited characters. That can be done by soaking the tomato seed for a short time before planting or by applying a dilute solution on the growing tip of the plant. If the latter method is followed, only the new growth on the treated stem will have the double number of chromosomes. When the flowers on the treated plants are used to cross with L. peruvianum, fertile seed develop.

Another approach to the problem has been embryo culture. When the cross between plants in which the chromosomes have not been doubled is made, normal fruit (but not seed) development occurs. About 30 to 40 days after pollination the tissue around the young developing embryo collapses. Shortly after, the embryo dies. Occasionally a few immature seeds have an embryo sufficiently developed to grow when aseptically removed from the seed and transferred to a nutrient medium and cultured at a warm temperature. The young seedlings are planted in soil later and handled in the routine manner.

Other techniques have aided vegetable hybridization. Some plants will flower only in a short day. Others will flower only in a long day. We now retard or induce flowering at will by controlling the length of day, either by using dark curtains to shorten the day or a few minutes of light in the middle of the night to lengthen the day. Growth regulators have been applied to the pedicels of flowers to aid in securing set of fruit and seed. Flies have been used to make controlled pollination in onions and carrots, thus saving the plant breeder many hours of work. The control of temperature, moisture, and nutrition have aided in flowering and fruit set.

Because of the expense involved, most of the disease-resistant varieties of vegetables are developed by public-supported State and Federal agencies or large seed companies. The search for a source of resistance may take years of testing and involve many thousands of plants.

For more than 20 years breeders have been striving to develop a tomato resistant to the curly top virus. Few private companies could have financed such a program. Yet the acute need for solving this as well as other difficult problems remains. Well-equipped laboratories, greenhouses, and field plots must be available. Often a particular disease can be studied only in a particular locality, and facilities must be provided in that area if they do not already exist. Besides the plant breeders, other trained specialists are needed for the work, among them pathologists to help eliminate susceptible plants, cytologists to study the chromosome behavior, and chemists to make analysis of the content of vitamins, sugar, and fiber.