J. C. Walker.
The cabbage tribe includes cabbage, cauliflower, broccoli, brussels sprouts, kohlrabi, kale, and collard. All have been derived from the leafy wild cabbage of Europe. They inter-cross readily with each other and with cabbage.
Somewhat removed from them botanically is radish, which does not ordinarily cross with members of the cabbage tribe. When it does, a rank-growing, sterile hybrid usually results.
Turnip and rutabaga are the other two important vegetables in this group. They are known to hybridize with each other. Their hybrids, too, generally are sterile.
All these vegetables belong to the family of plants known as crucifers, which includes also Chinese cabbage, water cress, rape, the wild and cultivated mustards, and many weeds, such as shepherds-purse and pennycress.
Some 60 diseases may affect one or more of the cruciferous vegetables. The most destructive are yellows, black rot, blackleg and dry rot, club-root, and mosaic.
YELLOWS, a warm-weather malady, is most destructive throughout the Corn Belt and as far north as central Wisconsin and northwestern New York. It is not important on the midwinter crop in the South but may be destructive on autumn-sown cabbage and on the part of the crop that grows into late spring.
Yellows appears on young plants, in the seedbed or after transplanting, as a lifeless, yellowish-green color of the foliage. Often the yellowing is more intense on one side of the leaf, and on it a warping develops. Lower leaves are affected first. As the disease progresses up the plant, the leaves become brown and brittle and drop prematurely. The water vessels of leaf and stem turn brown. Severely diseased plants die early. Entire fields may be destroyed if susceptible varieties are used on infested soil. Yellows is especially severe on cabbage, a crop often grown in climates favorable for the disease. It is less common on other susceptible crops, which usually are grown in cooler regions.
The causal fungus, Fusarium oxysporum f. conglutinans, produces white or cream-colored threads, which form a cottony mass on culture media. On the threads are borne numerous microscopic spores, which propagate the fungus. The fungus can exist indefinitely in the soil. It is not seed-borne but is transported in transplants and in soil moved about by wind, water, implements, and animals. The fungus in the soil enters the plant through the young roots at or near their growing tips. Without noticeably injuring the outer parts of the root, it becomes established in the water vessels and progresses upward through them to the taproot and into the stem and leaves. The fungus remains confined in the water vessels until the plant is dead. Then it grows to the surface and produces spores.
How it acts on the plant is not known. It probably plugs the vessels and produces toxic materials, which cause discoloration of the bundles, yellowing of leaves, leaf drop, and death.
Yellows was so destructive by 1910 that it became the subject of a research project initiated by L. R. Jones of the Wisconsin Agricultural Experiment Station. The program was continued as a cooperative project with the Department of Agriculture.
It was one of the first studies of the relationship between soil temperature and plant disease. It brought out that when the soil was kept at 65 F. or lower little or no disease developed in plants grown in infested soil. As the temperature rose, the intensity of the disease increased up to about 81 . Then it declined. This temperature curve followed closely that of growth of the fungus in pure culture. The results explained why the disease increased in intensity as summer temperatures rose and also why it was severe in places where cabbage grew in warm months and was little noticed where the crop was grown in winter,
All attempts at practical control by chemical treatment of soils were unsuccessful. Dr. Jones noticed, however, that in any field that was almost completely destroyed occasional healthy plants survived. By growing seed from such plants and returning their progenies to infested soil, he got higher percentages of resistant plants in succeeding generations. After three generations of selection within the variety known as Hollander or Danish Ballhead, he obtained a strain sufficiently resistant to be of commercial value. It was released in 1916 under the name Wisconsin Hollander. It was still widely used on yellows-infested soil in 1953.
Two genetic types of resistance to yellows exist. One, known as type B, is found in Wisconsin Hollander. Wisconsin Hollander may show a considerable percentage of diseased plants in hot summers, but most of the plants are only mildly affected.
In the parent variety, Hollander, and in some other susceptible varieties, another type of resistance was found later. A single dominant gene controls it. This resistance, known as type A, has two advantages over type B resistance. As it is controlled by one gene, it is readily fixed and transferred from one variety to another. It also is stable at constant soil temperatures as high as 78 F.
Because breeding lines may carry both types of resistance and the two are hard to tell apart in the field, a way ,,as developed to separate them in breeding programs. Type A is expressed more strikingly than type B in young seedlings and in old plants.
By making use of the soil temperature effects studied earlier, technicians discovered that if young seedlings are grown in heavily infested soils at 75 , those that carry only type B resistance succumb to yellows while those which carry the gene for type A resistance are not affected. By growing seedlings in metal pans in thermostatically controlled water baths, known as Wisconsin soil temperature tanks, plants that carry type A resistance can be sorted out. The test is accurate and can be applied quickly and in small space to thousands of plants.
By selection and hybridization, type A resistance has been incorporated in many types of cabbage. Yellows-resistant varieties to suit most seasons and market requirements are available, but the work of breeding new varieties to meet changing requirements continues.
The chief yellows-resistant varieties now in use are Jersey Queen, pointed head, early; Wisconsin Golden Acre, round head, early; Resistant Detroit, round head, second early; Racine Market, round head, second early; Wisconsin Copenhagen, round head, midseason; Marion Market, round head, midseason; Globe, round head, midseason; All Head Select, flat head, midseason-, Improved Wisconsin All Seasons, flat head, late; Improved Wisconsin Ballhead, round head, late; Wisconsin Hollander, a semi-round head, very late; Bugner, semi-round head very late; Resistant Red Hollander, round head, late.
BLACK ROT, much like yellows in appearance, has been known since 1890 or so. It occurs on the same crops as yellows. It also affects turnip, rutabaga, radish, Chinese cabbage, rape, and other leafy crucifers. It occurs wherever those crops are grown in the United States except in places in the Pacific coast region where the crops are produced in dry seasons. The disease starts, often inconspicuously, at the water pores of the leaves. The organism progresses throughout the plant by way of the water vessels. The organism is known also to enter through the root system and through wounds made by chewing insects.
Marginal leaf infection is followed by browning and dying of the tissue. The lesion often forms a V, pointed toward the midrib. In the lesion the veinlets become black. As the organisms advance, the invaded vessels continue to turn black through leaf, stem, and heads of cabbage and through roots of fleshy crucifers. Non-marginal yellowish places next appear on leaves; as they turn brown and drop prematurely the disease closely resembles yellows and is often confused with it on cabbage and other hosts susceptible to yellows. The chief points of distinction are: The veins in yellows tend to be brown and those in black rot are black; V-shaped lesions at the leaf margins are not so distinct in yellows as in black rot; soft rot of cabbage heads commonly follows black rot but seldom does in yellows.
The causal organism, Xanthomonas campestris, is a motile bacterium, which produces yellowish growth on culture medium. It was studied intensively in the 1890's by Erwin F. Smith, of the Department of Agriculture, and H. L. Russell, of the Wisconsin Agricultural Experiment Station. They worked out some of the essential features of the disease cycle. They learned that the bacteria entered the plant chiefly through the hydathodes at the edges of leaves on which guttation drops commonly accumulate during periods of high humidity, especially during cool nights which follow warm days. The bacteria soon gain access to the extremities of water vessels in that way. Only rarely do the bacteria enter stomata, the "breathing" pores that open and close probably because in cabbage the substomatal chambers in leaves and stems seldom become filled with water in nature. When that condition is provided artificially, however, abundant stomatal invasion can be secured. In the first leaves of cabbage seedlings, a few stomata cease to open and close and serve more or less as hydathodes. Charles Drechsler at the Wisconsin Agricultural Experiment Station showed that that was the chief point of initial entry of the plant by bacteria carried with the seed. The seed coat is carried above ground as the seed germinates. It remains attached to the cotyledon, where it provides the infectious bacteria that penetrate the stomata.
Infected seed has been regarded as a major source of inoculum, but the disease cycle was not fully understood until the work of Allyn Cook, R. H. Larson, and me, of the Department of Agriculture and the Wisconsin Agricultural Experiment Station. It had been observed repeatedly that black rot epidemics appeared suddenly in cabbage fields approaching maturity, although there was no obvious source of the bacteria other than the seed used the previous spring. Plants that become infected in the seedbed commonly lose the infected leaves early. External signs of the disease disappear, although some bacteria have advanced into the stem before the infected leaf or cotyledon dropped. The plant may grow for weeks or months without any perceptible reduction in growth, but the bacteria become distributed more or less generally throughout the plant. At some later time, for reasons still not well understood, the bacteria multiply rapidly and cause many lesions along the leaf margins and in many plants in a field. The abundance of inoculum so produced may be spread rapidly in warm, moist weather, and the destructive disease epidemic is at hand.