To control black rot, it is necessary first to practice a 3-year rotation with non-cruciferous crops to avoid infection by organisms in plant debris. The next remedy is to prevent introduction of inoculum with the seed. Liquid or dust chemical treatments are not effective in this case. Immersion of the seed for 30 minutes in water at 122 F. is effective against black rot germs in the seed. That is a standard seed treatment. It is unnecessary to treat seeds of cabbage, cauliflower, turnip, and rutabaga grown in the Pacific Coast States, where the dry spell in midsummer keeps the disease from spreading in seedbeds.

BLACKLEG dry rot affects cabbage, cauliflower, broccoli, brussels sprouts, kohlrabi, kale, collard, Chinese cabbage, turnip, and rutabaga. It is incited by a fungus, Phoma lingam, which like the organism of black rot, subsists on infected plant debris for 1 or 2 years in the soil and in infected seed. All parts of the plants above or below ground may be affected. Spots appear on leaves as inconspicuous, indefinite, pallid areas. They gradually become well defined with ashen-gray centers, in which innumerable black dots, much smaller than a pinhead, are scattered irregularly; they are the fruiting bodies of the fungus, within which innumerable spores are formed and exude to the surface only in moist weather.

On the cabbage stems similar spots occur. Often they have a purplish border. When they are near the soil line, they gradually extend below ground, where the fungus eventually destroys the fibrous root system. That causes the plant to wilt or to topple over as the head increases in weight. When plants are carried through the winter to produce tall, branching seed plants, stems, branches, and seed pods are similarly affected. Here the fungus gains access to the young seeds. It burrows into the seed coat, where it remains dormant without necessarily stopping germination.

When turnip and rutabaga crops are infected, similar leaf lesions appear, and dry rot cankers on the fleshy roots develop before harvest or appear later in storage where heavy losses may follow.

The fungus of blackleg depends entirely upon dew and rain to promote discharge of spores and to spread them to uninfected plants. The disease starts from infection of young seedlings by the fungus from infected seed or from trash in the soil. Humid, rainy weather is essential for an epidemic and conversely in areas with dry growing seasons the disease is relatively rare. In Pacific coast areas the disease is so rare that seed produced there is considered safe without treatment. The only effective treatment, when it becomes necessary, is keeping the seed for 30 minutes in water at 122 F.

No resistant varieties are known, but the disease is controlled by 3-year rotation with non-cruciferous crops and the use of seed grown on the Pacific coast or treated with hot water. One should not plant crucifers in fields next to fields where crucifers were grown the year before, because surface-water may spread infected debris.

CLUBROOT, a troublesome disease for centuries throughout Europe, is acute in many intensive crucifer-growing areas in our Northern States.

Most crucifers, wild and cultivated, are susceptible and thus the mustards, shepherds-purse, and pennycress can serve as perpetuating hosts of the causal fungus, Plasmodiophora brassicae.

As clubroot affects only the below-ground parts of the plant, it may run part of its course after infection without causing any noticeable symptoms above ground. Such signs, when they appear, are likely to be in the form of slowly reduced growth, sometimes temporary wilting, and occasionally premature death. When affected plants are pulled, various types and stages of root enlargement and malformation are found. The club may consist of a fleshy enlarged root in the shape of a spindle or it may consist of a spheroid gall. When many infections occur close together, most of the root system is transformed into variously shaped malformations. The clubbed tissue after a time is invaded by soft-rotting organisms. If the plant is not protected by production of additional secondary roots it may now wilt and die or, depending on the extent of root damage, slow down to stunted, unproductive growth.

The club tissue is permeated by the fungus, which first stimulates the growth of the host and then goes over into production of innumerable spores, which are released to the soil as secondary organisms decay the roots. The next spring the spores germinate and produce motile bodies, which enter the young roots of crucifers. Since the fungus does not gain access to the seed, it is not seed-borne. It is transported in diseased roots and transplants and in soil in various ways. There is no evidence that it lives as a saprophyte, but infested soil is known to remain infested for 10 years or more. Infection is favored by relatively high soil moisture.

Clubroot is hard to control. Alkaline soil is unfavorable to it. In places where the soil is naturally alkaline or can be made so by liming, the disease can be held in abeyance. Heavy liming, however, is not desirable for such crops as potatoes, which scab most readily on such soil. It is important to select noninfested soil for the seedbed. The disease may be reduced somewhat by dissolving 1 ounce of mercuric chloride (corrosive sublimate) in 16 gallons of the transplanting water. That is not a complete remedy, however, and can be applied only in small cultures.

Varieties of turnip that are somewhat resistant are used in Europe. The strains of the clubroot organism prevalent in the United States appear to be relatively mild on turnip and rutabaga, however, and the disease is thus of little import on those crops here.

A NUMBER OF VIRUSES infect crucifers. Disease is commonly caused on cabbage by the combined effect of two. One is a strain of turnip virus 1 and the other a strain of cauliflower virus 1. The first causes mottling of the foliage. The second causes a clearing or chlorosis along the veins of the leaves. Together they produce lifeless spots on leaves, stems, and pods, pronounced stunting, and premature defoliation. The viruses carry over from season to season in cruciferous weeds and in plants kept for seed production. They are both transmitted to healthy plants by the cabbage aphid. The mottle phase is most noticeable in warm weather and the vein-clearing phase is favored by cool weather.

Mosaic caused great damage in the early 1940's in cabbage seed crops growing near Puget Sound. By 1942 the reduction in seed production was so great that special measures were taken to study and control the disease. The research men found that the intensity of the disease was brought about by the fact that in seed production the viruses infect the cabbage during the first season, winter over in such plants, and bring about stunting and sparse seed production the second year. Moreover, aphids leave the maturing seed plants in midsummer and migrate to seedbeds nearby and infect the new crop. After several seasons of this close cycle, the first-year crop was almost completely infected before going into the winter.

Control was accomplished by breaking the cycle. Seedbeds were removed to isolated areas, and the first-year crop was thus protected from infection. The seed plants in the second year were found to be relatively free from disease, and the yields of seed were normal. In other areas relatively successful control may be assured by moving the seedbed to locations where cabbage or other cruciferous crops have not been grown recently and where weeds have not become heavily infected with the viruses.

Varieties of cabbage differ in their resistance to mosaic. The resistant plants become infected as do susceptible plants, but they produce nearly normal growth despite the virus. A program of improvement was started in the Department of Agriculture in cooperation with the Wisconsin Agricultural Experiment Station. By continuous selection of tolerant plants with yellows-resistant varieties, strains resistant to both diseases can be obtained. One such variety, Improved Wisconsin All Seasons, has been made available to growers.

J. C. WALKER is professor of plant pathology in the University of Wisconsin. He has been a member of the faculty there since 1914. He also is a pathologist and agent for the Bureau of Plant Industry, Soils, and Agricultural Engineering. Dr. Walker holds three degrees from the University of Wisconsin.