J. T. Presley.
Verticillium wilt of cotton is caused by a soil-borne fungus. The disease occurs across the entire Cotton Belt from South Carolina to California. It is of major importance in the lower Mississippi Valley and the irrigated areas of the Southwest. Losses in yield from it may reach 50 percent. Average losses of 10 to 15 percent over large areas are not uncommon. Cool, wet weather favors the disease and plants may be attacked at any stage of development. The fiber from wilted plants is often damaged to the extent that the number of neps and manufacturing waste are increased, and the yarns are inferior in grade and looks.
The cotyledons of infected cotton plants become yellowish and quickly dry out. Young plants with three to five true leaves suffer considerable stunting. The leaves appear darker green than those of a normal plant and become somewhat crinkled between the veins. The amount of stunting apparently depends on the stage of development of the plant when it becomes infected. The outstanding symptom is the chlorotic areas on the leaf margins and between the principal veins, which make it look mottled.
In older plants the symptoms usually occur in the lower leaves first. They spread to the middle and upper leaves of the plant later in the season. The chlorotic areas gradually become larger and paler. Severely affected plants shed all the leaves and most of the bolls. Older plants may nevertheless survive the entire season and sometimes send up sprouts from the base of the plant.
Sometimes it is hard to tell verticillium wilt from fusarium wilt. In the early stages of verticillium wilt the vascular discoloration appears to be more evenly distributed through the stele of the plant at the ground line and to be lighter brown than the discoloration produced by fusarium wilt. In fusarium-affected plants, one or more leaves near the crown may wilt suddenly and die, while the other leaves remain apparently healthy. That condition has never been found in verticillium wilt.
In hop plantings in England, wilt appeared to spread in the direction of cultivation, since the spread of the disease was more rapid in cross-cultivated fields than in those cultivated in only one direction. Technicians at Shafter, Calif., found that the fungus spread more rapidly in the heavier soils. The spread usually was against the flow of irrigation water. In Mississippi, where the spread of the fungus was carefully studied, the fungus moved approximately 3 feet a year and spread out from centers of infection.
Diseased cotton stalks and leaf and stem trash from diseased stalks can carry the fungus over from one season to the next and spread the disease into new parts of a field.
MOST OF THE COMMERCIAL VARIETIES of upland cotton (Gossypium hirsutum) appear to be susceptible to verticillium wilt. Varieties of Egyptian, Pima, sea-island, and some South American cottons (G. barbadense) have a high degree of resistance or tolerance. Numerous observations in the field indicate that the nutrition of the host has a pronounced effect on the development of verticillium wilt. Workers have found that it is particularly severe on soils high in organic matter and also that the disease is favored by heavy application of organic matter to the soil. Efforts were made at Shafter to control the disease by applications of chemicals and soil amendments, but none of the practices appeared promising.
Despite the extreme susceptibility of most commercial upland varieties, some progress has been made through selection and breeding toward a high degree of tolerance or resistance. George J. Harrison at the United States Cotton Field Station, Shafter, has produced Acala 4-42, which has some tolerance to the disease. Further selections have been made that are even more tolerant.
A. R. Leding at the United States Cotton Field Station, State College, N. Mex., selected a wilt-tolerant variety Acala 1517 W. R. from Acala 1517. It was made available to growers in 1949. Of the cottons adapted to the Southeastern States, none has a high degree of tolerance to verticillium wilt. The fusarium-resistant varieties, such as Coker loo wilt, Coker 4-in-1, Empire, and Plains, however, have consistently yielded better than wilt-susceptible varieties when grown on soil infested with verticillium wilt.
At the Mississippi Agricultural Experiment Station, Hartsville was found to be highly resistant to verticillium wilt, but the variety was agronomically undesirable. Workers at several State agricultural experiment stations have used Hartsville in a selection and breeding program designed to develop an agronomically acceptable variety that tolerates the wilt.
A grower should choose the most tolerant variety adapted to his particular location to be planted in fields infested with verticillium wilt. Any cultural practice that produces and maintains a higher soil temperature tends to reduce the amount of wilt. Mr. Harrison at Shafter, by carefully controlling soil moisture after the first of July reduced the incidence and severity of wilt. His method is based upon frequent light irrigations which permit more rapid warming of the soil following each irrigation.
L. M. Blank and P. J. Leyendecker, at the New Mexico Agricultural Experiment Station, found that by planting cotton on high ridges a certain degree of control was obtained due to increased soil temperature. They also found that dry fallow for one year and a rotation with barley or barley-Hubam mixture reduced the percentage of infected plants in the first cotton crop.
Alfalfa is also used in rotation with cotton on wilt soil and the first crop following the alfalfa generally has less wilt. Succeeding cotton crops, however, are often more severely attacked than on land where no alfalfa has been grown.
J. T. PRESLEY is project leader for plant pathology in the division of cotton and other fiber crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering. He received degrees from the University of Maryland and the University of Minnesota. Since 1935 he has devoted the major part of his time to investigations of cotton diseases, primarily phymatotrichum root rot and verticillium wilt.
Septoria leaf spot on tomato.