Potassium deficiency symptoms and verticillium dahliae symptoms appear during late August and early September in many cotton fields in the San Joaquin Valley of California. The foliar symptoms occur in various degrees of severity from one field to another and from one location to another in the valley. Also, the severity tends to increase when an unseasonably cool spell occurs early in the growing season.

Direct measurements of yield reduction are difficult to make, but estimates range from 20%-30% of potential due to this malady. Years 1980 and 1982 were especially conducive to Apparent Potassium deficiency and verticillium wilt.

Earlier studies showed that a response to fertilizer K was possible, but foliar symptoms persisted even at high application rates. Field studies conducted during the growing season of 1982 indicated that a disease organism might be involved. The test consisted of three cotton cultivars (Gossypium hirsutum, cltvs Acala SJ-2, Acala SJ-5 and Acala C-1. A factorial design was used to test eight potassium rates with each cultivar. Treatments included a control, 250, 500 and 1000 lbs. K₂O/acre split applied, preplant and sidedressed; 250 and 500 lbs. K₂O/acre side dressed; 250 and 500 lbs. K₂O/acre preplant plus foliar K applied every three weeks during the growing season. Another area received the K treatments after the soil had been fumigated with 100 lbs./acre picchlor 60.

Significant yield responses were realized from any K treatment when compared with the control in the nonfumigated area. Soil fumigation had the effect of substituting for the need for extra K. Acala SJ-5 yielded the same with or without K fertilizer when planted to the fumigated soil.

It has been observed that other crops do not show K deficiency when grown on the same soil in which cotton shows the symptoms. Also, laboratory analyses reveal that most soils are not deficient in K. Since fumigation results in an increase in yields, it is possible that benefits are due to the control of a pathogenic organism.

Our work is far from completed and subsequent tests will be directed toward solving the unanswered questions which arose as a result of this year's efforts.