Numerous studies have reported interactions between cotton K nutrition and the incidence of several diseases. For example, increased leaf K concentration has been associated with reduced foliar lesions caused by Alternaria leaf spot. Research has suggested that expression of K deficiency symptoms is a prerequisite for an Alternaria leaf spot epidemic. Recently, an increased level of occurrence and severity of Alternaria has been reported in the north central Texas Blacklands region. The problem has been observed across all major varieties (Collin County Cotton Variety Trial 1996), with only limited variation in the degree of severity. In general, all varieties suffered almost complete defoliation with a projected but unquantified level of yield reduction. Although various causes of late-season K deficiency have been proposed, better information is needed regarding soil test K levels in susceptible soils and crop response to variable rates of K fertilization.