Alternaria Leaf Spot of Cotton

Alternaria leaf spot of cotton is also known as Alternaria leaf blight. The disease was first identified in cotton in the US in 1918 and is now distributed worldwide. Alternaria leaf spot has been considered a minor disease in the cotton growing areas of Arizona. The disease is frequently associated with senescing tissue of cotton under physiological stress (heavy boll load) or nutritional stress (potassium deficiency) late in the growing season. On rare occasion it can also affect seedlings. In recent years, several disease outbreaks that led to severe defoliation in late-season cotton were reported from Graham County. The disease can severely affect susceptible Pima cotton varieties and also attack upland cotton varieties. Susceptible varieties have had nearly 100% of leaves infected in years when weather conditions are conducive for disease infection and development. The impact on yield in Arizona was estimated to be 10% to 15% in highly susceptible cotton varieties.