Steve Koenning, Extension Plant Pathologist and Keith Edmisten Cotton Extension Specialist
Leaf spot on cotton is especially common throughout North Carolina A variety of fungi can be found in these spots including: Alternaria, Cercospora, Stemphyllium, and Colletrotrichum. Most commonly we are finding Ascochyta this year, which is not usually damaging to cotton. Though target spot has not been found on cotton in North Carolina at this date, it has been reported from Alabama, Georgia, and South Carolina. Fungicides can control these diseases, but rarely result in improved lint yield or quality. Furthermore, fungicides usually do not impact boll rot fungi.
Corynespora leaf spot or target spot of cotton was first identified as a possible problem in Georgia in 2005. In 2012, it was found in nearly all Georgia cotton fields and on rank cotton in North Carolina. The disease typically starts in the lower canopy where it causes 0.25 to 1 inch reddish-brown target spots with light and dark brown concentric bands or rings. Other fungi that can be associated with these spots include Stemphylium, Alternaria, Cercospora, and Colletotrichum. These fungi are most commonly associated with cotton under physiological stress or environmental factors such as frequent rainfall and overcast days such as seen most of the month of August in Virginia.
Quadris and Headline are strobilurin type fungicides that can be applied to cotton and should provide control for ten days to two weeks. The Efficacy of other fungicides on cotton for control of this disease is not known, but many triazole fungicides have not proved to be very effective against this disease.