Target spot (Corynespora cassicola) has been verified on cotton in Eastern North Carolina, and possibly in South Carolina. The geographic extent of the disease and any varietal resistance to Corynespora are not known at this time. Warm humid conditions favor the development of this disease. 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.
We have other leaf spot diseases (Stemphylium, Alternaria, Cercospera) that we commonly see in North Carolina. These diseases are often associated with dry periods and/or potassium deficiency. We have not found any benefit to treat for these diseases. These diseases are normally seen first on the top of the plant compared to target leaf spot which tends to start on the lower portion of the plant.
The decision to make a fungicide application is difficult. Below are some guidelines that may help:
If you need to apply an insecticide, then adding a fungicide makes sense, application costs may be greater than fungicide costs.
Late maturing susceptible varieties may benefit more from a fungicide than early maturing varieties.
Consider applying a higher rate of strobilurin type fungicide (Quadris or Headline) for late maturing varieties. This may prevent the need for another application.
On some varieties this disease will actually aid in defoliation, whereas fungicide applications may make defoliation more difficult. If the disease defoliates the cotton prematurely then one would expect reduced micronaire and yields.
More information and pictures can be found here:
Alabama Cotton Leaf Spot Information