Originally published: August 2, 2013

Recently, there have been numerous reports of leaf spot in cotton. Individuals who frequent cotton fields should be monitoring for this disease. Currently, an automatic fungicide application is not recommended. The decision to use a fungicide should be made on a field-by-field basis and should take into account the severity of the disease, growth stage of the crop and yield potential. Late season (late flowering/open bolls) fungicide applications may be effective for arresting disease development, but may have limited impact on yield preservation. Fungicides have not been evaluated in LSU AgCenter tests when disease pressure was severe and cotton was just beginning to flower; therefore, yield loss data is limited. The following information will aid in identification of several leaf spots affecting cotton.

Several fungi affect foliage, petioles, stems, bracts and bolls. The two most common leaf spot diseases are Alternaria and Cercospora (see figures). This is not the same Cercospora that infects soybean. These leaf spots are usually found on stressed cotton (fertility or drought stress). Other less frequently occurring leaf spot diseases on Louisiana cotton are Ascochyta blight, Stemphylium leaf spot, target leaf spot and bacterial blight.

Alternaria Leaf Spot is favored by ambient temperatures of 68-86 degrees F and high relative humidity. Infection can occur early season on the cotyledons, but typically symptoms are usually evident late season. Spots begin as pale green to yellow spots on the leaves and other plant tissue (stems, petioles and bolls). Mature spots usually have concentric rings that are irregularly circular with straw-colored or reddish brown centers. During humid conditions the centers may appear sooty (these are the spores of the fungus). In some cases the centers may deteriorate and fall out giving the leaf a shot-hole appearance. The fungus overwinters in infected plant debris from the previous season. Other sources of inoculum are adjacent cotton and weeds.

Cercospora Leaf Spot is favored by the same conditions needed for Alternaria leaf spot to develop. Symptoms are similar to those caused by Alternaria. The initial symptoms are usually present late season and occur as small red lesions on the leaves. As the lesions mature, the centers are tan with a red margin and concentric rings can be present. The fungus can overwinter in infected plant debris.

In most cases Alternaria and Cercospora leaf spots are present on cotton growing in less-fertile areas of the field. Cotton growing in areas deficient in potassium or drought-stressed is at risk to these leaf spots.Alternaria, Cercospora and Stemphylium may form a complex and may be observed within the same field. Generally, yield losses are thought to be minimal because they typically occur in areas of low production. However, under optimal conditions (susceptible variety, drought, potassium deficiency, high relative humidity), these pathogens may heavily infest cotton fields.

The risk of these diseases can be reduced by plowing under infected plant debris before planting, planting a tolerant variety if available, maintaining adequate fertility, providing adequate moisture (irrigation), and conducting any practices that maintain plant vigor. Fungicides have been used to reduce these diseases, but they are usually cost-prohibitive.