Boyd Padgett, LSU AgCenter
Recently, I have received reports of leaf spot in cotton fields located in several areas of the state. After examining some plants, I confirmed the leaf spot to be caused by Alternaria. Another common leaf spotter is Cercospora. These leaf spots are usually present every year, and it is usually on cotton that is stressed, usually due to a nutrient deficiency (usually potassium). However, last year these diseases were present in fields where cotton WAS NOT STRESSED and had high yield potential. 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. The following information will aid in identification of several leaf spots affecting cotton.
There are several fungi that 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), as well as, in non-stressed cotton. Other less frequently occurring leaf spotters on Louisiana cotton are Ascochyta blight and bacterial blight.
Alternaria Leaf Spot is favored by ambient temperatures of 68 to 86o 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 spots on the leaves and other plant tissue (stems, petioles, and bolls). Mature spots usually have concentric rings that are irregularly circular with straw 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 poor areas of the field. Cotton growing in areas in the field that are deficient in potassium or drought-stressed is at risk to these leaf spots. Yield losses are thought to be minimal because they typically occur in areas of low production.
The risk of these fungi can be reduced by plowing under infected plant debris before planting, maintaining adequate fertility, provide adequate moisture (irrigation), and any practices that maintain plant vigor. Fungicides have been used to reduce these diseases, but they are usually cost-prohibitive.