Trey Price, Assistant Professor, Field Crop Pathology, Macon Ridge Research Station
Dan Fromme, Associate Professor, Cotton and Corn Specialist, Dean Lee Research Station
Target spot, a fungal foliar disease caused by Corynespora cassicola, has been observed in Franklin, Madison, Tensas, Red River, Natchitoches, and Rapides parishes on many different varieties. I suspect that the disease is at least present in all cotton-producing areas. Target spot is different from other leaf spots that affect cotton (http://louisianacrops.com/2013/08/02/cotton-leaf-spots/) and is usually more aggressive. If your cotton crop is late this year, you should scout for this disease.
The disease starts in the lower canopy, and is usually more severe in rank (tall) cotton with a dense canopy. During initial infection leaves will display a water-soaking appearance or slight discoloration. Shortly afterwards, lesions up to thumbnail-sized are noticeable on the lower leaves (Photo 2). Lesions are tan to brown in color and will usually have concentric rings giving them a “bulls eye-like” appearance (Photo 2). Occasionally, black specks may be observed within lesions, which may be reproductive structures of the pathogen or secondary invaders (Photo 3). Lesions may appear on bracts as well (Photo 4). Only a few spots are visible at first; however, disease can quickly progress causing significant defoliation (Photo 5). I have observed fields that have gone from just a few spots to 50% defoliated within two weeks. The tricky part about spotting this disease is that it is not visible from the turnrow. Fields will appear to be healthy from the road, yet when you enter the field, symptoms are obvious and often moderate.
There are no known target spot-resistant varieties. Canopy management (PGR applications) earlier in the season is critical for reducing plant height and leaf area. Disease severity will likely be much worse in rank cotton with dense canopies. Disease management at this point will be difficult. If a fungicide application is warranted, coverage is key. Apply fungicides within recommended rate ranges and use as much total spray volume as possible (15 gallons/A is ideal). Therefore, ground application is preferred. If ground application is not possible, 5 gallons/A by air is preferred. Three products are currently labeled in cotton: Quadris, Headline, and Twinline. Read and follow label instructions when applying these fungicides to cotton. In other states where target spot is an annual problem, fungicide applications between bloom and 2 weeks after have been the most effective. Fungicide efficacy data for “rescue” or late-season applications is limited. Late applications may slow disease development and prevent further defoliation, but may or may not be economical. In some cases, the disease may be actually helping by increasing air/light and decreasing moisture in the lower canopy reducing boll rot. We currently have trials at the Northeast, Macon Ridge, and Dean Lee Research Stations that have moderate to heavy levels of target spot. Hopefully, these trials will provide useful information for next year. Below are links to more useful information concerning target spot. As always, please do not hesitate to contact your county agent, specialist, or nearest research station for more information.