Foliar Leaf Spots Observed in Cotton
Severe potassium-related leaf spot damage.
Over the past several years we’ve had to deal with several different foliar leaf spots in cotton. Bacterial blight has been observed on an extremely limited basis this year, in two fields to date. But, fungal leaf spots, in the form of the “potash complex†as well as target spot (Corynespora leaf spot) can also be identified in some cotton fields. The potash complex contains foliar leaf spots of cotton that typically occur in combination with a foliar potash deficiency. Alternaria leaf spot, Cercospora leaf spot, and Stemphyllium leaf spot are the three main diseases that can be associated with a foliar potassium deficiency. However, telling them apart from one another is difficult. The potash complex associated fungi typically produce small, oval leaf spots, with a tan center and maroon/purple border. When most mature the center of the leaf spot can fall out giving the appearance of insect feeding. Target spot appears quite different. The name says a lot about the appearance of the lesions associated with this particular disease. Concentric rings are one of the most common elements and are normally observed on the most mature lesions. In addition, more often than not the lesions reside in the bottom of the canopy. Rarely have I observed the lesions progressing up into the top of the canopy.
Management options are limited for either members of the potash complex or even target spot. Limited data, from other states, suggests that a carefully timed fungicide application can prevent yield loss as a results of target spot. Data from MS fungicide trials on the management of target spot is extremely limited. Trials conducted in Stoneville last year were not infected with target spot so little guidance from local trials can be relied upon. However, fungicide applications have not been proven to effectively combat or reduce the likelihood of yield loss as a result of the potash complex.
Diagnostically speaking the single most important event is to make sure that a disease is properly diagnosed at the field level. In some cases leaf samples will need to be submitted to a diagnostic laboratory if a high-powered hand lens isn’t available. Typically I used a 20x hand lens but I also have a 50x with me for some of the diseases (e.g., grey leaf spot of corn) that might require additional magnification to observe fungal structures. The photos in the attached table are similar to those that would be observed with the aid of a 20x hand lens. As always, if you need specific help with a proper plant disease diagnosis, feel free to call any time (Cotton foliar leaf spot complex2).
