Over the past few weeks we’ve observed several cotton fields with Stemphylium leaf spot. The ultimate cause is a depletion of potassium (K) in leaf tissues.
Potassium is rapidly exported from leaves and stems to fill developing bolls, and as K becomes scarce in leaves, plants become vulnerable to fungi such as Stemphylium and, less commonly, other secondary pathogens such Alternaria and Cercospera.
Potassium adds strength to leaf cells and the lack of potassium in leaf tissues makes them weak and susceptible to these fungal infections. These secondary invaders initially appear as small brown lesions, and as they enlarge, they can lead to massive premature defoliation.
Incidence of Stemphylium is most common in dry land fields under drought stress. Lack of soil moisture reduces uptake of K during vegetative growth and early flowering, and thus the reserves in stem and leaves are insufficient to sustain the leaves during boll fill.
Adding to the challenge is that root activity tends to decline as plant resources are directed towards reproduction. Short season varieties can sometimes be more susceptible to Stemphylium since they often have an intense demand for K in a short time.
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These secondary pathogens can also be a problem under irrigation, particularly in the latter stages of flowering (4th week or later) in the presence of a heavy boll load and if heavy rainfall or watering has promoted K leaching. In recent years there have been a few cases of Stemphylium attributed to high soil magnesium levels competing with K for uptake in plant.
Note, foliar fungicides do not help with these secondary invaders. Potassium deficiency is the issue. Other foliar diseases such as target spot and areolate mildew may respond to timely applications of fungicides, but they are ineffective for Stemphylium and related pathogens.