I have confirmed two cotton fields recently with stemphylium leaf spot. Stemphylium starts off as small brown lesions, and as they enlarge, they lead to massive premature defoliation.
Stemphylium is actually the secondary problem as it is brought on by potassium deficiency. Potassium adds strength to the leaf cells, and the lack of potassium in the leaf cells makes them weak and susceptible to secondary fungal infections such as stemphylium.
Most of the cases of stemphylium I have seen (both this year and in the past) have been on full-season varieties under irrigation. Stemphylium leaf spots start to appear around the fourth week of bloom with a heavy fruit/boll load which correspond to a heavy demand for potassium. The roots of the plant also start to decline at this time due to competition for carbohydrates by developing bolls. This adds to the challenge of taking up soil potassium at this time.
The drought may have promoted leaching of potassium under heavy irrigation this year as the dry soils pulled the water and potassium through the soil more quickly.
In the past, some cases of stemphylium have occurred in dryland fields where low soil moisture was suspected of reducing uptake of potassium. Some cases have occurred on short-season varieties where there was intense demand for potassium in a short period of time. A few cases occurred with high soil magnesium levels which competed with potassium for uptake in the plant.
Fungicide sprays do not help as potassium deficiency is the primary problem. Petiole testing could help avoid this problem since it will help detect potassium deficiencies up to two weeks in advance, especially as the crop moves toward peak bloom.
If potash deficiencies are caught early enough (before the fourth week of bloom) foliar potash may lessen the damage. Unfortunately, if potash deficiency is detected late in the growing season (sixth week of bloom) foliar potash sprays will likely not be a benefit.