I have visited several cotton fields with Stemphylium leaf spot. Stemphylium starts off as small brown lesions, and as they enlarge, they can lead to massive premature defoliation.
There are several cotton leaf diseases that have similar visual symptoms as Stemphylium so it is important to correctly diagnose which leaf disease you are dealing with because Stemphylium is actually the secondary problem 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.
This year most of the Stemphylium I have seen is in dry land fields under drought stress. The low soil moisture is reducing the uptake of potash in the plant and as the bolls, which are potash sinks, draw the potash out of the leaves they become susceptible to Stemphylium.
Short season varieties can sometimes be more susceptible to Stemphylium since they often have an intense demand for potash in a short time.
Stemphylium can also be a problem 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.
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This adds to the challenge of taking up soil potassium.The drought may have promoted leaching of potassium under heavy irrigation this year as the dry soils pull the water and potassium through the soil profile more quickly. In recent years there has also been a few cases of Stemphylium documented where high soil magnesium levels competed with potassium for uptake in plant.
Fungicides do not help as potassium deficiency is the primary problem.
Growers should soil test fields with Stemphylium to make sure there is adequate potassium. Petiole testing can 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) in irrigated fields or dry land fields with adequate moisture 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.
I would not make foliar fertilizer applications to fields under drought stress as the plant may not be able to take it up and there is an increase risk of foliar burn.
Eddie McGriff is the Alabama Extension Regional Agent in NE Alabama. He can be contacted at 256-557-2375 or email@example.com.