The cotton plant’s peak demand for Potassium occurs during boll filling. Potassium is mobile within the plant. If the levels of Potassium in the soil are inadequate or if prevailing weather conditions are not conducive to Potassium uptake from the soil, then there is a relocation of Potassium within the plant from younger leaves to maturing bolls. The younger leaves towards the top of the plant turn red and senesce prematurely. Well shaded leaves and branches remain green. Plants with a heavy boll load are affected first while plants with few bolls are usually unaffected. The plants in the row along the edge of the field are also less affected because of less competition for available soil nutrients.
Premature senescence often develops during extended periods of overcast, cool weather late in the season when bolls are filling. Crops affected by premature senescence are very susceptible to epidemics of Alternaria leaf spot if exposed to further periods of wet weather Premature senescence occurs when potassium is relocated from the upper parts of the plant to maturing bolls.
Rows at the edge of fields remain green (above).
High fruit load increases the demand for potassium and the severity of symptoms (right).
Leaves develop reddening on the upper surface (below).
Single plants that have filled all bolls will senesce prematurely and may defoliate . Alternaria leaf spot, if present, is more severe.