Premature senescence of cotton (Gossypium hirsutum L.) – Predominantly a potassium disorder caused by an imbalance of source and sink

Abstract

Premature senescence has been occurring with increasing frequency in Australian cotton crops. The work reported in this paper was carried out to characterise the symptoms of the disorder and to make a preliminary examination of its causes. Observations of typical foliar symptoms were made and photographed during the period from 1992 to 1998 in the Australian cotton production areas. On three occasions in 1998, during late boll filling, detailed comparisons were made on ten pairs of adjacent or near adjacent (within 20 cm) plants, with and without symptoms. Plants with severe symptoms had 55–66% heavier total boll mass and their leaves had only about half the potassium (K) and three-quarters the phosphorus (P) concentration of unaffected plants. Hence, affected plants had less leaf K and P to meet the demand of a bigger boll load (e.g. in one sampling unaffected plants had 5.4 mg K g boll^-1 and 2.0 mg P g boll^-1 while affected plants had only 1.3 mg K g boll^-1 and 0.8 mg P g boll^-1). The work concludes that large boll loads, rather than plant disease or poor rooting structure, are likely to constitute the most important factor in predisposing Australian cotton crops to premature senescence. A visual and written description of the symptoms is also provided as an aid to field diagnosis.