Abstract:In order to elucidate the cause of cotton red leaf disease, a series of experiments were conducted to study correlation between sink/source (boll/leaf) ratio and the disease severity. The results showed that the grades of the diseased plants were positively correlated with their boll/leaf ratios significantly. The early-maturing cotton variety had an earlier flowering date, higher boll/leaf ratio and a more serious disease severity than those of the late-maturing one. The air temperature before full squaring date was found more important on cotton growth and the occurrence of red leaf disease than soil moisture. Compared to the normal temperature treatment, increasing air temperature from the 3rd leaf date to full squaring date promoted cotton vegetative growth and reduced the sink/source ratio, thus, the peak stage of the disease was postponed by about 45 days, the average disease index decreased by 72.9% and the average biological yield of cotton increased by 123.8%. Decreased boll/leaf ratio by removing squares and delayed topping could reduce the incidence of red leaf disease. The treatment of removing all squares from cotton plants resulted in the least disease severity. Foliar application of mepiquat chloride increased cotton boll/leaf ratio and was thus favorable to the development of red leaf disease. Those findings indicate that the imbalanced sink/source ratio is the main cause of red leaf disease in cotton. The growth of metabolic source (leaf) before flowering date determines the onset and the severity of the disease.