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Development and Evaluation of Guidelines for the Initiation of Chemical Control of Alternaria Leaf Spot in Pima Cotton in Israel. D. Shtienberg, Department of Plant Pathology and Microbiology, Faculty of Agriculture, P.O. Box 12, Rehovot 76100, Israel. . Plant Dis. 76:1164-1168. Accepted for publication 27 May 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1164.

The optimal time to initiate spraying for management of Alternaria macrospora in Pima cotton (Gossypium barbadense) was determined in two field trials in 1990. Sprays applied prior to flowering did not contribute substantially to overall disease suppression irrespective of disease intensity at the time of spraying. Two field trials conducted in 1991 revealed that when disease was present in a field a spraying program initiated on flowering was preferable to one initiated according to the current action threshold (i.e., when an average of one lesion per 10-m row of plants was detected on true leaves), because it required fewer applications. Reduction in the number of applications did not affect control efficiency or decrease yield. Accordingly, the following guidelines are proposed: fungicides should not be applied prior to flowering, irrespective of disease intensity. Starting at the time of flowering, the crop should be inspected periodically and sprayed when disease is present in the field as determined by the action threshold level. By following these guidelines, growers can expect to achieve a saving of one to three spray applications with no substantial increase in disease development.

Keyword(s): epidemiology, yield loss.