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Variables Associated with Intensity of Alternaria Leaf Spot in Pima Cotton. D. Shtienberg, Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel; Phytopathology 86:123-128. Accepted for publication 23 August 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-123.

The effect of variables associated with the intensity of Alternaria leaf spot (caused by Alternaria macrospora) in Pima cotton (Gossypium barbadense) was investigated over three years in two growing regions in Israel. Alternaria leaf spot intensity was recorded at the initiation of flowering and at advanced stages of boll development. Variables characterizing the field and its surroundings (seven variables), the crop (seven variables), and plant protection actions (five variables) were recorded. The data were analyzed with a general linear model (GLM), and the relative contribution of each variable to disease incidence or disease-induced defoliation was determined. The first GLM explained 72% of the variance in disease incidence at the initiation of flowering (P ? 0.0001). Of the variables examined, only four significantly contributed to the explanation of variance in disease incidence: the previous crop (4.4% of the explained variance, 3.2% of the total variance), the growing season (6.1 and 4.4%), the farm (42.4 and 30.5%), and the interaction between farm and soil type (47.1 and 33.9%). The second GLM explained 62% of the variance of disease-induced defoliation at advanced stages of boll development (P ? 0.0001). Of the variables examined, only four had a significant effect: disease incidence at flowering (9.9 and 6.2%), type of irrigation system (10.1 and 6.3%), region (37.4 and 23.2%), and farm (42.6 and 26.4%).

Additional keywords: integrated pest management, survey.