VARIABLES ASSOCIATED WITH INTENSITY OF ALTERNARIA LEAF-SPOT IN PIMA COTTON

The effect of variables associated with the intensity of Alternaria le af spot (caused by Alternaria macrospora) in Pima cotton (Gossypium ba rbadense) 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 boil development. Variables cha racterizing 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), a nd 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 less than or equal to 0.0001). Of the variables examined, only four significantly contributed to the explanation of variance in disease i ncidence: the previous crop (4.4% of the explained variance, 3.2% of t he 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 3 3.9%). The second GLM explained 62% of the variance of disease-induced defoliation at advanced stages of boil development (P less than or eq ual to 0.0001). Of the variables examined, only four had a significant effect: disease incidence at flowering (9.9 and 6.2%), type of irriga tion system (10.1 and 6.3%), region (37.4 and 23.2%), and farm (42.6 a nd 26.4%).