J. E. Ephrath - Department of Field and Vegetable Crops, Faculty of Agriculture, 76100, Rehovot, Israel
A. Marani - Department of Field and Vegetable Crops, Faculty of Agriculture, 76100, Rehovot, Israel. 
D. Shteinberg - Department of Plant Pathology and Microbiology, Faculty of Agriculture, 76100, Rehovot, Israel
A. Dinoor - Department of Plant Pathology and Microbiology, Faculty of Agriculture, 76100, Rehovot, Israel

J. Drieshpoun - Department of Plant Protection, Ministry of Agriculture, Israel

The effect ofAlternaria alternata on gas exchange proceses, on total yield and on yield components in cottonGossypium hirsutum cv. Acala SJ-2, was examined under field conditions. Variations in disease severity levels were achieved by using three fungicide treatments (Fentin Acetate). Disease developed more rapidly on the upper canopy layer (>661 cm height) than on lower ones. Infections were not accompanied by leaf shedding. The main effect of the disease was a reduction on bolls' number. A significant increase in seedcotton and lint yield, as compared to the untreated control, was achieved by applying fungicides. It was found that omitting early and late applications was not followed by significant yield reduction. Alternaria leaf spot reduced photosynthetic rate more and transpiration rate less, than could be explained by the extent of infected leaf area alone.