Effects of Alternaria macrospora on the yield components of Pima cotton in Israel

Effects of Alternaria leaf spot (caused by Alternaria macrospora ) on the yield components of Pima cotton ( Gossypium barbadense ) were examined in four field trials, for the lower (< 30–50 cm), middle (30–50 to 70–90 cm) and upper (>70–90 cm) layers of the canopy. The influence of the disease on yield was determined by comparing measurements taken in fungicide-treated plots with those taken in untreated plots. Alternaria -induced defoliation was most severe in the lower canopy layer and gradually decreased towards the upper canopy layers. Significantly fewer bolls (26·0%) were picked in untreated plots than in fungicide-treated plots, but differences in average boll size (reduction of 1·07%) among the treatments were non-significant. The relationship between disease intensity and the loss of bolls was linear. Of the differences in yield between fungicide-treated and untreated plots, 43·5% was observed in the lower canopy layer, 37·6% in the middle canopy layer and only 18·9% in the upper canopy layer.