INCREASED AGGRESSIVENESS OF ALTERNARIA-MACROSPORA, A CAUSAL AGENT OF LEAF-BLIGHT IN COTTON MONOCULTURE

Consecutive, single-crop cultivation continues to be practiced worldwide as a matter of necessity. The monoculture of cotton now practiced in Israel results in annual outbreaks of leaf blight epidemics. Pathogenic isolates of the causal agent, Alternaria macrospora, were field collected for 6 years and inoculated onto cotton plants under controlled environmental conditions. We were able to demonstrate that the aggressiveness (virulence and fitness) of the pathogenic isolates increased over the years. The modified aggressiveness was manifested as follows: (i) an increase in the number of lesions per leaf; (ii) an increase in the size of the lesions; (iii) a decrease in the time required for symptom expression; (iv) an increase in the viability of the pathogen spores after exposure to a normally lethal temperature; and (v) a decrease in the wetting period required for symptom expression. We conclude that agricultural monoculture can accelerate evolution in the aggressiveness of this plant leaf pathogen.