Adaptation to host (chickpea) genotype by isolates of Didymella rabiei following repeated cycles of infection

Ascochyta blight of chickpea (Cicer arietinum), caused by Didymella rabiei, is an important disease of chickpea world- wide. Under favourable conditions, the pathogen completes several disease cycles in one cropping season. To assess the effect of multiple infection cycles on the aggressiveness of D. rabiei, two isolates of the pathogen were inoculated onto each of three chickpea cultivars, then re-isolated and re-inoc- ulated onto the same cultivar for five generations. Subcultures of each isolate were retained after each generation, and the aggressiveness of each subculture was assessed at the end of the study by measuring blight severity when re-inoculated onto all three chickpea cultivars. The effect of isolate genera- tion was small but significant. Blight severity on a cultivar increased with repeated culturing on that cultivar. This indi- cates that the aggressiveness of the pathogen population in the field is likely to increase on a cultivar as a result of repeat- ed infection of that cultivar. Thus, avoiding repeated cultiva- tion of a cultivar in a field may reduce the potential severity of Ascochyta blight outbreaks.