Distribution of mating types and diversity in virulence of Didymella rabiei in Israel

The distribution of mating types and diversity in virulence of Didymella rabiei populations were studied in Israel from 1997 to 1999. Forty-one monoconidial D. rabiei isolates from 18 commercial fields distributed among all the chickpea production areas of the country were paired with MAT1-1 and MAT1-2 mating type tester isolates of D. rabiei. Both mating types were found in all chickpea production areas of the country. Of the 18 fields sampled, MAT1-1 was observed in 44%, and MAT1-2 in 88% of the sites. In some sites both mating types were present in close proximity, suggesting that sexual reproduction of the pathogen was feasible. The contribution of sexual reproduction of the fungus to virulence diversity was tested on detached leaves of six differential chickpea cultivars. Nine isolates were derived from different well separated foci (derived from ascospores as inoculum) and eight isolates were derived from a single, well defined infection focus (derived from sister conidia). In the analyses of variance the cultivar x isolate interaction showed no significant (P of F>0.09) effect on disease incidence; the chickpea cultivars differed significantly (P of F<0.0001) in their response to D. rabiei; and the isolate effect was highly significant (P of F = 0.0007) for the conidial population, but not significant (P of F>0.1) among isolates of the ascosporic population. Nevertheless, when comparing a cultivar at a time, the ascosporic and conidial populations did not differ significantly (P of F>0.1) in their virulence diversity. Virulence of 41 isolates collected from the different chickpea fields was tested on detached leaves of four Israeli cultivars that differ in their field response to D. rabiei. The cultivar x isolate interaction showed no significant effect (P of F = 0.95) on disease incidence. The main effects of cultivar and isolate on disease incidence were highly significant (P of F<0.0001). Accordingly, our data do not support the hypothesis that there is pathogenic specialization in the D. rabiei-C. arietinum pathosystem in Israel.