Mating system and role of pycnidiospores in biology of Polystigma amygdalinum, the causal agent of almond red leaf blotch

Polystigma amygdalinum is a serious leaf pathogen of almonds. Ascospores are believed to be the only inoculum of this pathogen. Ascocarp initials develop in infected leaves in contact with, or in close proximity to, filimentous spore-bearing bodies, which are the first to appear. While the aseptate filiform spores (scolecospores), are suspected of involvement in the sexual cycle of P. amygdalinum, there is no direct evidence of this. In order to study the role of the scolecospores, infected almond leaves, collected during the period from first appearance of symptoms to perithecium development, were sectioned by freezing microtome and the developmental anatomy was examined. Since ascogonia were not evident prior to the appearance of scolecospores, it was concluded that these were part of the sexual cycle of P. amygdalinum. This is supported by observations that perithecium initials always developed beside scolecospore fruit bodies, scolecospores failed to germinate in vitro, and almond leaves inoculated with them never became infected. We conclude that the filiform spores are spermatia essential to the sexual process and not asexual spores. Moreover, P. amygdalinum is likely to be homothallic, because single lesions in leaves infected by P. amygdalinum, assumed to have arisen from single ascospore infections, produced perithecia as abundantly as occurred in coalescing lesions caused by multiple ascospore infections.