Venturia inaequalis Can Initiate Sexual Reproduction Prior to Leaf‐Fall: New Electron Microscopy Evidence

Apple scab, caused by Venturia inaequalis , is a major disease of cultivated apples ( Malus  ×  domestica ) worldwide. Annually, there is one cycle of sexual reproduction (ascospores) in V . inaequalis between strains of two opposite mating types. Sexual reproduction mainly occurs on leaf litter over the winter, and mature ascospores readily infect young leaves the following spring. We do not yet know whether sexual reproduction is initiated before or after leaf‐fall in autumn. The timing of sexual reproduction initiation can significantly impact the exchange of genetic information among pathogen strains. In this study, we used scanning electron microscopy (SEM) to determine the timing of sex initiation, in particular, whether sex can initiate between lesions on the same leaf prior to leaf‐fall. SEM images showed that pseudothecial structures begin to develop before leaf‐fall on leaves with either multiple lesions or a single lesion. Following an overwintering period under laboratory conditions, however, the pseudothecia on leaves with multiple lesions were larger and more mature than those on leaves with a single lesion, where initial pseudothecial structures were present but did not develop further. Thus, the results suggest that the initiation of sexual reproduction in V . inaequalis can occur prior to leaf‐fall in autumn, limiting the opportunities for sexual recombination among strains from different leaves.