Analysis of conidiogenesis and lifelong conidial production from single conidiophores of Podosphaera aphanis on strawberry leaves using digital microscopic and electrostatic techniques

In this study, strawberry plants ( Fragaria × ananassa Duchesne ex Rozier) grown via elevated cultivation in a greenhouse were heavily infested with powdery mildew. We isolated the powdery mildew fungus from strawberry leaves and identified the isolate as Podosphaera aphanis (Wallroth) U. Braun & S. Takamatsu var. aphanis KSP-7 N, based on its morphological characteristics and sequencing of the ribosomal DNA internal transcribed spacer region. The host range of KSP-7 N was assessed by inoculating 17 plant families comprising 49 species (a total of 110 cultivars) with conidia. The fungus caused severe powdery mildew on the tested strawberry plants (commercial, wild, and false strawberries), produced scattered conidia on conidiophores, and formed completely catenated conidia within approximately 27 h from conidiophore erection to the first release of mature conidia. Six conidia were produced on each conidiophore; only those at the apex reached maturity. The cycles of conidial release were repeated 17 to 21 times at intervals of approximately 6 h, during gradual upward elongation of the conidiophores. At the final stage, conidia were released without growth or septation of the generative cells. Conidiophores produced an average of 38 conidia during a 96-h period. These results will help to clarify the modes of conidiogenesis, the lifetime of conidiophores, and the production of conidia on a conidiophore among powdery mildew pathogens isolated from strawberry leaves.