Effects of blue and/or red light on conidial release, conidiophore morphology, and colony development in strawberry powdery mildew

In this study, we investigated the effects of blue and/or red light-emitting diode (LED) irradiation on conidial release, conidiophore morphology, and colony development of strawberry powdery mildew (PM) fungi ( Podosphaera aphanis var. aphanis , Japanese isolate KSP-7N) on strawberry leaves ( Fragaria × ananassa Duchesne ex Rozier) using digital microscopy and electrostatic techniques. Asexual conidia released from single PM colonies subjected to blue, red, or combined blue and red LED light, as well as greenhouse, growth chamber, or dark conditions, were collected electrostatically. A large conidial release was observed under blue light, combined blue and red light, greenhouse, and growth chamber conditions; only a few conidia were collected from colonies irradiated with red light or kept in darkness. After electrostatic collection, the number of conidia attached to conidiophores was determined. The maximum conidial chain length on conidiophores was six conidia under blue light, compared with a maximum of 15 conidia under red light. Consequently, conidiophores were longer and bore more conidia under red light and darkness than under other light treatments. These morphological differences corresponded with colony structure: colonies exposed to blue light were flat; those under red light exhibited a stacked, more three-dimensional morphology. Notably, conidiophores with chains of fewer than six conidia produced immature conidia that failed to germinate on host leaves, whereas chains exceeding seven conidia produced mature conidia capable of germination. Germinated conidia successfully infected host leaves, resulting in vigorous hyphal elongation and the subsequent formation of new conidiophores. Overall, this study demonstrates that conidiophore morphology and conidial release in the strawberry PM fungus are significantly influenced by the spectral quality of visible light.