Inhibitory Effects of Blue Light-Emitting Diode Irradiation on Podosphaera xanthii Conidial Release and Infection of Melon Seedlings

Powdery mildew fungi infect plant leaves, reducing the yield of infected melon plants. Therefore, an eco-friendly method of controlling powdery mildew in melon plants needs to be developed. A previous study described how the morphological characteristics of the conidiophores of the melon powdery mildew fungus Podosphaera xanthii Pollacci (designated KMP-6N) grown under greenhouse (natural) conditions and red light-emitting diode (LED) irradiation differed from those grown under growth chamber conditions and blue LED irradiation. In the present study, conidiophores with unconstricted conidia under blue light were collected and inoculated onto host leaves through micromanipulation; the unconstricted conidia germinated and infected the leaves, producing vigorously elongated hyphae. The number of conidia collected, the initial times of conidial release from single colonies, and the number of conidia remaining in chains on conidiophores were examined with electrostatic techniques. Under red light, the number of collected conidia gradually increased with the light irradiation period. The initial conidial release occurred between 2 to 4 h; the number of conidia remaining on the conidiophores gradually decreased and, eventually, the conidiophore lengths became shorter. In contrast, under blue light, few conidia were collected at any given time; the number of conidia on the conidiophores gradually increased and, eventually, the conidiophore lengths became longer. Next, the effects of red and blue light on the spread of powdery mildew infection by placing a KMP-6N-infected melon seedling at the centre of a tray containing healthy melon seedlings were examined. Almost all healthy seedlings caused powdery mildew symptoms at ca. 21 days after red light irradiation, whereas only healthy seedlings near the infected seedlings showed symptoms after blue light irradiation. Thus, the spread of melon powdery mildew infection clearly differed between red and blue light irradiation. This is the first report describing the effects of red and blue light on the spread of P. xanthii infection from a single infected seedling to healthy host seedlings; their results provide insight into the ecological mechanisms of powdery mildew conidial scatter from conidiophores.