Innovative assessment of the fungicide sensitivity of the melon powdery mildew fungus, Podosphaera xanthii, on host leaves using electrostatic and digital microscopy techniques

Abstract Powdery mildew (PM) fungi expand their infection of host plants by releasing progeny conidia, which form colonies that become subsequent sources of infection. In this study, 36 commercial fungicides and one biofungicide were sprayed on single 7-day-old colonies of melon PM fungi ( Podosphaera xanthii ) infecting the leaves of host seedlings ( Cucumis melo L.). To quantitatively estimate the effectiveness of fungicides on Japanese melon PM isolates ( P. xanthii KMP-6N) and to establish a fungicide sensitivity evaluation method, asexual conidia produced from single KMP-6N colonies spray-treated with different types of fungicides in a natural environment were collected using a dielectrically polarized insulator plate. Most of the tested fungicides strongly inhibited the production and/or germination of asexual conidia, determined as the amount of attraction to the insulator plate. However, the attracted conidia exhibited high germination rates in the presence of kresoxim-methyl, a quinone outside inhibitor fungicide; fenarimol and triforine, which are demethylation inhibitor fungicides; and thiophanate-methyl, a methyl benzimidazole carbamate fungicide. Moreover, the electrostatically collected conidia of fungicide-resistant KMP-6N colonies developed normally on host leaves previously spray-treated with the same fungicide. The morphologic and cytological characteristics of the conidiophores of single KMP-6N colonies spray-treated with the different fungicides were observed. Abnormal conidiophores were classified into five types based on their characteristics. These results demonstrate that methodologies assessing the number of conidia released from single colonies, conidial germination rates, and colony development in the early fungal growth stages can facilitate the screening of effective fungicides against cucurbit PMs, as well as the estimation of PM fungicide sensitivity.