Biotrophic Interactions Between a Mycoparasitic Ampelomyces Fungus and Podosphaera xanthii on Fungicide‐Treated Melon Leaves

We investigated the effects of commercial fungicides on the Japanese mycoparasitic Ampelomyces sp. (AM) strain Xs‐q, considered to be a promising biocontrol agent of powdery mildew (PM) fungi, in two settings: (a) in vitro assays; and (b) in vivo assays with a melon PM isolate ( Podosphaera xanthii KMP‐6N) on leaves of host seedlings ( Cucumis melo ‘Earl's Favourite’). When Xs‐q was inoculated onto nutrient media supplemented with one of 37 fungicides, 12 of these, including pyriofenone and mepanipyrim, failed to inhibit its growth and the production of pycnidia. We also used a digital microscope to monitor Xs‐q in fungicide‐resistant KMP‐6N colonies. In the presence of fungicides, Xs‐q hyphae penetrated the KMP‐6N hyphae and grew internally. Under greenhouse conditions (low RH, 30%–55%), at 7 days post‐inoculation (dpi) with Xs‐q spores, the KMP‐6N conidiophores and hyphae atrophied and collapsed. Under growth chamber conditions (high RH, 80%–90%), at approximately 7–10 dpi, Xs‐q hyphae produced intracellular pycnidia in KMP‐6N conidiophores and at approximately 10–14 dpi, Xs‐q released spores from its mature pycnidia. Pycnidia of Xs‐q were formed in KMP‐6N colonies pretreated with the fungicide kresoxim‐methyl. When combined with fungicides, the mycoparasitic fungus inhibited the development of the fungicide‐resistant fungus, which failed to release progeny conidia from the PM colonies, regardless of the relative humidity. Our study is the first to demonstrate biotrophic interactions between an AM strain and a fungicide‐resistant P. xanthii in the presence of a fungicide on host leaves, by combining digital microscopic and electrostatic techniques.