Studies on developmental stages of Venturia nashicola in Asian pear and on the interaction of the fungicidal preparation Clarinet in stages of the life cycle of the pathogen

摘要

In the life cycle of Venturia nashicola, causing scab of Asian pears, especially the stages of colonization and sporulation in the susceptible pear cultivar Suli as well as the interference of the fungicide mixture Clarinet combining the anilinopyrimidine derivative pyrimethanil and the triazole compound fluquinconazole in different stages of the life cycle of the pathogen were studied by means of scanning and transmission electron microscopy. The results showed that conidia of the pathogen germinated and developed appressoria on leaves 1 day after inoculation. After penetration of the leaf cuticle, hyphae developed between cuticle and epidermal cells and beyond hyphae also spread intercellularly into the leaf tissues. Hyphae did not penetrate into the cytoplasm. The ultrastructure of host cells which had been contacted by hyphal cells showed marked alterations: their cytoplasm was disorganized, cell wall layers became indistinct, showed electron transparent areas and some host cells were collapsed. Sporulation of the fungus started with the formation of conidia 12 days after inoculation. Subcuticular stroma produced a group of associated conidiophores (fascicles) that sometimes emerged through stomata. The cytoplasm of hyphal cells was dense with cell organelles and lipid droplets. Concentric bodies were found in the cytoplasm of conidiophores. Preinfectional treatment with the fungicidal mixture Clarinet showed that the product only exhibited a short protective action on preinfectional stages of V. nashicola. On the other hand, postinfectional treatments displayed pronounced ultrastructural changes such as disorganization and increased vacuolization, necrosis and irregularly thickened cell walls in stroma, subcuticular and intercellular fungal cells. Even when Clarinet was applied 12 days after inoculation cytoplasm of subcuticular hyphae and fungal cells in the intercellular space of mesophyll tissue became disorganized and showed large vacuoles. The fungicide mixture severely interfered in development of conidiophores and production of conidia by causing morphological alterations. The specific modes of action and interference of both fungicides in the life cycle of V. nashicola as well as aspects of an antiresistance strategy mediated by application of the mixture are discussed.