Morpho-toxicological study of tebuconazole against Valsa mali

Tebuconazole serves as a common agent to prevent and control apple tree Valsa canker. In order to reveal the germicidal mechanism of tebuconazole, the effects of tebuconazole to the germination, hyphal morphology and cell structure of the spores were investigated through microscopic techniques. It was found that tebuconazole could inhibit spore germination, mainly by inhibiting the elongation of germ tubes, causing malformation, thickening, and branching of germ tubes. However, it did not affect the swelling of spores, resulting in the unsuccessful invasion of the spores. The morphology and cell structure of the mycelium also changed significantly when they were treated by the fungicide. The morphological changes of mycelium were mainly swelling, increased hyphal branches and coarsened hyphae, and etc. The cytological changes mainly included increasing and thickening of cell septum, irregular thickening of cell wall, increased and membrane, thickened or irregular contracture of mitochondria, increased nucleus, nucleoli dispersion, severe cell vacuolation, extravasation of protoplasts, cell necrosis, and etc. Meanwhile, progeny mycelium was found in the necrotic hyphae, and it also exhibited abnormal phenomena, including irregular thickening of cell wall, increased number of mitochondria, and cytoplasmic necrosis, and etc. In this study, it was found that tebuconazole could not only affect the germination of Valsa mali, but also cause malformation of germ tubes and hyphae cells, which could significantly inhibit the successful infection of V. mali. The results provide an important theoretical basis for disease prevention by using tebuconazole to spray tree trunks in the field.