Mycosystema. 2014, 33(1): 36-46.
The infection process and ultrastructure of Botryosphaeria dothidea (Bd) on apple fruit were observed by light and electron microscopy in detail. The results observed by scanning electron microscopy showed that Bd conidia in the lenticel of apple fruit initially germinated from one or both ends, and the germ tubes directly penetrated through lenticel at 3 hours after inoculation (hai). The infection process normally completed at 9h. Disease symptoms could be observed in 30 days after inoculation (dai) and the pycnidium generated in 45dai. Subsequent observation through light and transmission electron microscopy showed that the filamentous Bd hyphae were septigenous and elongated in host cell gap, cell wall and the space between cell wall and cytomembrane. Several organelles were also observed in Bd hypha cells, such as nucleus, mitochondria and vacuole. Some granules with high electron density distributed radially around the hyphae. Huge changes in apple fruit tissue occurred with the expand of Bd hyphae, such as swollen distortion of cell wall, separation and fracture of middle layer, reduction of cell wall electron density, breakdown of vacuole, plasmolysis, coagulation and deposition of cytoplasm. In some case, the dead cytoplasm was transported from one cell to another by impaired cell wall plasmodesma. Pycnidia were generated by coagulation of hyphae under epidermis. The inwall cells of pycnidium were arranged tightly and a structure assembled by several filaments in parallel was observed. The fat granule around and this specific structure may function together in nutrition transportion and accumulation.
