Histological and cytological investigations of the infection and colonization of apple bark by Valsa mali var. mali

The infection process of Valsa mali var. mali (Vmm), causal agent of canker, in apple (Malus domestica cv. Fuji) tissue was investigated by light and electron microscopy. The germination pattern of conidia was similar on wounded or intact twigs, but there was a time delay on intact bark. On intact bark tissue, conidia swelling could be observed at 16 h post inoculation (hpi) and conidia germinated at 20 hpi, however, no infection was observed. On wounded twig surface, conidia swelled at 6 hpi, and germinated at 16 hpi. Direct penetration of germ tubes was observed by 20 hpi and the pathogen caused canker lesions 4 days post inoculation (dpi) on wounded twigs. Electron microscopy observation of the canker region showed that fungal hyphae mainly spread intercellularly causing severe tissue maceration and cell necrosis. By 15 dpi, hyphae colonized all bark tissues and xylem vessels and generated pycnidia on the canker region. Fungal colonization resulted in marked alterations in host tissues including plasmolysis and degeneration of protoplasts and cell walls. Based on enzyme- and immuno-gold labeling of cell wall components, pectinases were involved in pathogenesis of Vmm of apple tissue.