Histopathological and ultrastructural observations of Camellia oleifera infected with Colletotrichum fructicola

To clarify the histopathological characteristics of Camellia oleifera that were infected by Colletotrichum fructicola, this study utilized frozen tissue sections, whole-stain clearing and transmission electron microscopy to observe the time of transformation from biotrophic to necrotrophic when Co. fructicola infects leaves, as well as the colonization and expansion of the pathogenic fungus at different stages on the leaves. The Co. fructicola conidia produced germ tubes 2 h post-inoculation, and appressoria began to form 4 h after inoculation. There were two ways that pathogens can use to invade the leaves of Ca. oleifera. One was to directly penetrate the host cuticle, while the other involved invasion from the stomata of the host using germ tubes or hyphae. A few infection vesicles could then be observed in the leaf tissue, and the infection vesicles produce thicker septal primary hyphae after 24 h of infection. When the infection reached the 40 h point, the fungus started to produce secondary hyphae with smaller diameter than that of the primary hyphae and gradually expanded into the adjacent cells. Transmission electron microscopy revealed that when the secondary hyphae entered the mesophyll cells, the contents in the cells were digested and a cavity gradually formed, indicating that the cells had been killed. Thus, the appearance of the secondary hyphae marked the point at which Co. fructicola entered into a destructive necrotrophic stage.