Cytological observation of the infection process of Venturia carpophila on peach leaves

Peach scab caused by Venturia carpophila, is one of the most destructive fungal diseases of peach worldwide, which seriously affects the peach production. Up to date, the infection process and pathogenesis of V. carpophila on peach remain unclear. Here, we present the infection behaviour of V. carpophila at the ultrastructural and cytological levels in peach leaves with combined microscopic investigations (e.g., light microscopy, confocal laser scanning microscopy, scanning electron microscopy and transmission electron microscopy). V. carpophila germinated at the tip of conidia and produced short germ tubes on peach leaf surfaces at 2 days post-inoculation (dpi). At 3 dpi, swollen tips of germ tubes differentiated into appressoria. At 5 dpi, penetration pegs produced by appressoria broke through the cuticle layer, and then differentiated into thick sub-cuticular hyphae in the pectin layer of the epidermal cell walls. At 10 dpi, the sub-cuticular hyphae extensively colonized in the pectin layer. The primary hyphae ramified into secondary hyphae and proliferated along with the incubation. At 15 dpi, the sub-cuticular hyphae divided laterally to form stromata between the cuticle layer and the cellulose layer of the epidermal cells. At 30 dpi, conidiophores developed from the sub-cuticular stromata. Finally, abundant conidiophores and new conidia appeared on leaf surfaces at 40 dpi. These results provide useful information for further understanding the V. carpophila pathogenesis.