Submitted to: European Journal of Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/2012
Publication Date: 5/1/2013
Citation: Zhao, H., Han, Q., Wang, J., Gao, X., Xiao, C., Liu, J., Huang, L. 2013. Cytology of infection of apple leaves by Diplocarpon mali. European Journal of Plant Pathology. 136:41-49. Interpretive Summary: Leaf blotch of apple caused by the fungus Diplocarpon mali is a significant production problem as the disease results in premature defoliation, leading to reduction in quantity and quality of apple fruit. In this study, we used light and electron microscopy to investigate how the fungus invade apple leaves . We found that the fungus can use both biotrophic and necrotrophic strategies to establish infections on apple leaves. Understanding the strategies that the fungus uses to establish infection in apple leaves would help develop relevant measures for disease control.
Technical Abstract: Diplocarpon mali, the causal agent of Marssonina leaf blotch of apple, causes severe defoliation during the growing season. Little information is available on the mode of infection and infection process. In this study, the infection strategies of D. mali in apple leaves were investigated using fluorescence and electron microscopy. Conidia attached to leaf surface apparently by mucilage and germinated on both sides of leaves 6 h post-inoculation (hpi). The pathogen penetrated the cuticle by infection pegs formed either in germ tubes or appressoria in 6 hpi, and then formed haustoria in host epidermal and mesophyll cells accompanied by extension of subcuticular and intercellular hyphae. Five days post-inoculation (dpi), the intracellular hyphae were observed. At the same time, the subcuticular hyphal strands (SHS) were produced as a means for fast expansion and reproduction. About 7 dpi, acervuli formed on inoculated leaves. This was the first observation that D. mali formed haustoria and SHS as infection strategies. Our results suggest that D. mali may behavior like a hemibiotroph, which can use both biotrophic and necrotrophic strategies to establish infections on apple leaves.