Histopathology of leaf infection by Marssonina coronaria on resistant and susceptible apple cultivars
冠盘二胞Marssonina coronaria侵染不同抗性苹果叶片的组织病理学研究

The infection process of Marssonina coronaria was investigated on leaves of susceptible and resistant apple cultivars by means of light and electron microscopy.Infection and colonization by the pathogen could be divided into six stages:spore germination and germ tube formation,appressorium formation,penetration,haustorium formation in mesophyll cells,expansion of mycelium between and in mesophyll cells,and acervulus formation.Cell wall swelling and degradation,plasmolysis,accumulation of starch grains and osmiphilic globules,disruption of grana lamellae and vacuolization of mitochondrion were observed during mycelial expansion in host tissues.Branching rate of germ tubes,formation rate of appressoria and expansion of mycelium were significantly different between susceptible and resistant cultivars;however,the conidial germination rate did not show significant differences.For resistant cultivar,the pathogen’s penetration and formation of subcuticular hyphae,haustoria in mesophyll cells,and acervuli were clearly delayed,and the number of germ tube branches and haustoria in the mesophyll cells were all decreased.Hypha grew more slowly and deposited more electron-dense material in the leaves of resistant cultivars.It is concluded that the resistance of apple cultivars against Marssonina coronaria is related to inhibition of fungal infection,colonization,and mycelial expansion processes in host tissues.