Abstract:In order to explore the characteristics of infection and damage of citrus black spot to Citrus grandis ‘Guanximiyou’, the changes in hypodermal cell structure were observed under scanning electron microscopy and transmission electron microscopy (TEM) after artificially inoculated with Phyllosticta citriasiana. After pomelo fruits were infected, the tonoplast in hypodermal cell wrinkled, fractured, and eventually vanished. The starch grains in chloroplast disappeared. The plasmolemma wrinkled, invaginated and partly ruptured. The number of mitochondria increased with clear visible grana. The thylakoid swelled and disintegrated. Afterwards, mitochondria underwent morphological changes, including disappearance of two layers of membrane and vacuolization. However, there were no intercellular or intracellular hyphae, and the cell wall remained structurally stable. Phyllosticta citriasiana infected hypodermal cells of ‘Guanximiyou’ through toxins, which probably attack cell membrane structure and cross over cell wall through plasmodesmata.