Abstract: Ultrastructural changes during the process of interactions between three different diseaseresistant, susceptible, moderately resistant and resistant cucumber (Cucumis sativus L. ) cuhivars and Phytophthora melonis Katsura were respectively compared and the associated factors in the observed differences in colonization were determined. During the process of interaction in the susceptible cultivar, both inter- and intracellular growth of hyphae associated with extensive colonization of healthy hyphae and marked alteration of mesophyll cells, such as aggregation of cytoplasm and disintegration of cellular organelle were demonstrated. The general disorganization of host cell was observed in areas directly adjacent to the fungal hyphae and in cells several layers from the nearest visible hyphae. In the moderately resistant cultivar during interaction the host showed limited hypha growth with responses of resistance including 1 ) breakage of plasmodesmata to prevent the host cells to contact the hyphae, 2) increasing the quantity of endoplasmic reticulum and Golgi apparatus so as to increase the quantity of secretory protein, 3) a plasmalemma invagination containing granular material at the site in contact with the intercellular hyphae. The interaction in the resistant cuhivar was characterized as a hypersensitive reaction (HR) which means a rapid necrosis of the infected cells and the cells surrounding the invaded hyphae, resulting in death of the invaded hyphae. Formation of cell wall appositions right at the penetration sites of hyphae and haustorium-like structure was observed. The results indicate that the mechanism of host response to P. melonis invasion was differ- ent between the moderately resistant and the resistant cucumber cultivars.

Key words: Cucumber, Phytophthora melonis, Ultrastructure