Ultrastructural changes in crowns of peppers resistant and susceptible to Phytophthora capsici

The ultrastructural examination of resistant and susceptible host-pathogen interactions was conducted in the pepper cultivars Ince Sivri-35, PM 217, and PM 702 = CM 334 following inoculation with Phytophthora capsici Leonian. Responses were characterized and compared with healthy non-inoculated controls at 2, 4 and 6 days after inoculation. After 2 days in susceptible interactions, the pathogen grew intercellularly in crowns, whereas in resistant interactions only a few intercellular hyphae were observed penetrating the host cells and forming haustoria. The consistent features of ultrastructural changes in susceptible interactions at 2, 4 and 6 days after inoculation were degeneration of cell organelles and dissolution of host cell walls. The host cell walls were poorly stained and swollen, possibly due to alteration by fungal macerating enzymes. In the resistant cultivar, PM 217, at 2 and 4 days after inoculation, the organelles appeared healthy, but at 6 days after inoculation organelles had degenerated. In another resistant cultivar, PM 702, at 2, 4 and 6 days after inoculation fungal hyphae grew usually intercellularly, and mitochondria and chloroplasts, but not microbodies, were degenerated and papilla had formed. In general, reactions of the resistant cultivars to P. capsici were intercellular, rather than intracellular fungal growth, papilla (callose) formation, resistance of the cell wall to dissolution and microbodies with normal appearing inclusions.