Ultrastructure at the Host‐Parasite Interface of Phytophthora capsici in Roots and Stems of Capsicum annuum

The infecting hyphae of Phytophthora capsici grew intercellularly in infected tissues of roots and stems of pepper ( Capsicum annuum ). The vascular tissues were not markedly disorganized even when heavily infected. Intercellularly growing hyphae penetrated the host cells by forming haustorium‐like bodies. The consistent features of ultrastructural changes in infected tissues of pepper roots and stems were degeneration of cell organelles and dissolution of host cell walls. The cytoplasm detached from the cell wall aggregated abundantly around some haustorium‐like bodies or the penetration sites of fungal hyphae. The host cell walls were palely stained, thinned and swollen, possibly being biochemically altered by the action of fungal macerating enzymes. Electron‐dense, wall‐like material was apposed on the outer wall of xylem vessel contacted by fungal hyphae. The infecting hyphae were also surrounded by granular, dark‐staining cytoplasm. Characteristics of host cell responses to the invading P. capsici were the deposition of papilla‐like material on host cell walls next to hyphae and the encasement of haustorium‐like bodies with wall appositions.