AbstractStems of pepper seedlings were inoculated with zoospores ofPhytophthora capsici18 h after a soil‐drench with metalaxyl solution (5 μg ml−1) or water. Infected stem tissues were examined by electron microscopy 24 h after inoculation. Compared with untreated controls, in which the fungal cells were generally normal in shape and ultrastructure, the most conspicuous effects of metalaxyl treatment on the fungal ultrastructure in the stem tissue were an abnormal shrinking of the fungal cell, a separation of the plasma membrane from the hyphal wall, a peculiar invagination and breakdown of the plasma membrane, the presence of vesicles in the invaginated spaces and within the damaged fungal cells, and an indistinct structure of cell organelles. In metalaxyl‐treated stems, an electron‐dense material was apposed in those sites of the host cell wall in most intimate contact with the fungal cell wall, indicating that the deposition of these substances from the host cell walls may function as a plant defence reaction to the fungus, whereas in untreated controls, the dark‐stained host cytoplasm did not aggregate around the sites of fungal contact. The haustorium was encased by the extra‐haustorial matrix in treated stems. These results suggest that metalaxyl treatment not only changes the fine structure ofP. capsici, but may also induce the plant defe
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