An ultrastructural study of the effect of metalaxyl on Phytophthora capsici infected stems of Capsicum annuum

Abstract

Stems of pepper seedlings were inoculated with zoospores of Phytophthora capsici 18 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 of P. capsici, but may also induce the plant defence reaction.

Light and Electron Microscopy Studies on the Infection of a Wild-type and Metalaxyl-resistant Isolate of Phytophthora sojae in Soybean Hypocotyls

“…sojae (Stössel et al. 1982) and pepper stems infected with P. capsici (Hwang et al 1990). These changes especially comprise: increased convolution of the plasmalemma, separation of the plasmalemma from the hyphal wall, accumulation of vesicles in invaginated spaces and in the damaged hyphal cells.…”

“…and their host plants have only been reported by Stössel et al. (1982) and Hwang et al (1990). However, no corresponding studies on the influence of metalaxyl during the infection process of fungicide‐resistant isolates of Phytophthora spp.…”

Curative and Eradicative Effects of Fungicides

“…Cell walls of Phytophthora infestans hyphae became thickened, and degeneration of haustoria was recorded (Jing & Grossmann, 1991). Abnormal shrinking of cells, separation of membrane from the hyphal wall, breakdown of cell membrane and indistinct structure of cell organelles were noted in Phytophthora capsici mycelium inside the pepper seedlings after treatment with metalaxyl (Hwang et al, 1990). Five days after metalaxyl application on pea plants infected with downy mildew, Peronospora pisi, haustoria of the parasite were deformed, and vacuoles were visible in the cytoplasm of haustorial cells (Hickey & Coffey, 1980).…”