Effects of oxadixyl treatment on the ultrastructure of Phytophthora infestans infecting tomato leaves

Abstract

AbstractOxadixyl is a systemic fungicide controlling diseases caused by Oomycetes. In this ultrastructural study of the mode of action, it was found that more than 40% of 60 observed haustoria of Phytophthora infestans were less than I am in diameter in tomato leaf inner spongy mesophyll tissues previously treated with the fungicide (8 μg ml‐1) and were on average smaller than in untreated tissues. Ultrastructural observations on treated tissues also revealed that dictyosome vesicles were fewer near dictyosomes in the region of the intercellular hyphae which subtended the haustorium and in haustoria, during haustorium development. Fungal nucleoli degenerated and. in contrast to previous observations of the fungitoxic effects, numbers of ribosomes decreased in the cytoplasm, indicating that synthesis of ribosomes was probably inhibited. In addition, fragmentation of the endoplasmic reticulum and increases in the number and size of vacuoles and lomasomes were also observed in treated tissues. These results were considered to be fungistatic effects of the fungicide on the fungus. In some infected plant cells, electron‐dense deposits appeared in the cytosol in the vicinity of the extrahaustorial membrane. Compared with the nature of the related compound, metalaxyl, there appear to be differences between their effects in some aspects, e.g. haustorium development and haustorial encasement formation.