VIEW ARTICLE

Disease Control and Pest Management

Light and Electron Microscopy of Phytophthora Rot in Soybeans Treated With Metalaxyl. P. Stössel, Research Centre, Agriculture Canada, University Sub P.O., London, Ont., Canada N6A 5B7, Present address: Société d'assistance technique pour produits Nestlé S.A., Ch-1814 La Tour-de-Peilz, Switzerland; G. Lazarovits(2), and E. W. B. Ward(3). (2)(3)Research Centre, Agriculture Canada, University Sub P.O., London, Ont., Canada N6A 5B7. Phytopathology 72:106-111. Accepted for publication 11 May 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-106.

Hypocotyls of soybean seedlings were inoculated with suspensions of zoospores of Phytophthora megasperma var. sojae after overnight immersion of the roots in a solution (2 μg/ml) of the systemic fungicide metalaxyl or water. Lesions were excised after 12 or 24 hr of incubation and examined by light and electron microscopy. Treatment with metalaxyl greatly decreased hyphal branching and spread in the host; however, relative numbers of haustoria were not reduced significantly. In contrast to controls, cytoplasm in hyphae in treated seedlings usually did not stain with toluidine blue. Changes in fungal ultrastructure included: increased convolution of the plasmalemma; separation of the plasmalemma from the hyphal wall, leaving protrusions and vesicles in the intervening space; and disintegration of cristae in mitochondria. The cytoplasm near hyphal tips in treated seedlings appeared to remain healthy and the fungus continued to grow slowly between 12 and 24 hr as indicated by increased numbers of penetrated host cells. In both treated and control seedlings, haustoria usually were encased by extrahaustorial matrix only. Structure of wall appositions was not influenced by treatment with metalaxyl. Most host cells in lesions were necrotic regardless of treatment. It appears that metalaxyl acts primarily on the fungus and that necrosis and glyceollin production associated with metalaxyl treatment is a secondary effect.