Effects of Metalaxyl onPeronospora tabacinaInfecting Tobacco

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Abstract
Soil drenches of metalaxyl applied to potted tobacco plants did not affect the germination of sporangia and subsequent penetration of leaf epidermal cells by P. tabacina. Development of the pathogen in plants treated with fungicide before inoculation was limited to infection structures within epidermal cells; no intercellular hyphae or haustoria were formed. When metalaxyl was applied by soil drenches to blue mold-infected plants, within 48 h, sporulation by P. tabacina was suppressed 96%, relative to the controls. Sporangiophores that developed on treated plants were incompletely formed and some sporangia showed abnormal morphology. Ultrastructural changes in the fungus were evident 24 h after treatment; intercellular hyphae and haustoria were vacuolated and nuclei were condensed. Forty-eight h after treatment, 52% of the haustoria and 77% of intercellular hyphae were either necrotic or extensively vacuolated. Of the haustoria from infected, untreated plant 94% were encased in a single layer of amorphous, moderately electron-dense appositional material and 6% were encased in 2 layers of appositional material. The inner layer of encasement material was composed of amorphous material; the outer layer consisted of both host wall-like and membranelike portions that stained more densely than the inner layer. The frequency of haustoria encased by bilayered appositions was four times greater in metalaxyl-treated plants than in control plants. Appositional material surrounding haustoria from both control and treated plants stained positively for callose and cellulose but not for lignin.