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Effects of Metalaxyl on Peronospora tabacina Infecting Tobacco. R. N. Trigiano, Former graduate research assistant, North Carolina State University, Raleigh, NC 27695-7616; C. G. Van Dyke(2), and H. W. Spurr, Jr.(3). (2)(3)Associate professor of botany and plant pathology, and professor (USDA) of plant pathology, respectively, North Carolina State University, Raleigh, NC 27695-7616. Phytopathology 74:1034-1040. Accepted for publication 16 April 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-1034.

Soil drenches of metalaxyl applied to potted tobacco plants did not affect the germination of sporangia and subsequent penetration of leaf epidermal cells by Peronospora 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 hr, 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 hr after treatment; intercellular hyphae and haustoria were vacuolated and nuclei were condensed. Forty-eight hours after treatment, 52% of the haustoria and 77% of intercellular hyphae were either necrotic or extensively vacuolated. Ninety-four percent of the haustoria from infected, untreated plants were encased in a single layer of amorphous, moderately electron-dense appositional material and 6% were encased in two layers of appositional material. The inner layer of encasement material was composed of amorphous material, whereas 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.

Additional keywords: Nicotiana tabacum.