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Infection and Colonization of Tobacco Callus by Peronospora tabacina. R. N. Trigiano, Graduate research assistant, Departments of Botany and Plant Pathology, North Carolina State University, Raleigh 27650; C. G. Van Dyke(2), H. W. Spurr, Jr.(3), and D. J. Gray(4). (2)(3)(4)Associate professor of Botany and Plant Pathology, professor (USDA) of Plant Pathology, and graduate research assistant, respectively, Departments of Botany and Plant Pathology, North Carolina State University, Raleigh 27650, (4)Present address: Research associate, Botany Department, University of Tennessee, Knoxville 37916. Phytopathology 74:280-285. Accepted for publication 14 September 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-280.

The infection and colonization of susceptible tobacco callus inoculated with sporangia of Peronospora tabacina was studied by using light-, scanning-, and transmission-electron microscopy. Germ tubes typically exceeded 50 μm in length and often grew in contact with the host cell wall for considerable distances. Prior to cellular penetration, the apex of most germ tubes expanded to form a well-defined, bulbous, appressoriumlike structure capable of producing intercellular hyphae, intracellular hyphae, and haustoria. Host cells were invaded by either intracellular hyphae or haustoria. Intracellular hyphae, 8- 9 μm in diameter, exhibited indeterminate growth and were surrounded by an electron-dense matrix. Haustoria, frequently observed in the same host cell as intracellular hyphae, grew determinately and were ~3 μm in diameter. Haustoria were also surrounded by an electron-dense matrix but, in addition, were encased in calloselike material. Haustoria in intact leaf cells were similar in dimensions and morphology to haustoria in callus cells. A scanty mycelium developed on the surface of the callus cultures after 10 days, but then declined so that after 20 days only a few hyphal cells remained. Sporangiophores and sporangia were not produced.

Additional keywords: blue mold, Nicotiana tabacum, ultrastructure.