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Ecology and Epidemiology

Relationships of Numbers of Spores of Phytophthora parasitica var. nicotianae to Infection and Mortality of Tobacco. M. E. Kannwischer, Plant Pathology Department, University of Florida, Gainesville 32611, Present address of senior author: Agronomy Department, University of Florida, Gainesville 32611; D. J. Mitchell, Plant Pathology Department, University of Florida, Gainesville 32611. Phytopathology 71:69-73. Accepted for publication 8 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-69.

Sonicated chlamydospores of Phytophthora parasitica var. nicotianae were mixed into autoclaved Astatula sand to achieve defined inoculum densities. Individual 4-wk-old susceptible tobacco plants were exposed to infested soil. After 50 days at 25 C in a growth room, the percentages of infection were 23, 27, 50, 81, 92, 98, and 100 for plants originally exposed to 50, 75, 100, 250, 500, 1,000, and 5,000 chlamydospores per kilogram of soil, respectively. Motile zoospores were added to individual plants growing in flooded sand. After 28 days at 25 C in a growth room, the percentages of infection were 10, 22, 35, 55, 82, 93, and 100 for plants originally exposed to 5, 15, 25, 50, 100, 200, and 300 zoospores per plant, respectively. Sonicated oospores that were frozen and either left untreated or treated with cellulase or Gluculase (glucuronidase and sulfatase) enzymes were mixed into autoclaved soil at densities of 500 to 100,000 spores per kilogram of soil. The oospores were plated on various media to determine germination percentages. None of the enzyme-treated or untreated oospores germinated after 96–128 hr at 25 C and 12 hr light on various media and none of the tobacco plants were infected after exposure to infested soil for 50 days at 25 C in a growth room.