Relationships of Numbers of Spores ofPhytophthora parasiticavar.nicotianaeto Infection and Mortality of Tobacco

Abstract

Sonicated chlamydospores of P. 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.degree. C in a growth room, the percentages of infection were 23, 27, 50, 81, 92, 98 and 1O0 for plants originally exposed to 50, 75, 100, 250, 500, 1000 and 5000 chlamydospores/kg of soil, respectively. Motile zoospores were added to individual plants growing in flooded sand. After 28 days at 25.degree. 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/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 500-100,00O spores/kg 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 h at 25.degree. C and 12 h light on various media and none of the tobacco plants were infected after exposure to infested soil for 50 days at 25.degree. C in a growth room.