VIEW ARTICLE

Ecology and Epidemiology

Interactive Effects of the Dark Period, Humid Period, Temperature, and Light on Sporulation of Peronospora destructor. P. D. Hildebrand, Department of Environmental Biology, Ontario Agricultural College, University of Guelph, Guelph, Ontario, Canada N1G 2W1, Present address: Agriculture Canada, Research Station, Kentville, Nova Scotia, Canada , B4N 1J5; J. C. Sutton, Department of Environmental Biology, Ontario Agricultural College, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Phytopathology 74:1444-1449. Accepted for publication 6 July 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1444.

Onion plants infected with Peronospora destructor were exposed in growth cabinets to light for 14 hr then to various conditions of darkness, high relative humidity (RH), and temperature. The end of the light period was designated as 2000 hours. The pathogen sporulated in the range of 6- 22 C when the humid period began at 2200 hours, but not at 6, 10, 14, and 22 C when humid periods at these respective temperatures began at 2400, 0100, 0300, or 0300 hours or later. When conditions favored sporulation, numbers of spores produced increased as the humid period was extended from 0400 to 0700 hours. Deferred onset of high humidity during the dark period delayed development of sporophores and spores, and usually reduced the numbers of spores produced. The optimum temperature for sporulation increased from 10 to 18 C when onset of high humidity was changed from 2200 hours to 0300 hours. Spores matured 2 hr earlier at 14 C when the dark period was started at 1800 hours instead of 2000 hours. Interruption of the humid period by RH 80- 90% for 0.5 hr at 0100- 0400 hours stopped sporulation. Exposure of young sporophores to light at 0100- 0400 hours resulted in production of deformed spores.