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Relation of Weather Variables and Periodicities of Airborne Spores of Alternaria dauci. W. J. Langenberg, Former Graduate Student, Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1, Present address of senior author: College of Agricultural Technology, Kemptville, Ontario K0G 1J0; J. C. Sutton(2), and T. J. Gillespie(3). (2)Associate Professor, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1; (3)Associate Professor, Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Phytopathology 67:879-883. Accepted for publication 5 January 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-879.

Trends in weather variables, leaf blight, and numbers of airborne spores of Alternaria dauci were monitored in carrot field-plots during two growing seasons. Weather factors during July frequently favored spore production, liberation, and dispersal, but low numbers of airborne spores were detected because there was a scarcity of spore-bearing carrot leaves. The numerous airborne spores which occurred during August coincided with periods favorable for spore production and release and increased blighting of carrot leaves. In September, low numbers of airborne spores frequently were observed, usually after nights when temperatures were too low for abundant sporulation during the period of leaf wetness. Numbers of airborne spores showed a characteristic circadian periodicity. Few spores were dispersed at night when host leaves were wet, the relative humidity (RH) high, the temperatures cool, and the wind speeds low. Numbers of spores increased after 0800 hours when the leaves dried, the RH decreased, and the temperature and wind speed increased. Peak populations occurred at about 1300 hr. Prolonged high winds, periods of rain or persistent leaf wetness, and cool temperatures during dew periods resulted in atypical periodicities and few airborne spores. Limited amounts of blighted leaves, low temperatures, and short dew periods were the major factors restricting spore production.

Additional keywords: epidemiology, Alternaria leaf blight of carrot.