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Effects of Temperature, Free Moisture, and Relative Humidity on the Occurrence of Walnut Anthracnose. W. M. Black, Formerly Graduate Research Assistant, Department of Plant Pathology, University of Illinois, Urbana, IL 61801, Present address of senior author: ChemScape Div., ChemLawn Corp., Lombard, IL 60148; Dan Neely, Plant Pathologist, Illinois Natural History Survey and Professor of Plant Pathology, University of Illinois, Urbana. Phytopathology 68:1054-1056. Accepted for publication 25 January 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1054.

Black walnut seedlings grown in a greenhouse were artifically inoculated by brushing an aqueous suspension of Marssonina juglandis conidia (perfect stage: Gnomonia leptostyla) onto the foliage. Little infection occurred at 27 C, none at 32 C, and symptom appearance was delayed at 15 and 10 C. At the optimum temperature (approximately 21 C) more than 6 hr with free moisture on the leaflets were required for the development of significant amounts of infection. Longer wetting periods were required at lower temperatures. No infection occurred at relative humidity levels below 98% (2%). Conidia survived for 2 wk on the leaflet surface, infecting the seedlings when moisture became available.

Additional keywords: epidemiology.