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Disease Severity Gradient of Soybean Downy Mildew from a Small Focus of Infection. S. M. Lim, Research plant pathologist, Federal Research, Science and Education Administration, U.S. Department of Agriculture, and Department of Plant Pathology, University of Illinois, Urbana, IL 61801; Phytopathology 68:1774-1778. Accepted for publication 23 June 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1774.

The spread of Peronospora manshurica from an initial infection focus was studied in a field plot of a susceptible soybean cultivar, Williams. Inoculum was introduced in the southwest corner of the plot on potted plants previously infected in the greenhouse with unidentified Illinois biotype(s) of P. manshurica. Disease severity was rated around arcs at five distances from the focus of infection. The regression line slope for disease severity gradients at 30 and 44 days after inoculation agreed with the slope of a theoretical line from a point source of inoculum. The slopes of gradient estimates made after 58 days were smaller than earlier ones, indicating a more uniform level of disease throughout the plot. These later gradients also were higher above the X-axis, indicating that disease was more severe. Mean of the proportion of disease in the plot was 0.21 by 58 days after inoculation. Disease increased more rapidly at the more distant areas from the infection focus, where initial severity was lower. Spores were trapped at least once a week during the period of disease ratings. The most spores were trapped during periods of no precipitation. Frequent rainfalls, low wind speeds (08 km/ hr), and aging of soybean leaves resulted in trapping few spores despite favorable temperatures, relative humidity, and wind directions.

Additional keywords: disease intensity, Glycine max, spore dispersal.