Disease Severity Gradient of Soybean Downy Mildew from a Small Focus of Infection

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Abstract
The spread of Peronospora manshurica from an initial infection focus was studied in a field plot of a susceptible soybean [Glycine max] cultivar, Williams. Inoculum was introduced in the southwest corner of the plot on potted plants previously infected in the greenhouse with unidentified Illinois [USA] biotype(s) of P. manshurica. Disease severity was rated around arcs at 5 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. he 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. More 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 (0-8 km/h), and aging of soybean leaves resulted in trapping few spores despite favorable temperatures, relative humidity and wind directions.

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