Effects of Free Moisture and Soybean Growth Stage on Focus Expansion of Rhizoctonia Aerial Blight

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Abstract
The effect of free moisture and plant growth stage on focus expansion of soybean aerial blight caused by Rhizoctonia solani was quantiifed with soybean planted in the polyethylene chambers in a greenhouse. An inoculum source was introduced into the chambers at growth stages V2, V5, or V9 and the chambers subjected to free moisture treatment of cycles of 12 hr/day; cycles of 24 hr/day followed by 2 days of 12 hr/day; cycles of 24 hr/day followed byu 1 day of 12 hr/day, and 24 hr/day of free moisture. Radius of the focus, number of diseased plants/focus, percentage of leaves diseased, and disease focus severity were measured. Simpler linear regression of the disease variables on days after inoculation showed increases in slopes as free moisture increased. Plant growth stage at inoculation also significantly affected the slopes. Models to predict the development of each diseaes variable were developed, with accumulated free moisture hours as the predictor. The radius of a disease focus was a linear function of accumulated free moisture hours. Diseased plants/focus and disease incidence were best fitted with equation Y = 3.142(Bt)2, where t is time and B are slopes and were proportional to plant growth stage. Correlation coefficients among disese incidence, focus radius, and diseased plants/focus ranged from 0.885 to 0.96. However, severity of disease foci was less correlated with the other three disease variables. Viability of the tips of aerial mycelia decreased exponentially against period of dryness, with a half-life period of 8.25 days.