VIEW ARTICLE

Ecology and Epidemiology

Effects of Free Moisture and Soybean Growth Stage on Focus Expansion of Rhizoctonia Aerial Blight. X. B. Yang, Former graduate research assistant, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experimental Station, Louisiana State University Agricultural Center, Baton Rouge 70803, Present address: USDA-ARS Foreign Disease-Weed Science Research, Frederick, MD 21701; G. T. Berggren, and J. P. Snow. Professors, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experimental Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Phytopathology 80:497-503. Accepted for publication 15 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-497.

The effect of free moisture and plant growth stage on focus expansion of soybean aerial blight caused by Rhizoctonia solani was quantified with soybeans planted in 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 treatments 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 by 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. Simple linear regressions 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 disease 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 disease 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.