Analysis of epidemics of Rhizoctonia aerial blight of soybean in Louisiana

Frequency of primary infections, disease focus establishment, expansion of disease foci, and seasonal progress of soybean aerial blight caused by Rhizoctonia solani were studied from 1986 to 1988. Primary infections from natural inoculum were quantified by counting and then removing diseased leaves at 3- to 5-day intervals. High numbers of primary infections were found early in the season. The correlation between average number of primary infections per day and average daily rainfall was highly significant. The relationship between plant growth stage and the establishment and expansion of disease foci was studied in fields without a history of aerial blight by periodically inoculating 10 leaves at the center of each of 10 subplots. Establishment ratios of disease foci were low before soybean stages V7 and V4 in 1986 and 1988, respectively. In 1987, high establishment ratios in early plant growth stages were associated with heavy and frequent rainfall. Significant expansion of disease foci occurred only after canopy closure in all three seasons. Development of disease was correlated with rainfall and soybean growth stage (R2 ranges from 0.73 to 0.96 for different year X row spacing combinations). Expansion of disease foci was predicted using the number of rain days after inoculation and the soybean growth stages at inoculation. Disease in fields with natural inoculum progressed erratically, and correlation coefficients among disease incidences rated at different soybean growth stages were significantly reduced as the time span increased. The epidemiology of aerial blight may be divided into two phases, one before and one after canopy closure. The first phase is soilborne and determines the number of potential disease foci in the crop canopy. The second phase is leafborne and is important to the expansion of disease foci