P-649 - Low pH and pydiflumetofen affect mycelial growth of Fusarium virguliforme, causal agent of sudden death syndrome of soybean

Sudden death syndrome (SDS), caused by Fusarium virguliforme (Fv), is a significant soybean disease widely distributed across soybean-producing regions of the U.S.A. and Ontario, Canada. The incidence and severity of SDS are influenced by multiple factors, including host resistance. Field observations have suggested reduced SDS severity in soils with low pH, a pattern that requires validation through experimental data. This in vitro study evaluated the effect of pH on the mycelial growth of Fv in the presence and absence of the active ingredient fungicide pydiflumetofen. Half-strength potato dextrose agar (PDA) media were prepared with and without pydiflumetofen, combined with pH of media ranging from 4.5 to 8.0 in 0.5-unit increments. A 6 mm mycelial plug, taken from the margin of a 10-day-old culture, was placed at the center of each plate. Radial mycelial growth was measured at 5, 10, and 15 days after inoculation (DAI). Results showed that pH 4.5 significantly reduced mycelial growth compared to pH at or above 5.5, in both fungicide-treated and untreated media. At 15 DAI, mycelial growth was significantly less at pH 5.5 and below compared to pH 7 to 8 in the presence of pydiflumetofen. In the absence of fungicide, pH 5.0 and below suppressed growth compared to pH 6.5 and above. Across all pH levels, pydiflumetofen consistently reduced mycelial growth relative to untreated controls. These findings support the hypothesis that lower soil pH may contribute to reduced SDS severity and highlight a potential role in SDS management.