Fusarium oxysporum (Fo) and Fusarium graminearum (Fg) are important components of the Fusarium root rot complex in soybean. Fo is one of the species most frequently associated with soybean root rot, and Fg isolates that colonize wheat and maize have been found to be highly pathogenic on soybean, in the United States. Fo and Fg cause seed decay, damping-off, crown and root rots and pod blight.

The goal of this research was to characterize the biology of Fo and Fg and determine their role as soybean seedling pathogens in the Fusarium root rot complex. The objectives were to: i) assess the phenotypic characteristics of Fo isolates from soybean, including the interaction between Fo isolates and soybean cultivars, growth characteristics in culture, and sensitivity to fungicides, ii) evaluate the effect of pH and temperature on the development of soybean root rot caused by Fo, and iii) determine the impact of soil texture, soil pH and soil water content on seedling disease caused by Fg.

For objective 1, pathogenicity of fourteen Fo isolates was evaluated on eleven soybean cultivars in rolled-towel and petri-dish assays. Our study revealed that cultivars differed in susceptibility to Fo, and there were significant isolate à  à  cultivar interactions. These results suggests that the pattern of resistance or susceptibility for each soybean cultivar differs among isolates. In addition, soybean cultivars differed in susceptibility to Fo, illustrating the variability among Fo isolates from soybean and the potential for their management through cultivar selection.

Fo isolates also differed in radial growth on PDA. Pyraclostrobin and trifloxystrobin effectively reduced conidial germination, and ipconazole effectively reduced fungal growth, but fludioxonil was ineffective against Fo fungal growth. These results illustrate the variability among Fo isolates from soybean and the potential for their management through cultivar selection or seed treatment.

For objective 2, a growth chamber study was performed to assess the effects of pH and temperature on Fo fungal growth and seedling disease. Fo isolates were grown on artificial culture media at four pH levels (4, 5, 6, 7, 8), and incubated at four temperatures (15 20, 25, or 30ᵒC). In a rolled-towel assay, seeds were inoculated with a suspension of a pathogenic or a non-pathogenic Fo isolate. We found that Fo isolates had the greatest radial growth at pH 6 and 25ᵒC, and caused the most severe root rot at pH 6 and 25ᵒC. In addition, a Gaussian model was performed to estimate optimal pH and temperature for fungal growth and disease severity. Optimal conditions estimated using a Gaussian model were pH 6.4 at 27.4 ᵒC for maximal fungal growth, and pH 5.9 at 30ᵒC for maximal root rot severity. These results indicate that optimal pH and temperature conditions for Fo growth are similar to optimal conditions for infection and disease in soybean seedlings, and suggest that Fo may be a more important seedling pathogen when soybeans are planted later, under warm conditions.

For objective 3, we tested the effect of four artificial soil textures (sand, loamy sand, sandy loam and loam), two levels of soil pH (6 and 8), and three levels of soil moisture (permanent wilting point, field capacity and saturation) on root rot of soybean caused by Fg. We found a significant interaction between soil moisture and soil texture for root rot. The greatest severity (~70%) was observed at pH 6 and permanent wilting point in sandy loam soils. In contrast, pot saturation resulted in the lowest levels of disease in sandy loam and loam soils (11.6 and 10.8%, respectively). Percentages of reduction on seedling growth parameters relative to the non-inoculated control, such as root length, foliar area, shoot and root dry weights and root tips were significantly higher in sandy loam soils. In contrast, there were no relative growth reductions in sandy soils. Our results suggest root disease caused by Fg increases in water-stressed plants, resulting in detrimental effects on plant development.