Isolate-Cultivar Interactions, In Vitro Growth, and Fungicide Sensitivity of Fusarium oxysporum Isolates Causing Seedling Disease on Soybean

Fusarium oxysporum is frequently associated with soybean root rot in the United States. Information about pathogenicity and other phenotypic characteristics of F. oxysporum populations is limited. The objective of the research described herein was to assess phenotypic characteristics of F, oxysporum isolates from soybean, including the interaction between isolates and soybean cultivars, fungal growth characteristics in culture, and sensitivity to fungicides commonly used as seed treatment products, The pathogenicity of 14 isolates was evaluated in rolled-towel and Petri-dish assays using 11 soybean cultivars. In the rolled-towel assay, seed were inoculated with a conidial suspension and disease severity was observed. In the Petri-dish assay, F. oxysporum isolates were grown on 2% water agar and seed were placed on the F. oxysporum colony to observe the symptoms that developed. Cultivars differed in susceptibility to F. oxysporum, and significant (P = 0.0140) isolate cultivar interactions were observed. F. oxysporum isolates differed in radial growth on potato dextrose agar at 25 degrees C. Pyraclostrobin and trifloxystrobin reduced conidial germination with average 50% effective concentration (EC50) of 0.15 and 0.20 lig active ingredient (a.i.)/ml, respectively. Ipconazole reduced fungal growth with average EC50 of 0.23 mu g a.i./ml, whereas fludioxonil was ineffective. Our results illustrate soybean F. oxysporum isolate variability and the potential for their management through cultivar selection or seed treatment.