Fungicide Sensitivity of Gaeumannomyces Species Associated with Take-All Root Rot of St. Augustinegrass in Texas

Gaeumannomyces species are soilborne fungi associated with take-all root rot (TARR) of St. Augustinegrass ( Stenotaphrum secundatum) and other warm-season turfgrasses. Penetrant fungicides, such as demethylation inhibitors (DMIs) and quinone outside inhibitors (QoIs), have been labeled and used for TARR management. Reports of turfgrass pathogens resistant to these fungicide groups are increasing. However, there is no fungicide sensitivity information for Gaeumannomyces species in turfgrass. In this study, sensitivities to QoI fungicide azoxystrobin and DMI fungicide tebuconazole were determined for Gaeumannomyces species ( G. graminicola, G. arxii, and G. floridanus) isolated from St. Augustinegrass in Texas. A discriminatory concentration of 0.1 µg a.i. ml−1 was deemed suitable for rapid screening and accurate prediction of the effective concentration of a fungicide that inhibits fungal growth by 50% (EC50) for tebuconazole and azoxystrobin. The EC50 values of Gaeumannomyces species ranged from 0.021 to 0.1 µg ml−1 for tebuconazole (with the exception of one isolate having an EC50 value = 0.296 µg ml−1) and from 0.014 to 0.399 µg ml−1 for azoxystrobin. To our knowledge, this is the first study to investigate fungicide sensitivities of Gaeumannomyces species associated with TARR in St. Augustinegrass to fungicides in the DMI and QoI chemical groups. Most isolates tested in this study can be considered sensitive to tebuconazole and azoxystrobin. This indicates that the fungicide resistance of Gaeumannomyces species from St. Augustinegrass in Texas lawns might not be prevalent at the moment. Future field validation needs to confirm in vitro sensitivities and field efficacy of tebuconazole and azoxystrobin.