Geographic Distribution and Severity of Strobilurin Fungicide Resistance among \u3ci\u3eRhizoctonia solani\u3c/i\u3e on Rice in Southwestern Louisiana

Strobilurin fungicide resistance threatens rice production in southwestern Louisiana. Strobilurins are the often relied upon method of managing sheath blight development due to the lack of strong genetic resistance to this disease in the currently available rice germplasm. However, isolates of Rhizoctonia solani AG 1-IA causing sheath blight were reported resistant to azoxystrobin near Mowata, LA in 2011. To determine the geographic distribution and severity of azoxystrobin resistance, 40 commercial rice fields in southwestern Louisiana were sampled for isolates of R. solani. Sensitivity to pyraclostrobin and trifloxystrobin was also assessed to investigate cross-resistance. A genetic test was used to confirm that all isolates belonged to AG 1-IA. One isolate from each field was tested using a traditional fungicide-amended solid medium assay measuring radial mycelial growth. A second, higher-throughput assay was developed to accommodate all 162 collected isolates. This assay used fungicide-amended liquid medium and measured light absorbance through suspended mycelium. Both assays were used to determine the EC50 (the effective concentration to inhibit mycelial growth halfway between an observed maximum and minimum response of individual dose-response curves) separately for the three strobilurins. Azoxystrobin resistance was found in a greater area than previously reported. Cross-resistance was only weakly supported between azoxystrobin and the other fungicides due to few significant differences in pyraclostrobin and trifloxystrobin responses across the isolate population. However, the sensitivity distributions were bimodal for both azoxystrobin and pyraclostrobin, while unimodal for trifloxystrobin. This clustering of higher-sensitivity and lower-sensitivity individuals at opposite ends of the spectrum suggests the presence or development of isolates resistant to azoxystrobin and pyraclostrobin, respectively. Fungicide resistance management will continue to be vital for rice production, and fungicide rotations utlizing trifloxystrobin may be useful against even azoxystrobin-resistant and possibly pyraclostrobin-resistant isolates of R. solani