APS Homepage

Poster: Biology & Disease Mgmt: Chemical Control


High-throughput strobilurin sensitivity testing of R. solani on Louisiana rice confirms reduced azoxystrobin but continued trifloxystobin sensitivity
A. LUNOS (1), C. Hollier (1), L. Brooks (1), S. Harding (1) (1) Dept. of Plant Pathology & Crop Physiology, LSU AgCenter, U.S.A.

Azoxystrobin resistance in the rice sheath blight pathogen, Rhizoctonia solani AG 1-IA, has compromised a widely effective disease management tool for rice production in southwestern Louisiana. We previously characterized azoxystrobin, pyraclostrobin, and trifloxystrobin sensitivity of 40 R. solani isolates, one from each of 40 rice fields, to identify locations with azoxystrobin resistance and to assess cross-resistance. A high-throughput fungicide sensitivity assay was developed and used to characterize 122 additional isolates from these 40 fields. Serial dilutions of commercial grade fungicides were made in potato dextrose broth and added to 96-well microtiter plates. Fungal mycelial growth was measured based on optical density. Growth curves were generated and used to determine EC50 values, resulting in sensitivity distributions that agree with previous results. The distributions of azoxystrobin and pyraclostrobin EC50 values were bimodal while trifloxystrobin was not. This and internal control isolates provide significant evidence of azoxystrobin resistance, reduced pyraclostrobin sensitivity, and continued sensitivity to trifloxystrobin in the sampled R. solani population. Growers need to prepare for the advent of azoxystrobin-resistant R. solani and could potentially substitute trifloxystrobin in fungicide rotations.