P-697 - Unveiling Azoxystrobin sensitivity in Pyricularia oryzae in Louisiana rice production

Rice blast, caused by Pyricularia oryzae, is one of the most damaging diseases affecting rice worldwide. In Louisiana, although genetic resistance has been effective, several popular cultivars are susceptible to blast. Overreliance on fungicides has raised concerns on fungicide-resistant populations. Therefore, we aimed to evaluate the sensitivity status of P. oryzae populations from Louisiana to quinone-outside inhibitor fungicides (QoIs). Thirty isolates of M. oryzae collected from rice fields in Louisiana in three different seasons were grown on PDA for five days. For each P. oryzae isolate, 5 mm mycelial plugs were transferred to PDA amended with salicylhydroxamic acid (SHAM) to inhibit alternate oxidative pathways and four azoxystrobin concentrations (0.001, 0.01,0.1, and 1 µg/mL). The experiment was conducted in a completely randomized design with three replications. Plates were incubated at 25 °C for seven days in the dark, and colony diameters were measured. Mycelia growth inhibition (MGI) was computed relative to untreated controls, and the effective concentrations able to inhibit 50% of fungal development (EC₅₀) were estimated. Our results indicate an average EC₅₀ of 0.05 µg/mL (from 0.01 to 0.25 µg/mL), and mean MGI of 83% (from 52 to 100%) at 1 ppm. Our results indicate that most isolates are sensitive to QoIs, although a variation of up to five-fold was observed in one isolate, possibly indicating reduced sensitivity. This study contributes to a broader understanding of the fungicide sensitivity status of P. oryzae in Louisiana.