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Poster: Biology & Disease Mgmt: Chemical Control


Fungicide sensitivity in the wild rice pathogen Bipolaris oryzae
C. CASTELL-MILLER (1), D. Samac (2) (1) PLPA/UMN, U.S.A.; (2) USDA ARS, U.S.A.

In recent years the occurrence of fungal brown spot, caused by Bipolaris oryzae has increased in cultivated wild rice (Zizania palustris) paddies in spite of the use of fungicides. To implement an efficient integrated disease management system, we are exploring whether field isolates have developed reduced sensitivity due to the recurrent use of strobilurin (i.e. inhibitors of the quinone outside site of the mitochondrial cytochrome b, QoI) and azole (i.e., C14 demethylation inhibitors, DMI) fungicides. We conducted in vitro spore germination assays to determine the EC50 of two sets of isolates collected in 2008 and 2015 using 0, 0.10, 0.15, 0.25, 0.35, 0.5, 1.0, 5.0, 10.0, and 100 ug/ml of the active ingredient azoxystrobin (QoI), and 0, 0.1, 0.15, 0.25, 0.5, 1, 5, 10, 35 ug/ml propiconazole (DMI). All isolates tested were found to express the mitochondrial alternative oxidation pathway (AOX) allowing the fungus to germinate at all the doses of azoxystrobin tested. When the AOX was blocked, an average EC50 value of 0.206 ug/ml was obtained for 2015 isolates and 0.158 ug/ml for isolates collected in 2008. Up to now, there is no evidence that isolates have become less sensitive to QoI fungicides. The EC50 obtained for propiconazole for all strains tested until now was lower than the dose-equivalent used in the field. Molecular and in planta tests are underway to test the efficacy of the AOX pathway in spore germination.