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Title: Sensitivity of Bipolaris oryzae isolates pathogenic on cultivated wild rice to the quinone outside inhibitor azoxystrobin

item CASTELL-MILLER, CLAUDIA - University Of Minnesota
item Samac, Deborah - Debby

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2019
Publication Date: 8/2/2019
Citation: Castell-Miller, C.V., Samac, D.A. 2019. Sensitivity of Bipolaris oryzae isolates pathogenic on cultivated wild rice to the quinone outside inhibitor azoxystrobin. Plant Disease. 103:1910-1917.

Interpretive Summary: The occurrence of fungal brown spot has increased in cultivated wild rice paddies in spite of the use of strobilurin type fungicides, raising the concern that the pathogens have developed resistance to the fungicide active ingredients. Fungi collected before application of fungicides were tested for sensitivity to the active ingredient and compared to fungi collected after nearly a decade of fungicide application. No significant difference in fungicide sensitivity was detected and all strains were sensitive to the active ingredient. DNA sequencing of the gene product targeted by the fungicide found that mutation to a form resistant to the fungicide would be lethal in the brown spot fungus. Thus, this fungus is unlikely to become resistant to strobilurin fungicides. Disease epidemics were most likely the result of a buildup of inoculum, conducive environmental conditions, and late application of fungicides rather than fungicide resistance. The information from this study will lead to better management of diseases, improved farm income, and provide a more stable supply of wild rice for domestic and international consumers.

Technical Abstract: The occurrence of fungal brown spot, caused by Bipolaris oryzae, has increased in cultivated wild rice (Zizania palustris) paddies in spite of use of azoxystrobin-based fungicides. These fungicides target the quinone outside inhibitor (QoI) sequence in mitochondrial cytochrome b (cyt b), thus obstructing respiration. The in vitro EC50 of baseline isolates collected in 2007 before widespread fungicide use was estimated to be 0.427 µg/ml and 0.394 µg/ml with a linear and a PROBIT regression analysis, respectively. Isolates collected during 2008, 2015, and 2016 had a range of sensitivity as measured by relative spore germination (RG) at a discriminatory dose of 0.4 µg/ml azoxystrobin but no highly resistant isolates were identified. Isolates with a higher (= 80%) or lower RG (= 40%) had the wild type sequences at amino acid positions F129, G137, and G143 of cyt b, the sites of mutations known to be associated with QoI fungicide resistance. Two Group I introns were found in the QoI target area. The splicing site for the second intron was found immediately after the codon for G143. A mutation for fungicide resistance at this location would hinder splicing and severely reduce fitness. B. oryzae expresses an alternative oxidase in vitro, which allows the fungus to survive inhibition of respiration by azoxystrobin. This research indicates that B. oryzae appears to be a low risk species for developing resistance to QoI fungicides. Thus, judicious use of QoI fungicides within an integrated disease management system will promote an effective and environmentally sound control of the pathogen in wild rice paddies.