Sensitivity and resistance of Cercospora kikuchii, causal agent of Cercospora leaf blight and purple seed stain of soybean, to selected fungicides

Abstract

Isolates of Cercospora kikuchii, the causal agent of Cercospora leaf blight (CLB) and purple seed stain (PSS), were used to determine baseline sensitivities to selected quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) fungicides by conducting radial growth assays on fungicide-amended media. The effective concentration to inhibit 50% radial growth (EC50) for each isolate was calculated by linear interpolation of the dose-response relationship. All baseline distributions were non-normal with outliers towards the less sensitive ends of the spectra, and median EC50 values for azoxystrobin, pyraclostrobin, trifloxystrobin, flutriafol, propiconazole, and tetraconazole were 0.081, 0.013, 0.012, 0.273, 0.143, 1.47 µg/ml, respectively. When compared to baseline sensitivities, median EC50 values for isolates exposed to azoxystrobin, pyraclostrobin, and trifloxystrobin in 2011/2012 were significantly higher at 37.2/57.6, 10.1/12.2, and 20.1/29.1 µg/ml, respectively. Cross-resistance to all three QoI fungicides was observed in the 2011 and 2012 populations. Discriminatory doses of 10 µg/ml were developed for all three QoI fungicides to distinguish between sensitive and resistant isolates. Approximately 83% of all isolates screened in 2011 and 2012 were resistant to QoI fungicides, and isolates from 21 of 27 parishes tested positive for resistance. Median EC50 values for isolates exposed to flutriafol, propiconazole, and tetraconazole in 2011/2012 were 0.41/0.54, 0.33/0.24, and 0.75/0.73 µg/ml. Significant shifts from the baseline towards less sensitivity were detected in isolates exposed to flutriafol and propiconazole. Additionally, outliers towards less DMI sensitivity were detected for all three DMI fungicides 2012. Strong, positive, and significant cross-sensitivity was observed among all three DMI fungicides. At a discriminatory dose of 5 µg/ml thiophanate methyl, methyl benzimidazole carbamate (MBC) resistance was detected in the 2000, 2011, and 2012 populations at 23.3, 44.8, and 35.7%, respectively, with resistant isolates in 19 of 27 parishes. Isolates exhibiting multiple resistance to QoI and MBC fungicides also were detected in 15 of 27 parishes. Ninety-eight percent of MBC-resistant isolates also were resistant to QoI fungicides. Based on results from this research, CLB/PSS management strategies with QoI and MBC fungicides should be reconsidered in areas where resistance has been confirmed, and C. kikuchii populations should be further monitored for shifts in DMI sensitivity.