Identification of Cercospora spp. on corn in North America and baseline flutriafol fungicide sensitivity.

Gray leaf spot (GLS) is an important corn disease reportedly caused by Cercospora zeae-maydis and C. zeina. Recently, flutriafol, a demethylation inhibitor (azole) fungicide received EPA registration as Xyway® LFR®, a product that is applied at planting for management of fungal diseases in corn, including suppression of GLS. In this study, 448 Cercospora spp. isolates were collected in 2020 and 2021 from symptomatic corn leaf samples submitted from the United States and Ontario, Canada. The Cercospora spp. were identified using multi-locus genotyping of the internal transcribe spacer (ITS), elongation factor 1-α (EF1), calmodulin (CAL), histone H3 (HIS), and actin (ACT) gene. Based on the multi-locus phylogenetic analyses, six species were identified; C. cf. flagellaris (n = 77), C. kikuchii (n = 4), C. zeae-maydis (n = 361), Cercospora sp. M (n = 2), Cercospora sp. Q (n = 1), and Cercospora sp. T (n = 3). In subsequent pathogenicity tests using selected isolates from each of these species, only C. zeae-maydis resulted in symptoms on corn with no disease symptoms observed after inoculation with C. cf. flagellaris, C. kikuchii, Cercospora sp. M, Cercospora sp. Q, and Cercospora sp. T. While disease symptoms were observed on soybean following inoculation with C. cf. flagellaris, C. kikuchii, and Cercospora sp. Q, but not the other three species. Fungicide sensitivity of Cercospora spp. to flutriafol was assessed using a subset of 340 isolates. The minimum inhibitory concentration (MIC) to inhibit the growth of Cercospora spp. completely was determined based on growth of each species on flutriafol-amended clarified V8 agar at nine concentrations. The EC50 was also calculated from the same trial by measuring relative growth as compared to the non-amended control. Cercospora zeae-maydis was sensitive to flutriafol with mean MIC values of 2.5 µg/mL and EC50 values ranging from 0.016 to 1.020 µg/mL with a mean of 0.346 µg/mL. Cercospora cf. flagellaris, C. kikuchii, Cercospora sp. M, Cercospora sp. Q, and Cercospora sp. T had mean EC50 values of 1.25 µg/mL, 7.14 µg/mL, 2.48 µg/mL, 1.81 µg/mL, and 2.24 µg/mL respectively. These findings will assist in monitoring the sensitivity to the flutriafol fungicide in Cercospora spp. populations.