RESUMEN
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.
RESUMEN
In late July of 2022, corn leaves with spots were observed in a field at North Carolina State University's Vernon James Research and Extension Center (35.873294º N, 76.658599º W; Plymouth NC). The affected leaves contained small (two to three millimeters in diameter) tan-colored circular to oval lesions with reddish-brown margins and chlorotic haloes. The disease severity approached 50% on affected leaves. Leaf tissue from the margins of the lesions was excised and surface sterilized by soaking in 10 % bleach solution for two minutes followed by rinsing in distilled water. Next, leaf tissue was transferred to alkaline water agar amended with streptomycin (100 mgliter-1) and the plates were incubated at 27º C in dark. After one week, conidia were observed. Conidia were curved, light brown in color and contained three transverse septa. The size of the conidia (n= 30) ranged from 21-25 µm (average 24 µm) long by 7 to 10 µm wide (average 9 µm). These characteristics were typical of Curvularia lunata (Munkvold and White 2016). DNA from pure cultures was extracted, internal transcribed space region (ITS) was amplified using the primers ITS4 and ITS5 (White et al. 1990). Sequences were obtained and deposited into GenBank (accession no. OP998306). BLAST search indicated that the sequence was 100 % identical to C. lunata from Louisiana (MG 971305.1, Garcia- Aroca et al. 2018). Pathogenicity was confirmed on field corn cultivar 'Yellow dent' by inoculating two pots of two-week-old greenhouse-grown corn plants (6 plants per pot) by spraying conidial suspension with a concentration of 6 x 104 conidia /ml. The control plants (n= 6) were sprayed with distilled water and the plants were covered with a plastic bag for 24 hours. The plastic bags were removed and plants were moved to the greenhouse at 23º C. Four days after inoculation small round tan-colored lesions were observed and no symptoms on control plants. The experiment was repeated once and the pathogen was recovered from the symptomatic tissue. The morphology of the recovered isolate was similar to the original isolate. This is the first report of C. lunata causing leaf spot on corn in North Carolina. Although this disease has not been demonstrated to cause yield loss it has been detected more frequently in corn production areas of U.S. and should continue to be monitored going forward.