First report of Curvularia leaf spot on Zea mays caused by Curvularia lunata in North Carolina.

Abstract

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.