First Report of Curvularia Leaf Spot of Corn, Caused by Curvularia lunata, in Kentucky

In July 2018, small circular light brown to tan lesions (0.25 to 2.5 mm diameter) were observed on corn (Zea mays) leaves at the University of Kentucky Research and Education Center in Princeton, KY. Lesions were surrounded by dark margins and chlorotic halos. Multiple hybrids were affected, and lesions were observed in the mid-upper canopy with disease severity ranging from 1 to 30% of leaf tissue affected. Small sections (1 cm²) of leaves with lesions were surface sterilized by soaking in water for 10 min, followed by a 0.05% NaOCl solution for 5 min, and rinsed with sterile water for 5 min. Leaf material was transferred to potato dextrose agar amended with rifamycin (25 mg/liter). After 7 days, dark brown to black colonies with orange margins were observed. Conidia were curved and melanized with three transverse septa, measuring 8 to 12 × 20 to 35 µm, consistent with previously published descriptions of Curvularia lunata (Garcia-Aroca et al. 2018; Mabadeje 1969). A pure culture was obtained, and the internal transcribed spacer (ITS) regions were amplified and sequence reads obtained for each primer (ITS4 and ITS5). DNA sequences of the representative isolate (KY18.1) were deposited into the GenBank database (ITS4, MK623264; ITS5, MK623265). BLAST searches of the resulting sequences had 100% similarity to reference sequences for ITS4 (LT631353.1; C. lunata type strain 730.96) and ITS5 (MG971305.1; Garcia-Aroca et al. 2018). Koch’s postulates were completed by inoculating greenhouse-grown corn plants (hybrid DKC62-08RIB) at the four-leaf collar growth stage. A conidial suspension was prepared by rinsing plates with 10 ml of sterile water and adjusted to a concentration of 5 × 10⁴ conidia/ml. Leaves of six plants were inoculated with 25 ml of the suspension. Two plants inoculated with water served as a control. Each plant was covered with a plastic bag for 24 h after inoculation. After 14 days, small, round, tan lesions (0.2 to 1.5 mm diameter) were observed on inoculated corn leaves. No symptoms were observed on the water-inoculated control plants. Symptomatic leaves were collected, and C. lunata was reisolated from symptomatic leaves as described above. Conidia size and colony morphology of isolates obtained from greenhouse-inoculated plants were identical to original isolates. ITS regions of isolates obtained from inoculated plants were sequenced and identical to C. lunata reference sequences described above. To our knowledge, this is the first report of Curvularia leaf spot on corn in Kentucky. The disease was reported for the first time in the United States in Louisiana in 2017 (Garcia-Aroca et al. 2018), and the economic impact in the United States is still unknown. Curvularia leaf spot did not limit yield in this field in 2018; however, the effect on yield in Kentucky is still being investigated, and the disease has been reported to cause significant damage in tropical climates (Carson 2016). Future monitoring and research will help to determine the impact of this disease in Kentucky.