In early September 2025, small, round to oval, tan lesions 1 to 3 mm in diameter and surrounded by a brown margin were observed on leaves of field corn (Zea mays L.) in Ramsey County, Minnesota. The lesions were in the upper canopy, with 1 to 5 lesions per leaf in the observed plants. Symptomatic leaves were collected, and spores were obtained from within the lesions. Spores observed under a microscope appeared curved with three septa, smooth and melanized, 20 to 27 μm in length and 8 to 12 μm wide at the broadest part (n = 4). The symptoms on the leaves matched those of Curvularia leaf spot of corn, and the conidia resembled the morphology of Curvularia lunata isolated from corn in the United States (Garcia-Aroca et al. 2018; Anderson et al. 2019; Henrickson and Koehler 2022; Jimenez Madrid 2022; Moparthi and Kleczewski 2023). To isolate the fungus, individual spores removed from lesions were cultured on potato dextrose agar amended with 12.5 mg/L rifampicin (PDAR) and grown at 25 °C in darkness. At 10 days, the colony of the representative isolate Cl1MN was dark brown, with white and gray on top and orange to reddish margins. Conidia were as described above, 17.5 to 27.5 μm × 9 to 15 μm (n = 20), consistent with the morphology of C. lunata. For further identification, genomic DNA was extracted from the isolate Cl1MN, and the nuclear ribosomal internal transcribed spacer (ITS, PX555854), the translation elongation factor 1-alpha (TEF1, PX529568), and the glyceraldehyde-3-phosphate dehydrogenase (GAPDH, PX529567) regions were sequenced (Manamgoda et al. 2014). BLASTn queries showed that the ITS sequence shared 100% identity with the ITS region of the reference genome of C. lunata strain W3 (PRJNA416716, 563/563 bp) and the isolate DMCC2087 (MG971304, 555/555 bp) used in the first report of C. lunata in the United States (Garcia-Aroca et al. 2018). More than 99% identity was shared with the TEF1 (MG979800, 901/903 bp) and GAPDH (MG979801, 550/554 bp) sequences of C. lunata DMCC2087 and those of the C. lunata W3 genome (TEF1, 900/901 bp and GAPDH, 550/554 bp). Maximum Likelihood phylogenetic analysis of concatenated ITS, TEF1, and GAPDH sequences of Curvularia species (Manamgoda et al. 2014) and other available in GenBank further supported the assignment of isolate Cl1MN to C. lunata. A pathogenicity test was conducted with the Cl1MN isolate following methods and conditions reported by Garcia-Aroca et al. (2018). Inoculum was applied to leaves on four plants of each corn line, B73 PI 550473, GC-103-58 RSS, and sweet corn 5456T.54 at growth-stage V4. The same number of plants per line served as non-inoculated controls. Lesions characteristic of Curvularia leaf spot appeared on the inoculated leaves 3 to 4 days after inoculation, similar to those observed on the symptomatic leaves in the field. Symptoms were not observed in the control group. Ten days after inoculation, symptomatic leaf sections (~5 mm²) were excised, sterilized in 0.6% NaOCl, rinsed with sterile water, and air-dried before placement on PDAR and incubation at 25 °C for 5 days in darkness. Characteristics of the reisolate, including colony and conidia morphology, and three sequences were identical to those of Cl1MN and the previous reports of C. lunata in the United States. This confirms for the first time that Curvularia leaf spot of corn caused by C. lunata occurs in Minnesota, adding to the known geographical distribution of this disease and fungus in the United States, which was first documented in Louisiana in 2017 (Garcia-Aroca et al. 2018).
Solórzano et al. (Mon,) studied this question.