Is Microdochium maydis associated with necrotic lesions in the tar spot disease complex? A culture-based survey of maize in Mexico and the midwestern United States

Authors: LUIS, JANE MARIAN - Virginia Tech; Mehl, Hillary; PLEWA, DIANNE - University Of Illinois; KLECZEWSKI, NATHAN - University Of Illinois

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2023
Publication Date: 11/7/2023
Citation: Luis, J., Mehl, H.L., Plewa, D., Kleczewski, N.M. 2023. Is Microdochium maydis associated with necrotic lesions in the tar spot disease complex? A culture-based survey of maize in Mexico and the midwestern United States. Phytopathology. 113(10):1890-1897. https://doi.org/10.1094/PHYTO-04-23-0109-R.
DOI: https://doi.org/10.1094/PHYTO-04-23-0109-R

Interpretive Summary: Tar spot of corn is an emerging disease in the United States caused by the fungus Phyllachora maydis. Tar spot can decrease corn yield and quality, but much about the biology and management of the disease is still unknown. In some cases, tar spot lesions are surrounded by a necrotic area known as a fisheye. Fisheye lesions were previously reported to be caused by a second fungus, Microdochium maydis. However, recent research has been unable to verify the relationship between fisheyes and the presence of this fungus. To gain insight into associations between fisheye lesions and the presence of M. maydis or other fungi, corn samples with tar spot fisheyes were collected from fields across Mexico, Florida, Illinois, and Wisconsin. None of the fungi isolated from lesions were identified as M. maydis, but 92% of the isolates were confirmed as Fusarium species. Results suggest that fisheye lesions are associated with a diversity of Fusarium spp., including species that produce mycotoxins. Additional research is needed to determine the potential impacts of increasing tar spot disease incidence and severity on mycotoxin contamination of corn.

Technical Abstract: Tar spot, caused by Phyllachora maydis, is an emerging disease of corn in the United States. Stromata of P. maydis are sometimes surrounded by necrotic lesions known as fisheyes and were previously reported to be caused by the fungus Microdochium maydis. The association of M. maydis with fisheye lesions has not been well documented outside of initial descriptions from the early 1980s. The objective of this work was to assess and identify Microdochium-like fungi associated with necrotic lesions surrounding P. maydis stromata using a culture-based method. In 2018, corn leaf samples with fisheye lesions associated with tar spot stromata were collected from 31 production fields across Mexico, Illinois, and Wisconsin. Cultures of pure isolates collected from Mexico believed to be M. maydis were included in the study. A total of 101 Microdochium/Fusarium-like isolates were obtained from the necrotic lesions, and 91% were identified as Fusarium spp., based on initial ITS sequence data. Multi-gene (ITS, TEF1-a, RPB1, and RPB2) phylogenies were constructed for a subset of 55 isolates; Microdochium, Cryptostroma, and Fusarium reference sequences were obtained from GenBank. All the necrotic lesion isolates clustered within Fusarium lineages and were phylogenetically distinct from the Microdochium clade. All Fusarium isolates from Mexico belonged to the F. incarnatum-equiseti species complex, whereas >85% of the U.S. isolates grouped within the F. sambucinum species complex. Our study suggests that initial reports of M. maydis were misidentifications of resident Fusarium spp.