Potential biocontrol agents of corn tar spot disease isolated from overwintered Phyllachora maydis stromata

Authors: Johnson, Eric; Dowd, Patrick; Ramirez, Jose; Behle, Robert

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2023
Publication Date: 6/10/2023
Citation: Johnson, E.T., Dowd, P.F., Ramirez, J.L., Behle, R.W. 2023. Potential biocontrol agents of corn tar spot disease isolated from overwintered Phyllachora maydis stromata. Microorganisms. 11(6). Article 1550. https://doi.org/10.3390/microorganisms11061550.
DOI: https://doi.org/10.3390/microorganisms11061550

Interpretive Summary: Tar spot disease of corn is a problem in the Midwestern United States and Florida. Fungicides are somewhat effective in controlling the disease, but alternative disease control products need to be developed for full season control and to prevent potential fungicide resistance in the pathogen. Winter survival structures of the tar spot organism, called stromata, are thought to be responsible for infecting corn plants in the spring. Stromata that do not release their reproductive structures in the spring might be infected with parasites. We isolated several potential parasites (various fungi and bacteria) from overwintered stromata in Central Illinois. The purified parasites were added to stromata on corn leaves that were collected in the summer in laboratory studies. Two of the parasites, as well as a commercial biofungicide, significantly reduced the number of stromata that released reproductive structures compared to stromata that received control treatments. These experiments suggest that some of the overwintered parasites, as well as a commercial biological control organism, could be utilized by farmers to control or prevent tar spot disease of corn.

Technical Abstract: Tar spot disease in corn, caused by Phyllachora maydis, can reduce grain yield by limiting the total photosynthetic area in leaves. Stromata of P. maydis are long-term survival structures that can germinate and release spores in a gelatinous matrix in the spring, which are thought to serve as inoculum in newly planted fields. In this study, overwintered stromata in corn leaves were collected in Central Illinois, surface sterilized, and caged on water agar medium. Fungi and bacteria were collected from the surface of stromata that did not germinate and showed microbial growth. Twenty-two Alternaria isolates and three Cladosporium isolates were collected. Eighteen bacteria, most frequently Pseudomonas and Pantoea species, were also isolated. Spores of Alternaria, Cladosporium and Gliocladium catenulatum (formulated as a commercial biofungicide) reduced the number of stromata that germinated compared to control untreated stromata. These data suggest that fungi collected from overwintered tar spot stromata can serve as biological control organisms against tar spot disease.