Progression of Corn Tar Spot and reproducibility assessment of an inoculation protocol in semicontrolled environments

Authors: JIMENEZ-BEITIA, FIDEL - Purdue University; ACOSTA, ALEX - Purdue University; CRUZ, ANDRES - Purdue University; GONGORA-CANUL, CARLOS - Tecnologico Nacional De México, Instituto Tecnológico De Conkal; FERNANDEZ-CAMPOS, MARIELA - Purdue University; SIC, WILEY - Purdue University; FALCONI, CESAR - Armed Forces University; ZAMBRANO, JOSE - National Institute For Agricultural Research (INIAP); Goodwin, Stephen; SOLORZANO, JOSE - University Of Minnesota; MALVICK, DEAN - University Of Minnesota; CRUZ, C - Purdue University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2025
Publication Date: 3/9/2026
Citation: Jimenez-Beitia, F.E., Acosta, A.G., Cruz-Sancan, A., Gongora-Canul, C., Fernandez-Campos, M., Sic-Hernandez, W., Falconi, C.E., Zambrano, J., Solorzano, J.E., Malvick, D.K., Goodwin, S.B., Cruz, C.D. 2026. Progression of Corn Tar Spot and reproducibility assessment of an inoculation protocol in semicontrolled environments. Plant Disease. https://doi.org/10.1094/PDIS-09-24-2046-RE.
DOI: https://doi.org/10.1094/PDIS-09-24-2046-RE

Interpretive Summary: Tar spot was not detected in the U.S. until 2015, but since then it has become the most serious foliar disease of corn, causing extensive losses in the major growing regions of the U.S. and Canada. Limited knowledge about the biology of the causal fungus, Phyllachora maydis, has hampered control strategies. Development of a controlled inoculation strategy for this pathogen is critical for future research, yet its life as an obligate pathogen means that it cannot be cultured on artificial media, which makes laboratory analyses extremely difficult; previous inoculation protocols have not provided enough disease pressure for confident identification of resistant lines. To test the hypothesis that we could develop a robust, repeatable inoculation protocol for infecting corn with P. maydis, parallel experiments were conducted in Indiana and Ecuador using spores collected from both fresh and stored leaves. Results in Ecuador were more consistent than those in the U.S. and spores collected from fresh leaves generally gave better infections than those from leaves that had been stored. Although more needs to be learned about the environmental conditions required by the fungus for successful infection, results in both countries were consistent enough to be used for genetic evaluations and other purposes. These results can be used by corn breeders and geneticists to test for resistance in corn lines, and by plant pathologists to better understand how the pathogen enters the host to cause disease.

Technical Abstract: The challenge of managing tar spot of corn is compounded by limited knowledge of the biology, ecology, and epidemiology of the disease. The responsible pathogen, P. maydis, is an obligately biotrophic fungus that relies on living host tissue, making it so far impossible to cultivate in artificial media and hindering controlled studies. We hypothesized that we could develop a non-invasive and reproducible inoculation protocol for systematically infecting maize with P. maydis, even under varying conditions. To test this hypothesis, our research was guided by three objectives: (1) Develop and describe a detailed, non-invasive inoculation protocol; (2) Assess the impact of inoculum source, cultivar susceptibility, and infection dynamics, including the influence of leaf position on disease progression; and (3) Test the reproducibility of the protocol across semi-controlled environments in Ecuador and the U.S. We employed a rigorous approach using a Zero-Inflated Negative Binomial (ZNBIN) model to manage the discrete nature and over-dispersion of stromata counts, which are critical challenges in tar spot research. Lin's concordance correlation analyses revealed that the protocol was highly reproducible in Ecuador, where favorable environmental conditions facilitated consistent infections. In contrast, reproducibility in the U.S. was less consistent, likely due to significant environmental variability such as differences in humidity and temperature. This study underscores the importance of developing standardized inoculation protocols tailored to diverse environments, contributing to a broader understanding of P. maydis infection dynamics, and providing a critical foundation for future research aimed at improving tar spot management in maize cultivation.