Potato Virus Y Restricts <i>Alternaria solani</i> Growth During Co-Infection

In the environment, multiple microbes can interact with each other in the plant phyllosphere. These associations can shape the plant's development, stress responses, and disease susceptibility, but the molecular mechanisms that govern this process remain unexplained. Of interest are the multiple or successive infections that crop plants are exposed to within a growing season. One of the most common and economically important viruses of potato is potato virus Y (PVY, <i>Potyviridae</i>). We show that PVY infection of potato limited the expansion of foliar necrotic lesions caused by the early blight fungus <i>Alternaria solani</i>. The reduced growth phenotype persisted when the fungal mycelium was transferred to solid growth media. RNA-seq analysis of responses in potato and <i>A. solani</i> to the presence of PVY suggested two mechanisms that can explain this interaction. First, in <i>A. solani</i> exposed to PVY-positive leaves, we observed a downregulation of fungal pathogenicity genes. Second, we found that, in the absence of PVY, <i>A. solani</i> downregulates ethylene-responsive defense in potato, but this effect was eliminated when the host was infected with PVY. Our findings expand our understanding of how pathogen virulence can be affected by other pathogens competing for the same host resources. The observation that PVY can alter <i>A. solani</i> infection illustrates the ecological role of viruses as a potential contributor to the development of disease outbreaks. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.