Identifying the Host Range of the Fungus Phyllachora maydis, the Corn Tar Spot Pathogen

From beneficial partners to harmful pathogens, fungi play diverse roles in ecosystems. Some fungi are generalists, capable of infecting many different organisms known as hosts, while others are specialists, limited to a single host or closely related hosts. Why is this important? In agriculture, this distinction is significant when it comes to fungi that affect crops. While both specialists and generalists can negatively impact crop production, fungal pathogens that can infect different hosts can pose a greater challenge. A broad host range makes them more challenging to manage and control, increasing the risk of crop loss.

An Emerging and Invasive Fungal Pathogen

Tar Spot of Corn, is caused by Phyllachora maydis, a fungus that produces black spots resembling tar on its corn host (Fig. 1, left), hence the name. The fungus was first discovered in the United States in 2015 and subsequently reached Minnesota in 2019. However, before being found in the U.S., Phyllachora maydis had been known in Latin American countries since 1904.

Why Are We Investigating the Host Range of This Fungus?

During its lifecycle, Phyllachora maydis is naturally obligated to grow and reproduce in corn. It survives the non-growing season in corn debris, which facilitates infection the following growing season. It is unknown if Phyllachora maydis infects other host plants during its lifecycle, which could make it a more challenging pathogen to manage. The recent emergence of this fungus in the U.S., however, raised the question of whether it represents a threat to native plants in addition to corn, as invasive organisms often impact hosts in new locations. On the other hand, a recent study based on molecular analyses indicated that other Phyllachora fungi, known to infect grasses, are genetically identical to Phyllachora maydis, suggesting that this corn pathogen likely has a broader host range than is currently known.