The U.S. Department of Agriculture's Agricultural Research Service scientists are leaving no stone -- or rather, leaf -- unturned in their search for new ways to counter the fungus that causes tar spot, a yield-robbing disease of field corn in the midwestern United States.
First reported in Illinois and Indiana in 2015, tar spot has now expanded to include other nearby states, as well as Florida and Canada. The disease manifests as raised black spots that mottle the leaves, husks and stalks of susceptible corn varieties, diminishing their photosynthetic ability and, in severe cases, killing the plants and inflicting grain yield losses of 20 to 60 bushels an acre.
Now, however, those same spots may reveal a hidden foe of the fungus that causes tar spot, Phyllachora maydis. The spots, called stromata, are a tough structural form of the fungus that enables it to survive the winter and release a bevy of spores the following spring that infect the next corn crop. But a team of sharp-eyed scientists with USDA Agricultural Research Service’s National Center for Agricultural Utilization Research in Peoria, Illinois, observed that some stromata specimens they collected failed to germinate -- the “handiwork” of other fungi and bacteria that parasitize the tar spot fungus, potentially opening the door to a biologically based approach to controlling it.
The scientists’ observation came while inspecting a research plot of corn near the Agricultural Research Service center in April 2022. Mild outbreaks of tar spot can generally be reduced with synthetic fungicide applications and corn varieties that can tolerate some damage from the fungus. But under the right weather conditions, severe outbreaks can overwhelm those defenses, exacting a costly toll on farmer profits and underscoring the need for additional countermeasures that can be deployed.
Fortunately, nature, with its system of checks and balances, offered several different species of fungi and bacteria that grow and reproduce on or inside the fungus’s stromata -- some of which appeared as a whitish fuzz on the stromata when researchers examined them under a microscope in the laboratory.
The researchers’ use of DNA-based identification methods revealed that some of the fungi and bacteria were known biological control agents of diseases affecting other crops. In trials, exposure to spores of Gliocladium catenulatum, which is a commercially available biocontrol fungus, prevented 88 percent of the tar spot fungus’ stromata from germinating.
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