USA- Agricultural Research Service (ARS) scientists are diligently seeking innovative ways to combat the fungus responsible for tar spot, a disease that significantly reduces the yield of field corn in the Midwestern United States.

Initially identified in Illinois and Indiana in 2015, tar spot has since spread to neighboring states, including Florida and Canada. This disease manifests as raised black spots on the leaves, husks, and stalks of susceptible corn varieties, impairing their photosynthetic capacity and, in severe cases, leading to plant death and grain yield losses ranging from 20 to 60 bushels per acre.

Recent developments, however, have uncovered a potential adversary for the fungus-causing tar spot, Phyllachora maydis. The spots, known as stromata, represent a resilient, structural form of the fungus that helps it survive the winter and release spores the following spring, infecting the subsequent corn crop.

A team of observant scientists from ARS’s National Center for Agricultural Utilization Research in Peoria, Illinois, noticed that certain collected stromata samples failed to germinate. The culprit behind this phenomenon appeared to be other fungi and bacteria parasitizing the tar spot fungus, offering the possibility of a biologically based approach to control it.

This discovery came while examining a research plot of corn near the ARS center in April 2022. Although synthetic fungicides can manage mild tar spot outbreaks and corn varieties that can withstand some damage from the fungus, severe outbreaks can overwhelm these defenses, resulting in substantial financial losses for farmers.

Fortunately, nature provided several species of fungi and bacteria that grow and reproduce on or within the fungus’s stromata. Some of these organisms even appeared as whitish fuzz on the stromata when observed under a microscope in the laboratory.

The researchers used DNA-based identification methods and found that some fungi and bacteria were known biological control agents for diseases that affect other crops. 

In trials, exposure to spores of Gliocladium catenulatum, a commercially available biocontrol fungus, prevented 88% of the tar spot fungus’s stromata from germinating. An Alternaria fungus isolated from a tar spot stroma prevented around 45% of stromata from germinating.

Eric Johnson, a research molecular biologist with the ARS center’s Crop Bioprotection Research Unit in Peoria, explained that previous research studies had demonstrated the effectiveness of certain strains of Alternaria alternata as biocontrol agents that can reduce damage caused by plant pathogens. 

Furthermore, laboratory tests indicated that the tested Alternaria strain did not cause disease in a susceptible corn variety when added to damaged leaf portions. This strain may be particularly useful in eliminating overwintering tar spot stromata, given that it thrives at cold temperatures.

The studies are still in their early stages and require further research to determine the full potential of these biological agents for tar spot control in commercial fields during the growing season or for overwintering eradication. 

Meanwhile, other approaches for managing the disease are being explored in Peoria and at ARS’s Crop Production and Pest Control Research Unit in West Lafayette, Indiana. 

These approaches include examining the fundamental biology and genetic foundations of the tar spot fungus to uncover new methods of control, developing molecular markers to expedite the search for new sources of tar spot resistance in corn, and exploring strategies to maximize the use of fungicides approved for tar spot control in corn as part of an integrated approach to disease management.

Details regarding the biocontrol potential of natural enemies of the tar spot fungus appeared in the June 2023 issue of the journal Microorganisms by Eric Johnson and co-authors Pat Dowd, Jose Ramirez, and Robert Behle, all associated with the ARS center’s Crop Bioprotection Research Unit in Peoria. 

Additional research on the biological control of tar spot disease is now receiving funding from the Illinois Corn Growers Association and the ARS National Plant Disease Recovery System.