Researchers tackle tar spot fungus which threatens US corn

Tar spot disease was first detected in northern Indiana and Illinois in 2015 and is now also found in corn-growing areas of Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, Wisconsin, Florida in the US, and in southwestern Ontario, Canada. Outbreaks can be costly – an outbreak experienced from 2018 to 2020 claimed an estimated 241 million bushels (6.1 million tonnes) of US corn.

A fungal disease

Appears as black, roughly circular discolorations on the leaves, husks and stalks of corn plants, tar spot is caused by the fungus Phyllachora maydis. A tan halo sometimes surrounds the spore-filled spots, creating what’s known as a fish-eye lesion.

Fungicides offer some protection from tar spot, however, according to plant pathologist with the ARS Crop Production and Pest Control Research unit in Indiana, Steve Goodwin, resistance to tar spot disease in corn is far more preferable.

Tar spot research

Goodwin is working in collaboration with fellow ARS scientists Raksha Singh, Matthew Helm and Charles Crane to manage tar spot through research work on several fronts:

• Screening existing commercial corn varieties and germplasm lines for their resistance or susceptibility to tar spot.
• Developing tools known as molecular markers to identify a gene known to confer tar spot resistance in corn, namely Qrtsc8. Identifying corn plants that lack the gene but are still resistant to the disease are also of interest, since an entirely new gene or genes unknown to science could be at play. Such sources of resistance could prove useful in shoring up the crop’s defenses even further.
• Determining the biocontrol potential of a community of microorganisms known as the microbiome that was observed on tar-spot-resistant but not susceptible corn plants. “The main goal is to understand how environmental factors, plant growth stage and the associated corn microbiome affect tar spot disease progression and how all these factors are interconnected,” noted Singh.
• Identification of several proteins the tar spot fungus uses to ‘short circuit’ the defenses of susceptible plants, and how these proteins could be exploited for better detection of different strains of the fungus and their severity in fields.

Other research efforts to reduce the severity of tar spot

University collaborators are also conducting research to optimise the timing of fungicide sprays and evaluate rotations of corn with non-host crops to reduce the disease’s severity and prevent the fungus from surviving the winter on debris from prior corn harvests.

Researchers are also pouring through existing literature on the biology of the tar spot fungus and building on what’s known about it with genomic sequencing, which is a kind of decoding of its DNA playbook for causing disease in corn. It is hoped that this will yield clues to new ways of controlling the fungus and pre-empting further outbreaks.

National Plant Disease Recovery System and partners

The effort is being carried out under the auspices of the National Plant Disease Recovery System, which aims to ensure that the tools, infrastructure, communication networks, and capacity required to mitigate the impact of high-consequence plant disease outbreaks are such that a reasonable level of crop production is maintained in the US.

Besides ARS, other partner organisations are Purdue University, Michigan State University, Iowa State University, Ohio State University, University of Missouri, University of Florida and the International Maize and Wheat Improvement Center in Mexico, where tar spot was first identified in 1904.