by Cameron Rudolph

Corn growers have witnessed firsthand how years with wet conditions are particularly problematic for tar spot, a fungal disease that was discovered in Michigan in 2016 and in Ohio in 2018.

Before 2015, tar spot was mostly found in Central and South America. Since then, however, the pathogen has taken hold across most of the U.S. Corn Belt — 19 total states thus far — and Ontario, Canada.

Tar spot presents as small, black lesions on leaves, which quickly begin to destroy plant tissue. 2018 and 2021 have been tar spot’s most prolific seasons in Michigan. Plentiful rain, morning dew and fog kept leaves wet, allowing the pathogen to thrive. Drier years such as 2022 saw far less incidence of disease.

Once tar spot has infested a field, mitigating damage becomes incredibly difficult. Previous research has shown that spread is rapid if mismanaged, from a few dots on one plant’s leaves to engulfing an entire field in less than three weeks.

“This is a devastating disease for growers, not only in Michigan but everywhere it’s present,” says Martin Chilvers, associate professor in the Michigan State University Department of Plant, Soil and Microbial Sciences. “In 2018, for example, we saw epidemic levels in areas of Michigan. Some growers were losing 50 to 100 bushels per acre. National estimates put losses at 183 million bushels. There were similar losses in 2021. This is something we need to figure out, and we’re doing that by studying the disease on a number of fronts.”

Chilvers has partnered with colleagues from MSU, other universities, agricultural commodity organizations and companies to perform an array of tests on fungicide efficacy and timing, hybrid resistance, planting density and spore trapping. The research teams have leaned on his experience with new diseases in both corn and soybean cropping systems.

“Weather is the key driver to tar spot development,” Chilvers says. “We know that much for certain, so most of our recommendations are based on the work we’ve done around that idea, and some are common sense or concepts borrowed from the management of other diseases. We are still trying to put data sets together to better inform or demonstrate management implications.”

Tips, tools for growers

A series of tips and tools have been created to assist growers, from a smartphone app to online resources such as fungicide efficacy tables and variety selection suggestions. All this information is shared via Extension services at partner universities and on the Crop Protection Network website, a joint effort of land-grant universities to distribute research findings.

On the technology side — using data from MSU researchers, among others — Chilvers’ colleague at the University of Wisconsin-Madison, Damon Smith, produced a disease-risk-prediction algorithm available for smartphones called Tarspotter.

The app uses GPS coordinates to identify if the weather in an area has been friendly to tar spot fungus development. Based on future weather predictions, crop development stage and characteristics, a disease forecast is generated.

More recently, led by graduate student Emily Roggenkamp in Chilvers’ lab, the team has developed a real-time PCR diagnostic technique that detects tar spot quickly, which will be shared with collaborators to use in future growing seasons. Chilvers and his team have also assembled a high-quality genome sequence of the tar spot pathogen, Phyllachora maydis.

“The diagnostic tool is obviously important for us in identification of the disease quickly, and it can be paired with spore traps to detect the fungus prior to disease symptoms,” Chilvers says. “The genome sequence is invaluable because it helps us potentially understand more about infection, resistance and biology of the pathogen. This information allows us to apply fungicides at the most ideal times, learn about the conditions that lead to disease, and even develop resistant varieties.”

Looking toward the future

Chilvers is part of a relatively new project led by Addie Thompson, an assistant professor of maize genetics and genomics in the MSU Department of Plant, Soil and Microbial Sciences, looking at tar spot resistance.

The group was funded in summer 2022 by a $590,000 grant from USDA’s National Institute of Food and Agriculture, building on previous USDA-funded work, the Great Lakes Tar Spot Initiative.

The project is focusing on characterizing the phenolic compounds that accumulate in corn in response to tar spot infection. Phenolic compounds are essential to plants’ defense from pests and diseases. Researchers will then test and validate previously identified resistance genes.

Using this information, along with remote-sensing data collected through multispectral and hyperspectral imaging, the group will create predictive models on tar spot severity and the relationship to phenolic compound accumulation. This can ideally help detect early onset of the disease.

“Understanding the relationship between compound accumulation and resistance will enable development of new varieties of corn that are less reliant on pesticides,” Thompson says. “Results will be highly applicable to breeders and growers, which is why this project is innovative and timely as this disease spreads at alarming rates each year throughout the upper Midwest.”

Additionally, Chilvers received funding in 2022 for a project through the Michigan Alliance for Animal Agriculture (M-AAA), an initiative that brings together MSU and animal agriculture industries. The research will address tar spot in corn silage, which is used for animal feed on many Michigan dairy farms.

Tar spot negatively affects corn silage by rendering it too dry, reducing quality and zapping nutrient content. This leads to less productive cows and lower profitability for producers. Working with dairy farmers and the Michigan Milk Producers Association, Chilvers will conduct variety susceptibility trials across hybrids, as well as fungicide efficacy, fungicide timing and silage chop timing experiments.

Rudolph serves as a communications manager with MSU AgBioResearch.