Tar spot is a significant challenge for corn, but seed companies and agronomists are stepping up with solutions.
For nearly a decade, corn seed developers have been addressing the rise of a new disease threatening the Midwest’s corn yields. Tar spot, a fungal disease caused by Phyllachora maydis, has emerged as one of the most significant challenges for corn production, presenting both risks and opportunities for seed companies to innovate and provide solutions.
First detected in the United States in 2015, tar spot has rapidly gained a foothold, thriving under the right environmental conditions and causing significant yield losses.
The Yield-Loss Reality
“I’ve been able to see tar spot firsthand for the last four straight seasons in central Iowa,” says Matt Vandehaar, field agronomist with Pioneer. “Most farmers in my geography now definitely know what tar spot looks like; however, some don’t quite know the full economic impacts if it gets a lot of pressure.”
The disease manifests as small, black lesions — tar-like spots — on corn leaves. These lesions can coalesce, reducing photosynthetic capacity and causing premature plant death. The economic implications are staggering. In 2021 alone, tar spot was responsible for an estimated reduction of 231.3 million bushels in U.S. corn yields, equating to nearly $1.25 billion in losses. For seed companies, this underscores the urgency of developing hybrids with robust tolerance.
The Science Behind Tar Spot
“Tar spot can shut down that corn plant early, and even if you’ve made a fungicide application, it can still cause significant damage,” Vandehaar explains. “We’ve seen areas with a lot of pressure where yields are hit hard.”
The spread and severity of tar spot are heavily influenced by weather. Cool, wet conditions, particularly in late summer, create the perfect environment for the disease to thrive. However, these conditions have not been consistent across the Midwest.
The Weather Wildcard
“We’ve lacked a cooler-than-average, wetter-than-average August for the past few years,” Vandehaar says. “But even so, there have been pockets—in Indiana, Illinois, Wisconsin, Northern Iowa, and Southern Minnesota—that have experienced significant tar spot outbreaks.”
Tar spot seems to be a bit of a wildcard in the field. At this point, the fungi itself is prevalent in fields and crop residue. Early scouting is likely to reveal tar spot stroma on leaves. Now, if the weather conditions do not favor tar spot growth, that is where the disease will stay, as a stroma.
Should the weather patterns shift and favor tar spot growth, then it can really take off and shut down corn plants before grain fill has completed.
The Race to Develop Resistant Hybrids
For seed developers, the goal is clear: create hybrids that offer improved tolerance to tar spot. Pioneer and other companies are making strides in this area.
“We don’t have genes that completely defend against it,” Vandehaar admits. “But we do have hybrids that show very strong tolerance. These hybrids have excellent yield potential, the agronomics farmers need, and the ability to handle tar spot pressure.”
One key trait in these hybrids is “stay-green,” a measure of a plant’s ability to maintain healthy, photosynthetic tissue late into the season.
“Stay-green correlates highly with tar spot tolerance in our lineup,” he says. “These plants remain healthy despite significant tar spot pressure, allowing them to maintain yields.”
To meet the evolving needs of the market, seed companies are intensifying their efforts to screen hybrids and identify those with superior performance against tar spot. This requires robust field trials, data analysis, and the integration of genetic insights.
“Each year, we screen hybrids to identify the ones that perform best against tar spot,” Vandehaar emphasizes. “We’re aiming to release hybrids with even higher scores for tolerance, so farmers have better tools to manage this disease.”
Fungicides: A Tool, Not a Silver Bullet
In addition to genetic improvements, understanding tar spot’s lifecycle is critical for seed companies when educating their customers. The fungus survives on corn residue and spreads via wind and rain splash. Early scouting often reveals tar spot stroma on leaves. While this stage does not always lead to significant outbreaks, weather shifts toward cool and wet conditions can trigger rapid disease progression, impacting grain fill and yield potential.
Fungicides play a complementary role in managing tar spot, but their success depends on application timing.
Vandehaar notes, “Just because the tar spot stroma is present doesn’t mean it will become an issue. Monitoring weather conditions is crucial to determine the right time for fungicide application. Sometimes, two applications during grain fill are necessary to hold back significant pressure.”
Innovations to Combat the Disease
For seed developers, the challenge is balancing the introduction of hybrids with high tolerance against tar spot while supporting integrated management practices. This includes educating farmers on the importance of field monitoring, fungicide use, and maintaining overall plant health through good fertility and residue management.
As tar spot continues to spread, its impact on the Midwest corn belt likely will grow. For seed companies, and Vandehaar says this presents an opportunity for the seed industry to lead the charge in providing innovative, resilient hybrids. By staying ahead of this disease through advanced breeding strategies and customer education, the industry can help secure the future of corn production in the face of this evolving threat.
