If routine corn field scouting results in the discovery of leaves with multiple black, unevenly shaped spots, it might be corn tar spot.
In a recent presentation, Andrew Kness, University of Maryland Extension, discussed this relatively new corn disease.
“Tar spot of corn is a disease caused by the fungus Phyllachora maydis,” said Kness. “It’s characterized by black, raised spots on leaves, husks and tassels.” It affects only sweet corn and dent corn.
Tar spot is not native to North America. It’s endemic in Central and South America and was first noted in the U.S. in 2015 in several counties in Illinois and Indiana. Since then, it has spread throughout the American Corn Belt and into other states.
The disease made a 300-mile jump from Ohio to Lancaster County, PA. Kness said that although tar spot can spread regionally short distances, it was probably moved unintentionally from Ohio to Pennsylvania with test seed that wasn’t well-cleaned. Tar spot is now confirmed in Pennsylvania, Maryland, Delaware, Virginia and New York.
“Every year plant pathologists across the country rank diseases based on losses,” said Kness. “Tar spot has been ranked the number one foliar disease of corn from 2021 to 2024.”
Kness describes the conditions for this disease to become established and thrive: cool weather with 30-day average temperatures below 75º. Ideal conditions for infection are between 63º and 74º.
The characteristic black, raised spots are stromata, which contain spores. As new spots form, more spores are produced. This cycle can repeat as long as weather conditions are conducive.
Tar spot can only infect living tissue, said Kness, “but at the end of the season, tar spot symptoms are still prevalent on leaves because the stromata don’t just disappear.”
A leaf at physiological maturity can have clear signs of tar spot because the lesions were formed when the tissue was green and alive.
A hand lens during scouting can help identify stromata, which are slightly raised and embedded in the tissue. Scratching at the spots with a pocketknife or fingernail won’t remove the spots, which is another good indicator. If black dots on the upper leaf surface come off fairly easily, the dots are likely insect frass. At harvest, there may be fuzzy black lesions on leaves, which are secondary decomposing fungi – a natural process.
A tar spot infection can result in significant yield loss – up to 60 bushels/acre. Tar spot can also lead to early senescence. A plant that’s green and healthy can turn brown and dead in about two weeks, which impacts grain yield and presents a challenge for making corn silage due to suboptimal moisture.
Having tar spot on the farm is no one’s fault – it can spread no matter what precautions are taken. “If you do get tar spot on your farm, it’s heavily weather-dependent,” said Kness. “It isn’t the end of the world. We’re still dealing with other diseases and some are far worse than tar spot. However, tar spot could be bad.”
When black layer is reached at physiological maturity, there’s still a lot of green tissue on the plant for several weeks. Tar spot may continue to multiply and grow on tissue, but it has no effect on yield because the plant has reached black layer. Kness added that the wide use of fungicides on corn keeps it green longer, which allows the disease to progress and multiply after black layer, making it look worse than it is.
“Temperature is the most crucial variable,” said Kness, reviewing a Midwest study. “Temperatures over 73º significantly reduced tar spot infections. They also found that humidity plays a role – under 90% humidity for two to three weeks inhibits growth. Early leaf wetness aids infection because it helps spores germinate. However, extended wet conditions inhibit further germination of more spores.”
When scouting for tar spot, be thorough. Field edges and ridges may show tar spot before it appears in the center of the field because spores from neighboring fields may be contributing to an outbreak. Since tar spot can be found anywhere on the plant, it’s important to check the entire plant.
Several management tools help manage tar spot. While there are no truly “resistant” hybrids, Kness said some tolerate tar spot better than others. Select hybrids with good ratings for tar spot, especially if corn is following corn because field residue is a source of tar spot inoculum.
Up until now, hybrid resistance was based on field observations, but the genes that play a role in tar spot resistance have been identified, and seed companies will be using that information to develop more resistant varieties.
Another management tool is selecting earlier maturing hybrids or modifying planting dates so corn isn’t in tassel or early grain fill in mid- to late August. “That’s when disease pressure for tar spot in a typical year is high,” said Kness. “Having corn pollinated in mid-July rather than later is better.” Weather forecasting and crop risk tools are useful for monitoring conditions for tar spot.
Because tar spot overwinters in old corn residue, Kness suggested thinking of residue as the initial primary inoculum for tar spot. Spores come out of the residue into the next crop, so corn after corn, especially in a no-till system, isn’t ideal. Accelerating decomposition of residue maybe be somewhat helpful; disking and vertical tillage can also help.
Using fungicides to manage tar spot isn’t settled science. Some believe two applications will be necessary; however, many data point toward a single, well-timed application made between VT (tassle) and R2 (blister) stages, which is the same application time for other corn diseases.
Remember to check labels to make sure the product is labeled for corn tar spot. Products registered for use on corn as a fungicide will control tar spot. Products with two or three modes of action tend to get a better response than single mode of action.
“Good hybrids and crop rotation will be your best practices,” said Kness. “Scout fields around tasseling to be sure you don’t have tar spot problems and look for other diseases as well. Use fungicides where they’re appropriate.”
This crop risk tool can help farmers with disease management decisions: cropprotectionnetwork.org/news/new-crop-risk-tool-enhances-disease-management-decisions.
