A new reality in Ohio corn: living with tar spot

A walk through an Ohio cornfield in mid to late summer can reveal a great deal about the season. Healthy green leaves stretching toward the sky are a welcome sight, but in recent years many farmers have become increasingly concerned when they notice tiny black specks scattered across the leaf surface. Those black spots may be tar spot, a disease that has rapidly become one of the most important threats to corn production in Ohio. Since its first confirmation in the state in 2018, tar spot has spread across much of Ohio and is now considered a disease that growers should expect to encounter whenever weather conditions are favorable. Tar spot is caused by the fungus Phyllachora maydis, which infects corn plants and can significantly reduce yields when disease development occurs early enough and reaches high levels. What once appeared to be a relatively minor disease has become an annual concern throughout much of the Corn Belt. Researchers and growers now recognize that under the right environmental conditions, tar spot can develop quickly and cause substantial yield loss. The disease gets its name from its distinctive appearance. Infected leaves develop small, raised black structures called stromata that resemble flecks of tar splashed onto the leaf. These spots may appear on both the upper and lower leaf surfaces and can eventually spread to leaf sheaths, husks, and other plant tissues. Unlike insect frass or dirt, the spots are embedded within the leaf tissue and cannot easily be rubbed off. A simple field test is to moisten a thumb and attempt to wipe the spot away. If it remains firmly attached, tar spot may be present. Many Ohio farmers first begin noticing symptoms after the crop canopy has closed and humidity levels increase within the field. The fungus thrives in environments characterized by moderate temperatures, high relative humidity, and extended periods of leaf wetness. Frequent rainfall, heavy dews, cloudy weather, and prolonged moisture on leaf surfaces create ideal conditions for infection and disease development. Temperatures generally ranging from the low 60s to low 70s Fahrenheit are particularly favorable for tar spot development. Because weather plays such an important role, disease severity can vary dramatically from year to year. Some seasons may see only scattered infections with little impact on yield, while others can experience widespread outbreaks. Recent Ohio observations have shown that dry conditions can slow disease progression, whereas wet summers often create conditions that allow tar spot to spread rapidly through fields. One reason tar spot continues to challenge growers is its ability to survive between growing seasons. The fungus can overwinter on infected corn residue left in the field. When favorable weather returns the following year, spores released from that residue can serve as an early source of inoculum for new infections. While researchers continue to study the relationship between residue levels and disease severity, fields with a history of tar spot generally warrant careful monitoring throughout the growing season. The consequences of severe tar spot infections can be significant. As disease severity increases, the amount of healthy leaf area available for photosynthesis declines. The plant loses its ability to efficiently capture sunlight and produce the carbohydrates needed for grain fill. In severe cases, leaves may die prematurely, reducing kernel weight and ultimately lowering yield. Researchers have observed that disease pressure can build rapidly on susceptible hybrids when weather conditions remain favorable over an extended period. Fortunately, growers have several tools available to help manage the disease. Ideally, hybrid selection would be the most important long-term management strategies. However, the disease is relatively new, and seed companies have not had enough time to develop resistant hybrids, but significant differences in susceptibility do exist among commercial hybrids. Choosing hybrids with stronger tar spot ratings can reduce disease severity and lower risk when favorable conditions develop. Scouting fields is very important. Regular field monitoring allows growers to determine when symptoms first appear and whether disease is progressing. Early detection helps inform management decisions and can prevent unnecessary fungicide applications. Extension specialists encourage growers to scout fields routinely, particularly around tasseling and silking. Fungicides have also become an important component of tar spot management. Research conducted across multiple states has shown that applications made between tasseling (VT) and the milk stage (R3) generally provide the most consistent disease suppression and yield protection. Products containing multiple modes of action tend to offer the greatest level of control while helping reduce the risk of fungicide resistance development. However, fungicide decisions should always consider disease risk, field history, hybrid susceptibility, application cost, and grain price. Not every field will benefit economically from treatment every year. If tar spot does not first appear until after the milk stage, the cost of the fungicide at this late of application may cost more than the potential yield loss. Today, tar spot is firmly established in Ohio corn production. Rather than viewing it as an occasional problem, growers should consider it a disease that requires annual awareness and monitoring. Success comes from combining multiple management practices, including hybrid selection, routine scouting, informed fungicide decisions, and sound agronomic management. While tar spot is unlikely to disappear from Ohio fields, farmers who stay vigilant and respond to changing conditions can substantially reduce its impact and protect yield potential throughout the growing season. Ed Lentz is agriculture and natural resources educator with Ohio State University Extension — Hancock County.