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Tar spot found in Lancaster County cornfield

TAGS: Corn
Photo courtesy of Holly Spangler Close up of tar spot scattered across the upper and lower leaf surfaces of corn
LOOK FOR BLACK SPOTS: Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces of corn. It’s easiest to see on green tissue but can also be found on dried leaves and fodder.
A 2018 tar spot outbreak caused widespread yield losses in the Midwest.

A widespread outbreak of tar spot in 2018 led to Midwest farmers losing up to 60 bushels an acre. Now, tar spot is in Pennsylvania.

Penn State Extension recently identified the fungal disease in a field in western Lancaster County near the Susquehanna River.

Alyssa Collins, plant pathologist and director of the Southeast Agricultural Research and Extension Center, says that other fields around the farm also have the disease but a field in Lebanon County that initially was suspected of having it has been ruled out.

Tar spot was first identified in the U.S. in 2015. Three years later, a major outbreak in northern Indiana spread to surrounding states. It has since been identified in Illinois, Iowa, Michigan, Minnesota and Wisconsin.

U.S. map of tar spot of corn (Phyllachora maydis)

Tar spot is caused by the pathogen Phyllachora maydis. Under favorable conditions it can cause severe yield losses to susceptible corn hybrids. Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces of corn. It’s easiest to see on green tissue but can also be found on dried leaves and fodder.

“The pathogen that causes tar spot is a fungus, which reproduces and spreads via airborne spores and in crop residue. We don’t yet have enough information to determine if it arrived with weather systems or if it may have moved here on equipment, with fodder, on seed, or in some other way,” Collins wrote in an email this week.

Tar spot is believed to have originated from Latin America — specifically Central America and Mexico — where conditions are ripe for its spread: Temperatures of between 59 and 70 degrees F, relative humidity of at least 85% and more than seven hours of leaf wetness.

Managing for next year

Since it was found so late this season there is little concern of a major outbreak, so growers should be looking at ways to prevent it next year.

Researchers at Purdue Extension believe the fungus that causes tar spot overwinters in infected corn debris on the soil surface. As a result, growers are often told to rotate away from corn the following year to allow the residue to decompose, or to incorporate tillage to bury the inoculum.

In Latin America, growers use resistant corn hybrids to control its spread.

“We do not yet know how well the pathogen may or may not survive in our cropping system and climate conditions,” Collins says. “We should, however, assume that now that it has arrived, there will be some level of inoculum available to cause infections in most areas.

“The next most important determining factor will likely be the weather in future seasons, as it is for most foliar diseases of corn. There is some evidence that if we experience persistently humid conditions and temperatures in the 60s, this could favor disease development. We have much to learn about the impacts of various crop management approaches such as rotation, tillage, residue management and hybrid choice.”

Fungicides can help control the disease once it’s in a field, but it’s unclear whether they’ll be effective in the Mid-Atlantic and Northeast.

“All of the information we know about the disease is based on limited research and the experiences of our counterparts in the Midwest over the last few years,” she says. “No research has yet been done in Mid-Atlantic/Northeast conditions with regionally adapted hybrids. But we can surmise from fungicide trials conducted by our colleagues that a number of existing fungicides can be effective in reducing tar spot when applied at appropriate timing.

“A late-developing infection like the ones we’re seeing in Pennsylvania does not warrant any kind of fungicide application for control. However, when tar spot develops prior to grain fill and weather conditions favorable to the fungus persist, we know it can cause yield and quality impacts, and a fungicide may become an economically important part of management.”

The Crop Protection Network’s Corn Disease Working Group has developed ratings for how well fungicides control major corn diseases in the U.S., including tar spot.

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