For Andrew Kness, agronomy Extension agent with the University of Maryland, getting a recent phone call about possible tar spot in a Maryland field wasn’t a surprise.
“This is probably the tip of the iceberg. I do think that the next couple of years, we're just going to start seeing it as very common. It could be as common as gray leaf spot, or even worse," he told a group of growers at The Mill’s annual crop showcase.
The state’s first confirmed positive test for tar spot was late last month at Clear Meadow Farm, where The Mill holds its annual crop showcase. The topic was added at the last minute to The Mill’s agenda, and Kness is urging growers who see it to report it as soon as possible.
“We want to know where it is; we want to know where it’s spreading,” he says.
Since July 13, tar spot has been confirmed in at least 19 counties in Michigan; five counties across south-central and eastern Pennsylvania, and Crawford County in northwest Pennsylvania; and now in Maryland, although Kness thinks it’s likely spread to other counties.
Although tar spot can be misdiagnosed, there are a few things that make it easy to identify, he says. For one thing, the raised black spots on the leaves, which can mimic other plant diseases, can’t be scraped off.
Soaking a leaf in a little bit of hand sanitizer, or alcohol, Kness says, is also an effective way to tell if what you’re seeing is tar spot or something else. If the spots come off, then it isn’t tar spot.
The disease is caused by the pathogen Phyllachora maydis and is believed to overwinter and spread in crop residue. It is believed to only affect corn, and experts recommend rotating away from corn for at least a year to break the disease cycle, especially in a field that has had tar spot before.
Some fungicides can be effective, but it really depends on timing.
“Fungicides from tasseling to R1 are likely effective,” Kness says. “This is where they are finding the biggest bang for their buck in the Midwest, and there are folks looking at earlier applications in the growing season.”
Under favorable conditions, tar spot can cause severe yield losses; more than 50% yield loss has been reported in the Midwest. Tar spot in the upper canopy of a plant is particularly worrisome, he says, because this is where it can most affect yield.
“This is where the factory is,” Kness says. “Lower in the canopy, it definitely could spread to the higher parts, if weather is good for it.”
According to the online Crop Protection Network, tar spot severity on ear leaves at R5 (dent stage) can exceed 50% in susceptible hybrids when conditions are right for disease development. The leaves of infected plants will prematurely die when severity is about 30% or more.
The best conditions for tar spot development are temperatures between 59 and 70 degrees F, relative humidity of at least 85%, and more than seven hours of leaf wetness.
Judging by the fact that the disease has returned to places where it was found a few years ago, such as Lancaster County, Kness says that it is safe to assume that the disease can thrive in the mid-Atlantic and Northeast.
Choosing hybrids that have some sort of resistance to tar spot will be crucial this fall, he says, if you’re in an area where the disease has been reported. The pathogen that causes tar spot overwinters in residue, so working the residue into the soil usually helps. Rotating to another crop to break up the disease cycle will likely also help.
The tar spot pathogen is believed to have come from Central America and Mexico where conditions are ripe for its development and spread. It was first identified in the U.S. in 2015, and in 2018 a major outbreak in northern Indiana spread to surrounding states.
In Latin America, growers use resistant corn hybrids to control its spread.