Kaitlyn Bissonnette, University of Missouri Extension plant pathologist, is warning farmers of two corn diseases that surfaced in 2019 but may be retuning this year.
In the latest MU Integrated Pest Management newsletter, Bissonette looks at bacterial leaf streak and tar spot on corn. These two diseases have been in many Midwestern states for five years, but Missouri growers noticed them just last year.
She shared a few quick facts and advice on where to scout.
Wet weather triggers bacterial disease
Bacterial leaf streak of corn was confirmed in samples from Ray and Chariton counties in July 2019, Bissonette says. The pathogen was first detected in the U.S. in 2014. It appeared in Nebraska in 2016 and quickly spread throughout the state. Today, bacterial leaf streak is in nine other states, including Kansas, Nebraska, Iowa, Illinois and Missouri.
“With this disease relatively new to the United States,” she says, “information is still being gathered about differences among hybrids and management strategies.”
Bissonette says bacterial leaf streak is likely to show up following windy, wet weather because bacterial diseases require wounds and water to infect. She notes that farmers may confuse bacterial leaf streak with gray leaf spot.
“If you are observing inadequate control of gray leaf spot in your field, check for the possibility of BLS,” she says. “Detecting areas where BLS is present is critical because the pathogen overwinters on residues at the soil surface and on weedy hosts.”
Bissonette says bacterial leaf streak can occur anywhere in the canopy at any time. However, she notes it generally follows “high winds, hail damage or other inclement weather, which allows for wounds to occur and water to splash the bacteria into the wounds on the corn leaves.”
Symptoms of bacterial leaf strike appear as long brown necrotic lesions with irregular lines. The disease also presents with a yellow halo, which farmers can see when they hold it up to a light source.
Little-known tar spot
Tar spot also arrived last year in Scotland, Lewis and Clark counties in northeast Missouri. Researchers are still trying to understand this fungal disease that appears as small, raised, black spots across the upper and lower corn leaf surface.
Bissonette says the disease spread rapidly throughout the Midwest since it first appeared in Indiana and Illinois in 2015. “With little known about this disease or the pathogens that cause it,” she says, “researchers are working to come up with management solutions.”
In areas where tar spot is well established, differences have been observed among hybrids, fungicides and cultural practices to control this disease. Missouri is on the developing front of tar spot, she notes, so disease levels are currently low, making the focus scouting and detection of new counties. “This is especially important as this pathogen overwinters on corn residue in the field," Bissonette says.
Farmers may confuse tar spot with just dirt on leaves, insect waste or southern rust late in the season as the pustules turn black from a change in spore phases.
Bissonette say farmers can find tar spot in the lower canopy of fields. Scouting should start in mid- to late August and continue through harvest, as tar spot can attack both green and brown leaf tissue.
Occasionally, there can be a tan "fish-eye" or tan diamond-shaped lesion surrounding the black dots (stroma) on the leaves. In severe cases, plants will begin to prematurely senesce with yield loss becoming significant.
If farmers suspect either tar spot or bacterial leaf streak, Bissonnette says to contact her at firstname.lastname@example.org or contact the local MU Extension office.