Tar Spot Continues to Spread in Corn

JEFFERSON CITY, Mo. (DTN) -- It's been three weeks since the first tar spot lesions of 2023 were found in central Iowa. Since then, the fungal disease has made its appearance in cornfields in six states and one Canadian province. Extension plant pathologists continue to urge growers to remain vigilant against the disease as the corn crop in many regions is now reaching critical reproductive stages.

In Missouri -- where the July 10 USDA Crop Progress report indicated that more than half of the state's corn was silking -- tar spot was most recently found on July 11 in west-central Lafayette County. It represented the first time the disease had been detected south of the Missouri River in the Show-Me State.

"Lafayette County might seem like a bit of an outlier, but I think it's just an indicator of how much of the state probably has the pathogen present," said Mandy Bish, coordinator of the University of Missouri Integrated Pest Management program, during a phone interview with DTN. "We know that Phyllachora maydis can survive Missouri winters, so it makes sense that it continues to increase."

Tar spot infects the upper side of corn leaves and appears as raised hard, black spots, known as stromata. The spots are typically one-sixteenth to three-quarters of an inch in diameter and can protrude through the tissue and be seen on the bottom side of leaves. Within each stroma, the fungus produces spores that continue to infect throughout the growing season. Occasionally, tan to brown lesions with dark borders can develop around the stromata. These are known as fisheye lesions because of their appearance.

With long periods of favorable environmental conditions -- such as morning dews, lingering fogs, and rainy days -- high disease severity can reduce yield and test weight and result in plant lodging. Yield losses to tar spot can vary depending on the time of disease onset, weather conditions and hybrid susceptibility. Losses of 50 bushels per acre (bpa) have been observed in previous seasons. An integrated management approach can help protect against losses due to tar spot.

Bish said growing season conditions in Missouri have made for a unique scenario for tar spot so far in 2023. While cooler-than-normal minimum air temperatures and cooler dew point temperatures in June favored tar spot development, dry conditions weren't conducive.

"We've seen more incidence earlier than normal, but we haven't seen the severity yet increase," she explained.

Dry spring conditions also meant that fields were fit for planting corn earlier than average. As a result, much of the state's corn crop was already close to tasseling as the first reports of tar spot began.

"So, because we got our corn crop up so early, the timing of our fungicide application almost serendipitously was at that recommended VT growth stage for tar spot," Bish said. "We're not out of the critical window yet, but people are already applying fungicide. So, I think by default, the applications that have gone out should be effective."

In Indiana, tar spot has been found in three northern counties -- La Porte, Marshall, and Porter -- so far in 2023. As in Missouri, the cases of the disease haven't yet been severe, said Darcy Telenko, Purdue University field crops pathologist.

"We know that it's active, but what we've found is extremely low and probably still hard to find," she said. "If you've had a history of the disease, be ready for it."

Telenko explained that for the disease to occur, the pathogen and host must be present together within a favorable environment. Recent rains may have made conditions conducive, so it's important for growers to scout.

"Use the Tarspotter app and get out in the field," she said. "Be looking at the leaves at knee level because it's going to come in from the lower canopy in those fields with a history."

Here are some tips for scouting and identifying tar spot.

-- Start by scouting cornfields where the disease has occurred before and where neighboring fields have previously had tar spot. Inoculum overwinters in corn residue and can survive extreme temperatures. Spores are dispersed via wind and rain splash.

-- Scout susceptible areas with cornfields where tar spot is more likely to be present. This includes areas where leaves may stay wet longer due to early morning fog, such as river bottoms, low-lying areas and near windbreaks. Check field edges as well.

-- Check leaves in the lower part of the canopy first and work up the plants. Look for the telltale stromata that appear as small, raised, irregular-shaped black spots. The spots won't rub off with water.

-- Confirm tar spot. While tar spot is distinct, it can be confused with corn rust infections and insect frass (poop). Tar spots are firm, appear mostly smooth, and do not rub off or break open. Send samples with suspected tar spot to your state plant diagnostic lab to help track.

Several years of data have shown applications of fungicides between VT to R3 corn growth stages manage tar spot effectively. If growers find the disease in the lower canopy while corn is at tassel and weather conditions look conducive, then a fungicide application would be recommended. Bish said growers can gauge their scouting skills on the Crop Protection Network website.

"There are quizzes you can take to determine 1% coverage versus 2% and so on," she said. "We need to protect the crop before tar spot reaches 5% coverage. At 5%, it's too late to prevent yield losses."

Bish noted that even though instances of tar spot haven't been severe so far this year in Missouri, growers still need to pay attention to the disease.

"It can be easy to get complacent," she said. "In Missouri, we seem to have two audiences of farmers. We have those farmers who've seen tar spot for a few years, but it's always come in later. So, they haven't seen yield hits.

"But it took about four years in states like Illinois and Indiana for there to be enough inoculum that they started seeing substantial yield losses," she continued. "So, I don't want those farmers to get complacent if they've seen it for three or four years because we know the inoculum is building.

"Then we have this other audience of farmers where the disease is really new," she said. "They see one lesion and want to spray. Somewhere in the middle is kind of where we need to be."

To track where tar spot has been confirmed so far this season, go here: https://cropprotectionnetwork.org/….

To quiz your tar spot scouting abilities, go here: https://severity.cropprotectionnetwork.org/….

For the latest field crop disease updated from Indiana, go here: https://indianafieldcroppathology.com/….

The Tarspotter app is available for both Apple and Android devices. Download for Apple devices here: https://apps.apple.com/….

Download for Android devices here: https://play.google.com/….

Jason Jenkins can be reached at jason.jenkins@dtn.com

Follow him on Twitter @JasonJenkinsDTN

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