Tar Spot Found in Iowa, Missouri Corn

Farmers Should Begin Scouting Cornfields for Tar Spot Where Risk is High

Jason Jenkins
By  Jason Jenkins , DTN Crops Editor
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The first tar spot of the 2023 season has been found in cornfields in six central Iowa counties and one in Missouri. A magnified view of a stroma displays the disease's telltale black spots. (Photos courtesy of Matthew Vandehaar)

JEFFERSON CITY, Mo. (DTN) -- An eagle-eyed crop scout in Iowa has found the first tar spot lesions of the year. An Iowa State University undergraduate student working with Pioneer field agronomist Matthew Vandehaar observed the disease on June 21. In all, tar spot has been found in six central Iowa counties, including Poweshiek, Tama, Marshall, Jasper, Story and Polk, as well as Holt County in northwest Missouri.

"It's a very low level of tar spot, literally one spot on one leaf of a single corn plant in the field," said Iowa State University Extension plant pathologist Alison Robertson during a phone interview with DTN. "Though it's an extremely low level, it does tell us that we have had conditions to favor the production of spores to start the disease."

Caused by the fungus Phyllachora maydis, tar spot infects corn leaves and creates black spots called stroma. Within the stroma, the fungus produces spores that continue to infect throughout the growing season. With long periods of conducive 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. An integrated management approach can help protect against losses due to tar spot.

The lesions appear as raised hard, black spots, known as stromata, that form on the upper side of corn leaves. 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. While tar spot is distinct, it can be confused with corn rust infections and insect frass.

"You have to be really careful that the black dots aren't bug poop," Robertson said. "Tar spot lesions are in the leaf, so if the black spots wipe off the leaf with a little water, it's bug poop."

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 have been observed in previous seasons.

While detection of tar spot on June 21 was the earliest that the disease has been found in Iowa, it has been found earlier in other states. The disease was first found in the United in 2015 in seven counties in northwest Indiana and 10 counties in north-central Illinois.

In a June 23 blog post, Robertson wrote that cycles of wet and dry weather appear to drive tar spot development. She told DTN that pathogens like tar spot are patient, waiting for favorable conditions for growth. At this time, she is not recommending a fungicide application.

"The disease develops exponentially. It starts off very slow before ramping up when the environment allows," she said. "Most of the corn here in Iowa is sitting at about the V12 growth stage, so there's really no point in going in and spraying now. If we can wait until tassel or the blister stage, that's going to be way better because then we're protecting the corn crop through grain fill. Several years of data have shown applications of fungicides between VT to R3 manage tar spot effectively."

Robertson recommended that farmers use the Tarspotter app to estimate disease risk.

"They can plug their field into the app and get an idea of what the risk is like currently," she said. "If you're at high risk, then it's a good idea to go in and scout. It does not mean spray necessarily. You need to watch to see if the disease is progressing."

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: Tar spots are firm, appear mostly smooth, and do not rub off or break open. Not all black spots are tar spot, though.

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

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

Jason Jenkins