Remember when your mom licked her thumb or finger to swipe at something stuck to your face? And, when she couldn't dislodge it, she'd dig a little deeper -- only to discover it was a freckle (or alas, permanent magic marker?)

It's time to take Mom's technique (or something akin to it) to the cornfield to scout for the disease called tar spot.

As with most corn diseases, early detection is crucial for best management practices. But tar spot can be tricky to decipher from other diseases, insect frass or simply dirt. That's where the "mommy spit" scouting technique comes in.

Kansas State University Row Crop Pathologist Rodrigo Onofre uses a version of it to sort the differences.

"For me, what really helps is you know if you get a wet cloth and try to clean that leaf," he said in a news release. (https://www.ksre.k-state.edu/…)

"If it's tar spot, it's not going to rub off, but everything else will clean off."

Tar spot showed up in early June in several Midwestern states and has been adding some territory since. There's a map that tracks where it is being found. Find it here: https://corn.ipmpipe.org/….

So far, counties in Indiana, Iowa, Nebraska, Kansas and Missouri have reported positive tar spot detections.

As DTN reported in an earlier article (https://www.dtnpf.com/…), the key environmental factors favoring tar spot are moderate temperatures, high humidity and prolonged leaf wetness, particularly in areas with a history of the disease and a presence of corn residue. Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces.

Looking back during the past few years of detections and reports, tar spot in June does not necessarily lead to widespread epidemics in July and August. Keep in mind that some corn hybrids are more susceptible than others, too.

The question of when to spray fungicides is covered at length in a report from the Crop Protection Network (CPN).(https://cropprotectionnetwork.org/…) As that report indicates, research from across the Midwest and Canada has consistently shown that fungicide applications at VT to R3 have been the best timing to manage the disease.

Iowa State University plant pathologist Alison Robertson covered tar spot in a blog titled "Tar Spot in Central Iowa Should Not Be a Surprise," that she posted recently: see https://crops.extension.iastate.edu/….

Darcy Telenko's lab at Purdue University tracks tar spot here: https://indianafieldcroppathology.com/….

The University of Missouri recently covered tar spot in this article along with photos to help identify it: https://extension.missouri.edu/…

Crop Lookout is a real-time interactive map that highlights emerging field crop issues (including tar spot) across the U.S. and Canada from the CPN. Updated throughout the growing season by university extension specialists and research partners, the tool visualizes reports of diseases, pests, and other agronomic challenges as they develop. (https://cropprotectionnetwork.org/…

DTN's article discussing a new crop risk calculator from CPN can be found here: https://www.dtnpf.com/…

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on social platform X @PamSmithDTN

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