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:max_bytes(150000):strip_icc()/Tar-Spot-Jared-Goplen-Wyffels-IMG_7763-5cb41072e7d34537b0992f26a3b2a00b.jpg "35 Iowa Counties Report Tar Spot Infections in Corn Crops")
According to the latest Crop Protection Network map, positive cases of tar spot have been reported in 35 Iowa counties so far this growing season.
First reported in the Corn Belt in 2015, the disease has been in Iowa since 2016. All 99 Iowa counties have a history with the disease.
This season, tar spot has also been confirmed in neighboring Nebraska, Missouri, Illinois, Minnesota, South Dakota, and Wisconsin. Farmers and agronomists in Kansas, Indiana, Ohio, Michigan, Kentucky, Pennsylvania, New York, and Maryland have reported positive cases in 2025 as well.
Weather
Six consecutive weeks of unseasonably wet conditions have increased the risk for disease.
“Last week … a heat dome settled over the Midwest, producing some sweltering heat and humidity,” said Iowa Secretary of Agriculture Mike Naig in a press statement. “Forecasts show chances for a few more thunderstorms this week, followed by a welcome break from the hot weather. With a few days to go in the month, preliminary estimates indicate it could rank among the top 10 wettest Julys on record.”
Positive 2025 Cases
Angie Rieck-Hinz is an Extension field agronomist for Iowa State covering north central Iowa. She observed tar spot in Franklin County last week, but noted, “across all the fields I was in last week, the disease pressure is still low and everything was below the ear leaf.” In the latest regional crop update blog Rieck-Hinz and her peers encouraged farmers to scout.
Crop Protection Network
The following Iowa counties have reported positive cases this year:
- Plymouth County
- Fayette County
- Grundy County
- Buchanan County
- Dubuque County
- Crawford County
- Story County
- Marshall County
- Tama County
- Jones County
- Harrison County
- Audubon County
- Guthrie County
- Polk County
- Jasper County
- Poweshiek County
- Iowa County
- Cedar County
- Muscatine County
- Keokuk County
- Mahaska County
- Marion County
- Warren County
- Madison County
- Adair County
- Cass County
- Pottawattamie County
- Montgomery County
- Adams County
- Union County
- Fremont County
- Page County
- Taylor County
- Wayne County
- Appanoose County
What Should Farmers Do
According to the Crop Protection Network, there is likely a benefit from spraying, if it hasn’t been done already, up until R3, or the milk growth stage. There could be benefit to spraying at R4, or the dough growth stage, in cases of severe disease pressure.
“Yield loss is primarily a result of premature plant death, reduced kernel weight and poor kernel fill,” explained Wyffels agronomist Jared Goplen in a recent Between The Rows article.
Matt Bennett is an Illinois based grain market analyst and consultant. On a recent Agri-Talk episode he said, “I know everyone’s tight on cash right now. We all know that. This is a tough game we’re playing, but tar spot, as we know, can take 50, 60 bushels an acre out of your corn crop.”
He added, “With this environment we’re in, let’s walk these fields. You’re probably not going to see near the loss in yield from this tassel wrap situation as what we will from disease if we don’t address it appropriately.”
Iowa Corn Progress
USDA published the latest Crop Progress report July 28.
According to the report, 84% of Iowa’s corn had reached the silking stage as of July 27. This is ahead of the five-year average and last year at this time by 1 percentage point.
As of July 27, 34% of the state’s corn crop had reached the dough stage. This is ahead of the five-year average by 8 percentage points and two points ahead of last year’s progress.
Iowa Corn Condition
Wyffels agronomist Eric Wilson noted some signs of nitrogen deficiency in areas that have received heavy rainfall, “which is concerning as we move into grain fill stages,” he said.
On the western side of the state, Crawford County farmer Kelly Garrett said, “The corn crop is the best I’ve seen.”
USDA rated Iowa corn 1% very poor, 2% poor, 10% fair, 59% good, and 28% excellent.
That is better than the national average rating of 2% very poor, 5% poor, 20% fair, 53% good, and 20% excellent.
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