-Submitted photo
A corn leaf with curvularia is shown in this photo.

SUTHERLAND — Sprayers and spray planes have been applying corn fungicide to fields in northwest Iowa since mid-July.

But Iowa State University Professor and Extension Field Crops Pathologist Alison Rob-ertson told attendees at a July gathering at the Northwest Research Farm near Sutherland that corn producers would be wise to weigh every pro and con, with tight margins on the line, to save money wherever they can.

She said the best time to apply fungicide to manage diseases like tar spot, southern rust, northern corn leaf blight and gray leaf spot is from the tassel stage (VT) through milk stage (R3). But she said producers should consider the risk of disease during grain fill.

“If conditions are hot and dry, you could delay your fungicide application at tassel by a couple of weeks so you could push it forward into grain fill, so it lasts throughout grain fill and more towards the end of it when you’re going to need it — when tar spot starts ramping up again,” said Robertson.

She said southern rust concerns came to the forefront after last year’s showing.

-Submitted photo
This corn field is showing the effects of southern rust.

“Everybody is concerned about (it), but at the moment, southern rust development in the south is very much delayed compared to what it was last year,” said Robertson, adding that it’s only been reported in four Texas counties. That compares to last year, when it had been reported in several Texas counties, along with counties in Mississippi, Alabama, Georgia and Arkansas by this point.

She said June’s cooler-than-normal temperatures would not have favored the arrival of southern rust. June of 2025 showed two to three times more precipitation than normal, but not enough heat. This year she said temperatures have been warming up, but without precipitation.

“That means I’m not concerned about southern rust,” said Robertson. “We may see it, but probably towards the end of the growing season when it doesn’t make an impact.”

She said tar spot has been reported in central Iowa, and that it favors the cooler temperatures experienced early in the growing season. Once the hotter temperatures of July and August arrive, Robertson said they most likely “drastically slow it down” or possibly even stop it from developing in the canopy.

Robertson said it will most likely start to move as the cooler temperatures of mid-August to early September arrive.

-Submitted photo
Tar spot has invaded this corn plant.

She said northern corn leaf blight and gray leaf spot are “always around,” but that modern hybrids have more disease resistance to those two threats.

Susceptibility

vs. resistance

Robertson said she conducted trials at three Iowa locations involving a hybrid that was resistant to — and one susceptible to — tar spot. Those same hybrids selected also featured one resistant to southern rust and one susceptible to it as well.

“We found that a fungicide would reduce disease on both hybrids, but the amount of yield response was greater on the susceptible hybrid (which tends to get more and severe diseases easier). That yield response gave us a positive return on investment from that fungicide application — whereas on the resistant hybrids (which still gets disease, but has defense mechanisms in place), we didn’t get a big enough yield response to pay for that application, giving us a negative return on investment.”

Robertson said fungicide will reduce disease on a susceptible hybrid from 50 percent to 10 percent, and will reduce disease on a resistant hybrid from 10 percent to 3 percent, for example.

“Because you’ve reduced the amount of disease, the plant is able to photosynthesize more effectively and produce more yield. But we found that with the susceptible hybrid we had an 80 percent reduction in disease, so you can imagine that that 50 percent of disease was impacting yield much more than the 10 percent in the resistant hybrid,” said Robertson.

She said yield on a susceptible hybrid was more protected because more of the disease was reduced. The yield on the resistant hybrid, she said, was still protected from spraying fungicide, but the significantly fewer bushels protected were not enough to pay for the fungicide application, as compared to bushels protected on the susceptible hybrids. Those yields (from susceptible hybrids) that were protected, she said, made it more sensible to pay for the fungicide application.

“Those findings were interesting because when farmers buy their hybrids, they buy ‘defensive’ hybrids (meaning they have disease resistance), whereas other hybrids are what farmers call ‘race horse’ hybrids — all about yield, so they don’t have a lot of disease resistance. So the data from last year suggests to me that under that very high disease pressure we had, we do not need to apply fungicides to defensive hybrids because that resistance is going to protect our yield enough, and we won’t get a return on investment from applying that fungicide.”

She said it would be more worth targeting a fungicide application on the “race horse” hybrids, since they are more susceptible to disease.

That being said, Robertson said corn producers should be looking at every corn field and making the decision according to that field’s needs.

“You don’t have to spray every field,” she said. “We should be targeting those fields that are planted to race horse hybrids, and those planted to defensive hybrids — it probably will not be worth it to spray. We actually have data now to support that.”

Robertson said seed companies use number scales to rate disease resistance, and using a resistant hybrid (as opposed to a susceptible one) should be the starting point of a crop management program.

Early vs. late planted

Robertson also did field trials last year at three Iowa locations to determine the implications of planting dates in regard to disease resistance. Findings showed that, based on planting the same hybrids at both planting times, diseases (such as southern rust) were more severe on corn planted in mid to late May compared to corn planted in mid to late April.

“Maybe you want to target those acres you planted late, and whether or not they are ‘race horse’ hybrids, as you make your decisions. You might want to consider spraying those fields first with fungicide — and maybe not even spray the early-planted fields, especially if they are planted to a defensive hybrid,” said Robertson.

Moreover, she said early-planted corn — with tasseling in mid-July and disease ramping up in August — means that producers are already halfway through grain-fill at that critical time in August.

“If you plant late and tassel the third or fourth week of July, you’ll start with grain fill just as that disease is ramping up. The earlier that disease occurs in grain fill, the more impact it will have on yields because there will be more cycles of disease throughout grain fill, said Robertson.

She said if disease increases halfway through grain fill, the number of disease cycles can be significantly reduced.

“It’s further along in its development, and once the plant matures, then there’s no green tissue to infect, so a disease stops; if there is green tissue to infect, a disease will just keep going,” said Robertson.

She reiterated the importance of also scouting fields to determine if fields need to have fungicide applied.

Corn producers can gain insight into their decision-making process regarding fungicide application by going to the Crop Protection Network’s (CPN) Crop Risk Tool, where some links will allow producers to enter their own information to see if the risk outweighs the benefits of applying fungicides on their own acres.

Those CPN links include: https://cropprotectionnetwork.org/crop-disease-forecasting), https://cropprotectinonetwork.org/publications/funficide-efficacy-for-control-of-corn-diseases, and also https://cropprotectionnetwork.org/fungicide-efficacy-tool.