Southern Rust Set To Take Big Bite Out Of Midwest Corn Crop?

If one picture is worth a thousand words, then the video Iowa farmer Dan Striegel shot last week must be worth thousands more. In the video, Striegel is shown harvesting a field of emerald-green corn enveloped in a cloud of orangish-red southern rust.

“We were just getting that field opened up, and I looked over and saw that dust boiling up out of the chopper, so I shot the video,” Striegel says.

To date, Striegel’s video has garnered more than 48,000 views on X, formerly Twitter.

“We’re in southeast Iowa, Keokuk County, and I think the southern rust is as bad here as it is anywhere,” Striegel adds. “Every field you walk in, if you’re wearing a white T-shirt, you’ll come out of there red.”

A Red Path Of Disease Mars The Midwest

Expect to see more red T-shirt-clad farmers walking out of cornfields across the upper Midwest, based on what the Crop Protection Network (CPN) southern rust map is showing.

The CPN continually updates its online, interactive map showing the counties by state where southern rust infections are confirmed. Now, in late August, the counties look like red steppingstones. They form a checkered path from southwest Michigan through northern Illinois and Indiana, into southern Wisconsin, across all of Iowa and nearly two-thirds of the way across Nebraska. Eastern South Dakota is also lit up with a string of red counties, as are parts of southern to central Minnesota.

The amount of southern rust present in the upper Midwest is worrisome to Ken Ferrie, Farm Journal Field Agronomist. In severe cases, the disease can wipe out 45% of the yield potential in a field, according to the CPN.

“At most, one in 10 growers in northern Iowa and Minnesota have seen the kind of southern rust some of them are seeing this year,” says Ferrie, who was working last week with corn growers in both states.

“It was a problem in probably eight out of every 10 fields I was in, and they’d all been sprayed at least once,” he says. “Minnesota has a corn crop that’ll knock your socks off – yield potential of 250, 270. I encouraged every grower to spray their field a second time except for two fields. One had been knocked down by hail, and the other had a hybrid that was clean.”

Hybrids Have Little To No Resistance To Southern Rust

A combination of early-season moisture, heat and wind formed the perfect storm for southern rust this season, allowing the disease-causing fungal spores (Puccinia polysora) to move from southern climes up to the Midwest, according to Kurt Maertens, BASF technical service representative for eastern Iowa and western Illinois.

“We’ve seen it all – southern rust, tar spot, northern corn leaf blight, gray leaf spot. Our corn has been inundated with all these fungal diseases, and we started seeing them early,” says Maertens.

If there’s a silver lining to southern rust, it’s that it does not overwinter in corn residue like tar spot does. But like tar spot, southern rust takes advantage of hybrids that have no built-in resistance. For many growers, that was an Achilles heel this season.

“When you’re dealing with a 117-day hybrid like they grow in southern Illinois, Tennessee, and Kentucky, you don’t grow corn that doesn’t have good southern rust resistance, because they deal with it every year,” Ferrie notes. “When you move to Minnesota, and you’re planting 102- to 95-day corn, you’re probably not going to find hybrids with southern rust resistance.”

Striegel says that was true for his neighbor’s cornfield, which he custom chopped for silage. “That field had two hybrids in it, one was worse than the other, and the field had been sprayed with a fungicide,” he says.

He adds that he also sprayed his own cornfields with fungicide, but they are still inundated with southern rust.

“We’ve had southern rust before, and it’s not usually something we have to worry about, but this is really bad,” Striegel says. “I’m standing on my deck looking at the cornfield next to my house, and you know, all of the leaves from the ears down in that field are covered with it.”

How Late Is A Fungicide Application Still Worthwhile?

Ferrie says the fields he scouted last week were at late R3 to early R4 and had already been sprayed with fungicide at least once, but the disease was rebuilding.

“Any field where farmers had sprayed two weeks previously, the southern rust and northern corn leaf blight, to a lesser degree, were coming back, especially the southern rust. It was resporating,” he says.

The intense disease pressure from southern rust, tar spot and others have kept fungicide use at high levels this season, despite poor commodity prices.

“Because of that [amount of disease pressure], we have seen increased demand for our fungicides this year,” says Maertens, who encouraged customers to get applications made at the beginning of tassel.

Maertens says he has fielded a lot of questions this summer from farmers, asking how late they could go with a fungicide application and still benefit.

“Our recommendation is to get in front of disease,” he says. “Generally, we stop applications before we get to dent (R5). That’s not to say a later application can’t have some benefit, but our best results have been before infection was able to take place.”

Southern rust is a yield enemy farmers routinely face in the Southeast, reports corn yield champion Randy Dowdy, Valdosta, Ga. He participated in the Pro Farmer Crop Tour last week and said on U.S. Farm Report he believes many Midwest farmers still have time to address disease.

“We need to implore the fungicides, the technologies out there and get after it and protect this crop, especially that crop that still has not reached dent,” he says.

Farm Journal Field Agronomist Missy Bauer likes to see farmers complete their fungicide applications on the front side of dough (early R4). “Once we get to early dent, I think it’s a little more challenging to get the payback consistently, though we’ve applied at early dent (R5), and seen a nice response,” says Bauer, who is based in south-central Michigan.

Under the tough disease pressure farmers are facing this year, Bauer is telling growers to scout fields and evaluate what growth stage their crop is in before they walk away or pull the fungicide trigger one last time.

She adds that farmers need to check the label to make sure the product used is able to address southern rust effectively. She describes these as “Cadillac” products containing the newest chemistry.

“When it comes to some of these diseases, especially southern rust and tar spot, I do believe a little bit of a Hail Mary pass can be effective,” she says. “Will it be as effective as an application you could have made on a more timely basis? Well, no, you could have made more money doing it timely, but you’re still protecting bushels and gaining ROI at the end.”

Ferrie adds that farmers might want to do the late-season fungicide application to keep their corn crop standing until they can put their harvest plan in place.

“Be doing the push test to check stalk quality,” he advises.

Striegel says some of the farmers around him are heading to fields to harvest their silage corn sooner than later, because of standability concerns. “Some of this corn got planted early, and we had a lot of heat. The crop matured quickly, and the diseases are kind of shutting it down. It’s just dying out, and guys are going to go get it,” he explains.

That’s the strategy Ferrie encourages farmers to use in regular production corn, too.

“Harvest the fields most at risk first. But if a field of corn goes down, go combine the fields where the corn is still standing and come back to that one later,” he recommends.

The reasoning is you don’t want to risk more corn going down while you’re harvesting the field of corn that already has.

“While I was driving through Iowa last week, I kept thinking that if I built corn reels to pick up down corn I’d bulk up my inventory, because I know where they’re going to get used,” Ferrie says, only half joking. “Yes, harvesting corn at 25% moisture is expensive, but down corn will kick your butt.”

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