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Serving: IL
corn leaves showing signs of gray leaf spot Courtesy of Stephanie Porter
EFFECTIVE CONTROL: The most effective way to deal with gray leaf spot is to spray fungicide preventatively at tassel, before the disease becomes too noticeable, says Dekalb agronomist Randy Niver.

Do you need a foliar fungicide?

While disease pressure, yield goals and production strategies vary in Illinois, experts say the need to scout is universal.

Illinois farmers have long taken a variety of approaches to fungicides: Some spray all their soybeans and half their corn preventatively before any symptoms occur. Others do the inverse with all their corn and some of their soybeans, while still others spray curatively in response to flare-ups of disease such as southern rust or gray leaf spot. They spray different corn hybrids and soybean varieties depending on their disease tolerances.

While disease pressure, goals and strategies vary, the need to scout is universal, says Randy Niver, technical agronomist for Dekalb/Asgrow in central Illinois. Southern rust could be a concern this year, but positive reports of it have stayed in Southern states, according to this map from the Corn Integrated Pest Management website. Frogeye leaf spot is the primary concern for soybeans.

Gray leaf spot is a concern for corn growers across the state, and in northern Illinois, tar spot is worth scouting for as well, says University of Illinois crop pathologist Nathan Kleczewski. Niver says scouting and spraying at tassel or within 14 days after tassel is important to break even while fighting these two corn diseases.

“The problem with gray leaf spot and tar spot is that if you wait until you see a lot of it, by the time you spray, you have less yield potential to protect,” Niver says.

Kleczewski says fungicides don’t increase yield, but instead protect yield potential. Farmers should scout before tassel to see if diseases are present to determine if they need to apply fungicide later in the season. Divide per-acre application cost by per-acre crop price to figure out how many bushels you’d need to pay for the fungicide application.

“Scout early and often,” Kleczewski says. “Keep the weather in mind. Know your cultivar’s and hybrid’s strengths and weaknesses for disease. Resistant hybrids or cultivars are not likely to benefit from a fungicide, whereas racehorse hybrids with low disease resistance are likely to need a fungicide in a disease-favorable year.”

Combating foliar diseases with fungicides and other tactics like resistance are part of what Kleczewski calls an integrated approach to protecting yield. For example, in northern Illinois, fight northern corn leaf blight with resistant hybrids; in southern Illinois, use southern rust-resistant hybrids. Tillage and rotation are also important.

“Selection of a resistant hybrid can reduce or negate the need for a fungicide application,” he says, noting in a year like 2018 with hot temperatures and rain, susceptible hybrids could have upward of 20% blighted tissue “that will probably need a fungicide.”

Corn stalk rot

If disease on the ear leaf and above reaches 5% to 10% severity or more during grain fill, Kleczewski says that’s often enough to start to limit carbohydrates to where starches move from lower tissues to the top of the plant. This could lead to stalk rot issues.

“Anytime we’re reducing green tissue while we’re still filling grain, that could potentially enhance our issue with stalk rots,” he says.

When it comes to yield losses in the state, 1.4% came from stalk rot in 2018, while 0.9% came from the problem in 2019.

“That doesn’t seem like a lot until you start extrapolating that across the acres we have,” Kleczewski says. “Specific fields can have issues that are much greater. You could easily have a 25- to 50-bushel yield loss or more in a field where you have lodging issues and you can’t get that corn up off the ground, or the corn starts to rot. That’s why we need to be out there scouting to see what problems we have.”

He says gibberella, fusarium stalk rot, anthracnose, charcoal rot, physoderma and diplodia stalk rot are the major fungal cornstalk rots farmers should be on the lookout for. Managing early-season leaf blight from anthracnose does not have any real effect on future anthracnose stalk rot issues, but it might indicate the potential for problems later in the season, Kleczewski says. Most other stalk rots can increase in incidence when foliar disease levels are sufficiently high during crucial grain fill periods.

“When you have severe foliar disease pressure, the application of fungicides for managing those foliar diseases can reduce your amount of most stalk rots,” Kleczewski says.

Trial results

In a 2003 trial from the University of Illinois, when no fungicide was sprayed, 65% of the ear leaf showed blighting, with a 3.5 out of 5 rating for stalk rot. There was a reduction to just 2% blighting when using Headline from BASF, which came with a lower stalk rot rating of 2.3.

In a 2018 trial in Monmouth, Ill., when fungal disease pressure was severe in the area, there was 40% blighted leaf area in nonfungicide-treated checks with an 8% incidence of stalk rot. An application of Delaro fungicide sprayed at R1 reduced leaf blight to 21%. It also brought the incidences of stalk rots down to zero. Miravis Neo resulted in 14% blighted leaf area and a 4% incidence of stalk rot, while Aproach Prima resulted in 27% blighted leaf area with a 5% incidence of stalk rot.

“In general, when we’re applying our fungicide, we are seeing a numerical reduction in stalk rot. But it’s just one site, one year, and these are small plots, so keep that in mind — although similar data have been obtained over the years from multiple research programs,” Kleczewski says, noting that in 2019, when there wasn’t a lot of foliar disease, there wasn’t much correlation between the incidence of foliar disease and the severity of stalk rot.

“Managing foliar diseases in cases where you could potentially have a lot of disease can be very important to managing your stalk rots, but if you’re in a low disease year, or if you have a hybrid that’s resistant to your common diseases that could be problematic in your area, that’s going to limit the utility of fungicides for managing stalk rots,” Kleczewski concludes.

Soybeans and fungicide

In 2018, soybean farmers in Douglas County, Ill., and surrounding counties were dealing with heavy frogeye leaf spot pressure. An Asgrow strip trial showed a 12- to 15-bushel improvement for soybeans sprayed with Delaro at the R3 growth stage compared to soybeans that received no fungicide.

Using current prices of $8.75 per bushel and the $20 per acre cost of buying and applying the fungicide at a rate of 8 ounces per acre, there was a $111 return on investment in the trial. For corn at $3.50 per bushel, the breakeven for yield preserved through an application of Delaro is 6 bushels per acre.

“For corn and soybeans both, fungicides are a good management practice to maintain your yield potential,” Niver says, noting it’s important for the farmers he works with who are pushing higher yield targets, but tighter margins make the decision more difficult.

Fungicides also help save time during busy harvests by improving standability in corn and helping to prevent lodging in soybeans. “I think it’s a good overall practice, even with a lack of disease pressure at the time of application,” he concludes.

TAGS: Scouting
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