BELLEVILLE — Some Illinois soybean growers are facing late-season threats that require on-time treatment.

The first consideration, however, is not what to do, but whether to do anything at all. And if so, when to do it. So says Syngenta agronomist Phil Krieg.

“With late-season management in soybeans, it is very important to know what growth stage you’re in,” Krieg told farmers at a field day here sponsored by the Illinois Soybean Association. “It’s a little more different than corn. That timing is spread out because of maturities.”

Some fields have been victim to heavy attacks of Japanese beetles. As with most crop problems, location and planting dates are key.

A field Krieg visited last week in northwestern Illinois was covered with the bugs, while a Southern Illinois University research plot at Belleville showed no signs of trouble the following day.

“This plot is squeaky clean compared to yesterday at Roseville,” Krieg said. “Japanese beetles there were stacked on top of one another like cordwood.”

Before devising a strategy to tackle late-season bean trouble, farmers may be rewarded for taking their scouting to a new level. Plants at growth stage R3 — when they are developing pods — may be most receptive to fungicide treatment, preparing them for healthy end-stage growth.

“I can’t stress enough knowing the growth stage in the soybean plant,” Krieg said. “The reason we concentrate on the R3 growth stage is we’re setting soybean up for R4. If you’re a soybean plant, R4 is as hard as you can be working. You can’t have anything else going on.”

Diseases to look out for include frogeye leaf spot, cercospora leaf blight and septoria. Frogeye leaf spot infection results in leaves of plants with spots that give the disease its name. Cercospora blight gives leaves a sunburned look. Septoria shows up as blotches on leaves.

Krieg advised that growers contemplating treatment should err on the side of early spraying. But he cautioned that, in general, they should be judicious with fungicide applications.

A more effective strategy is the planting of resistant varieties. Breeders have found success in developing cultivars that avoid frogeye leaf spot, for example.

“I’m not a paint-brush-the-world kind of guy with fungicides. You have to balance this,” he said. “In that seed-selection process, you want a variety that has a very good frogeye score. I know a variety now that we can inoculate frogeye on it and it won’t develop it.

“The best place to go scouting is in the office, with those varieties you can plant. If we go into a field and we see disease lesions already, then we have to hurry up. If we go at R3 and see frogeye, we need to be in a hurry.”

Disease resistance is a growing concern. As with herbicides, producers should use chemicals with more than one mode of action.

“To just apply a strobiluron or triazole is only getting half the job done,” Krieg said.

“In southern Illinois and parts of the South we have strobiluron-resistant frogeye. In southern Illinois we don’t have the luxury of providing one product and, say, use it on corn, beans and wheat.”

But north of the Macomb field Krieg visited, he said they don’t have the strobiluron resistance, so they can use one product.

“We’re not that lucky here,” he said.

In short, he said, modern farming methods require creative techniques in battling pests.

“Who is farming today the same way they did 10 years ago? We need to be aware of what’s happening and be ready to react,” Krieg said. “Back in the day, when we used the moldboard plow, we buried a lot of our problems.”

Nat Williams writes for Illinois Farmer Today, a Lee Enterprises sister publication of The Southern Illinoisan.