Shape up soybeans with scouting

It’s a warm, sun-soaked, and humid July day, one in which soybeans are tapping water and nutrients in order to pack seed-filled pods later in the summer. Unfortunately, such days can also encourage diseases that can squelch bumper yields come fall. 

Kaitlyn Bissonnette often encounters this scene when she starts scouting a soybean field. “When I scout soybeans, I want to start a little ways into the field,” says the University of Missouri Extension plant pathologist. 

Scouting field edges can be misleading because several diseases can blow in and then stop in the first few rows. Starting 20 to 30 yards into the field can help glean a better picture of a field’s status while scouting, Bissonnette points out. 

Low-Canopy Look

If resistant varieties could deter every disease, raising soybeans would be easy. Unfortunately, not all do, and that’s why scouting during the growing season paired with sound variety selection is vital for high soybean yields. 

“When scouting for soybean disease, I start looking in the lower canopy,” says Bissonnette, because that’s where many diseases start. 

Some diseases look much worse than they are. The brown-spotted yellow leaves pegged to Septoria brown spot start on the lower parts of plants and stealthily sneak upward.

“This is something that happens year in and year out,” says Bissonnette. “Generally, though, it’s not something to worry about as it is not a yield-limiting disease.

“Another disease we look for is Cercospora leaf blight,” she adds. This disease, characterized by bronzed leaves, can show up in the late reproductive stages. 

“This bronzing is the result of toxins accumulating in the plant cells that react to sunlight,” Bissonnette says. “Oftentimes, you will see these bronzed leaves in the area of the plant closest to the sun. The leaves end up falling off if the disease progresses.” 

Cercospora leaf blight can also be associated with purple seed stain, a disease known to affect seed quality at harvest.

The good news is that Cercospora leaf blight rarely impacts yield. ”You can get some losses in the Deep South states like Louisiana, but not in the Midwest,” she says. However, Bissonnette adds that quality losses can be an issue in years when purple seed stain levels are high.

Frogeye Leaf Spot

Interspersed between these diseases, though, is a more serious fungal disease – frogeye leaf spot. Frogeye leaf spot can be identified by the semicircular spots with a dark purple to brown rim. 

“With this disease, you want to look into the mid- to-upper canopy,” Bissonnette adds. “Many times, they’re going to be on some of the newer leaves.”

On the underside of the leaf, there’s a gray fluffy growth. “This is the fungal mass that occurs with frogeye leaf spot,” Bissonnette says.

Resistant varieties are a good place to start when it comes to managing frogeye leaf spot. In most years, they can provide sufficient control, says Bissonnette. 

If warm, humid weather persists, though, frogeye leaf spot infestations can slice yields if a fungicide is not applied. Because the fungus that causes frogeye leaf spot generally resists strobilurin fungicides in the Midwest, Bissonnette recommends fungicides with multiple modes of action applied at full label rates from R3 (beginning pod) to R5 (beginning seed). 

High-Tech Scouting 

Walking through field after field may turn up maladies like these diseases, but it’s hot, sweaty work on sultry summer days. Scouting with drones has the potential to make crop scouting easier and more efficient. Drone technology can be particularly helpful in scouting for fields impacted by soybean cyst nematode.

“That’s especially true in areas where you can see visual symptoms,” says Bissonnette. “In areas where visual symptoms are not apparent, though, that might not be the case.”

An exception, though, is in fields plagued with sudden death syndrome (SDS), which can be linked to SCN infestations, she adds. 

A good resource is an independent crop consultant who can employ scouting technologies that farmers may not use or be able to afford, says Greg Tylka, an Iowa State University Extension nematologist.

“Farmers have to be everything from a mechanic to a biologist to an economist to a marketer, and it’s hard to know every little detail about pests like soybean cyst nematodes,” says Tylka. “Crop consultants can keep the records and make the meticulous observations needed to help farmers with their management plan.”

Don’t sweat short soybeans

If you’re worried that short soybeans don’t yield as much as taller ones, chill. As long as they’re packed with pods, short stature shouldn’t impact yield, says Ryan Van Roekel, a Pioneer field agronomist.

In some cases, soybean varieties are shorter than others because of genetics. In other cases, weather impacts plant height. Earlier this year, soybeans that struggled to grow in cold weather in many areas formed tight spaces between internodes less than 1 inch apart. 

“This created shorter-than-normal soybeans,” says Van Roekel. When temperatures warmed in June and July, soybeans rapidly grew and spaces between internodes lengthened. 

“I always like to remind growers they are not trying to grow big, beautiful bushes,” he says. “They’re trying to put more beans into the bin.” 

In some cases, tall bushy soybean varieties backfire since they can lodge and create a harvest nightmare, he says. 

Pods Pump Yields

“It’s all about the pods,” says Van Roekel. Short beans still can have plenty of pods, driven by warm weather with ample soil moisture.

However, August drought and heat can squelch pod development. This was best exhibited in areas like western Iowa in 2020. In these fields, fewer pods formed near the top of soybean plants compared with the number of pods at the plant’s bottom, which had formed earlier in the year.

Get in the Weeds

Weed patches are, well, filled with weeds. They may signal other problems in a farmer's soybean production strategy, though.

Greg Tylka, an Iowa State University Extension nematologist, disccovered a connection between soybean cyst nematode (SCN) and weed infestations while working with Ron Heck, a Perry, Iowa, farmer.

“We grid mapped 50 acres in 100 half-acre cells," recalls Tylka. “We measured soil pH and SCN numbers, but we also made a map of weedy areas. Ron pointed out that where we identified the high-pH, high-SCN areas, that was also where we had problems with weeds. We surmised it’s because the canopies weren’t closing over the weeds that got full sunshine.

“So, weed patches can be an indirect symptom of SCN that field scouting can detect,” he says.

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