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What attacked this soybean plant?

Soybean Watch: Look for signs of disease in late-season soybeans. Follow along as a seasoned agronomist determines the potential cause of this plant’s affliction.

Tom J. Bechman, Midwest Crops Editor

October 4, 2024

5 Min Read
diseased soybean plant
CHARCOAL ROT? It seemed unusual to find charcoal rot in an irrigated field, but initially, visible symptoms pointed to this dry-weather disease. Photos by Tom J. Bechman

Eyes trained through years of examining soybean fields and a copy of the Purdue Corn and Soybean Field Guide assist Steve Gauck as he tracks down probable causes of many soybean ailments. While sending samples to a university lab is necessary for confirmation, Gauck uses his agronomist’s intuition and the Purdue guide to arrive at possible causes.

“It is good to know what disease or insect issues you’re dealing with, in case it is something that you can prepare for the next time that field is in soybeans,” says Gauck, a regional agronomy manager for Beck’s. He’s based near Greensburg, Ind. Beck’s sponsors Soybean Watch’ 24.

When Gauck visited the Soybean Watch field late in this season, overall, the crop was healthy. However, even healthy fields usually have a few blemishes. He found scattered plants like the one pictured above. He used the guide and his own knowledge base to arrive at a logical conclusion.

1st step: Visual observation

Visual observation can unveil multiple clues. “The outside, visual symptoms on the stem looked similar to charcoal rot,” Gauck says. The stem was dark and slightly discolored.

The next step is to cut the stem lengthwise and observe both the inside of the cortes, or outer stem, and the pith, or inner portion of the stem. Both were clear of discoloration or presence of foreign bodies, likely eliminating charcoal rot as a cause.

Related:Irrigation pays for soybeans in 2024

Charcoal rot, usually expressed during hot, dry spells after infection earlier in the season, forms microsclerotia that appear as tiny black specks. These microsclerotia are usually evident inside the stem when it is cut open. There were no microsclerotia inside the stem in this case.

Zero in on cause

Because tissue inside the stem was clean, Gauck examined the outside of the rest of the plant more carefully again. His second examination included a closer look at the lower stem leading to the roots.

“There was some indication of symptoms like rhizoctonia,” Gauck recalls. “We saw some reddish tint, but it did not match the reddish symptoms you would expect to see from red crown rot, a relatively new disease in the Midwest.

“We would have had to send off a sample for confirmation to be sure,” Gauck continues. “However, what follows is the most likely possible explanation.

“It appears that this plant may have succumbed to a rhizoctonia infection, which initially occurred earlier in the season. It was apparently strong enough to withstand it then, but the disease eventually won out. Fortunately, only a small number of random plants were affected.”

soybean stem split open with clean pith and reddening around the roots

From the field:
Ups, downs of Iowa weather in 2024

by Greg Shepherd

We have enjoyed nearly ideal growing conditions in southeast Iowa for most of the 2024 crop year. Early this spring, the soil profile was dry. But that didn’t last long, as heavy rains came the last week of April. Fields that were planted in April benefited from soil temperatures that rose to 50 degrees F during the second week in April and held at that temperature or higher. Having consistently warm soils brought early-planted soybeans out of the ground rapidly. They were up and running.

More moderate storms continued in May. We began a cycle of a few weeks of wetter, cooler weather followed by a few weeks of warmer, drier conditions. There were pockets that received too much rain, with 8 inches in April for Davis County. And yet there were areas in east-central Iowa where the dry soil profile soaked up the 2 to 3 inches received right away and would have liked more. Early spring rains were good for the crop but presented challenges for weed control. A rainy and windy weather pattern made it challenging to get the first herbicide pass applied.

Weeds seemed to really put up a fight this year. We had to knock waterhemp down multiple times before finally knocking it out! We lost a few battles along the way. But in the end, farmers won this year’s war on weeds. I am thankful for multiple postemergence herbicide options that farmers have to keep weeds under control in soybean fields. This year reminded me of two important weed control principles:

  1. Stay ahead of weeds by starting weed control early.

  2. The best weed is the weed you don’t see. So, use residual herbicides in both passes.

Rest of the season

In June, rainfall amounts got back to normal for southeast Iowa. Northern Iowa continued to get heavy rains. Temperatures in late June cooled down. This continued into July, making for perfect conditions for soybeans to begin reproductive growth stages. Cooler temperatures in July and August made for very little stress. Timely rains in July recharged the soil profile. Thanks to desirable temperatures and ample but not excessive rainfall in July and August, we’re seeing high pod counts in our soybean crop.

In August, parts of southeast Iowa ran a little short on moisture as the soybean crop filled pods. Yet minimal heat stress allowed crops to remain happy and healthy. Limiting factors to our soybean crop appear to be bushels robbed by weed escapes and waterlogged soils that impacted nodulation and nitrogen fixation due to heavy spring rains.

Thankfully, those are not widespread problems. By and large, we have a terrific soybean crop out there. We have so much to be thankful for as harvest builds up to full steam.

Shepherd is a Beck’s field agronomist in eastern Iowa. He contributed to Soybean Watch ’24 all season.

About the Author

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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