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Know Your Soybean Diseases

Leaf symptoms can be confusing, especially for old nemesis brown stem rot.

You’ve seen those big brown patches in soybean fields in August, where the plants die early. It’s sudden death syndrome (SDS), right?

Not so fast, says Dean Malvick, University of Minnesota Extension plant pathologist. Yes, SDS gets much of the attention and is probably still growing in severity across the Midwest. Still, those brown patches may be caused by several diseases, including a soilborne fungal disease that has fallen off some radar screens in recent years: brown stem rot (BSR).

“It was first identified in Illinois in the 1940s,” says Malvick. “It got a lot of attention in the 1960s to the 1990s, but not so much now. Much of the recent research has gone to sudden death syndrome.”

That doesn’t mean that BSR has gone away. A recent survey in the Midwest pegged BSR as present in 70% of soybean fields. Yield losses associated with the fewer pods and smaller beans has been up to 27% for the entire field, although 10% to 15% (or at least 5 bushels an acre) is probably more common, says Malvick.

Split the Stem

People often confuse BSR and SDS (along with some others) because they are both caused by fungi that linger in crop residue and soil. Both can show similar symptoms when patches within a field begin to die and turn yellow early. Sometimes, even a close-up inspection of the leaves can be confusing. (See photo.)

To know if it is BSR, split the stem of an infected plant near its base, Malvick says. “Inside the stem, you will see the brown rot in the pith that tells you it is BSR,” he says. SDS and other diseases produce no such rot.

BSR thrives in cool and wet soils during the plant’s reproductive stage. The fungus carries over especially well in old soybean crop residue, so rotating crops helps. Still, Wisconsin scientists found that BSR can survive for 2½ years on surface residue, but just one year if it’s buried.  

“You will usually see a BSR outbreak after podding in August,” says Malvick. “It was likely there all along – since May or June – but it was sort of latent until erupting in August.”

Malvick wonders if BSR is showing up as a culprit more frequently now because of weather changes (wetter and cooler), or possibly the fungus has changed. 

“We see two types of BSR,” he says. “Type A is more aggressive, and it always shows symptoms on the leaves with the early browning. Type B is more common now. It’s a little milder. Often Type B has no leaf symptoms at all. Both types can produce yield losses of 10% or more.”

So what can you do? Crop rotation helps and perhaps modest tillage. Delaying soybean planting in the spring can also reduce BSR severity later in the season, Malvick says. However, weigh this against reduced yield potential from delayed planting.

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