Identifying and managing sudden death syndrome in soybeans

It's farmers' worst nightmare. A seemingly healthy soybean field dies right before their eyes, just months before harvest from sudden death syndrome (SDS). The disease is consistently rated one of the most damaging to U.S. soybeans, reducing yields by up to 25%.

Disease Life Cycle

The fungal disease starts soon after planting, rotting the roots but not causing obvious harm to the plant. Symptoms can go un-detected until the plant reaches the reproductive stage, when toxins produced by the fungus move to the leaves.

"It sounds like a really slow death," says Daren Mueller, an extension plant pathologist at Iowa State University. "The name sudden death syndrome comes from how quickly the foliar stage of the disease develops once that toxin moves into the leaves."

The disease dramatically reduces yield in two stages. The root rot stage prevents plants from taking in adequate nutrients, causing stress on the plants, while the foliar stage prevents photosynthesis, Mueller says.

Identifying SDS

SDS can be hard to identify in fields even after symptoms appear.

"It's tricky because SDS has a lot of look-alikes," Mueller says. "To really confirm it, you have to get out of the vehicle and look at the symptoms on each individual plant because there are subtle differences."

Common look-alikes include brown stem rot, stem canker, and red crown rot. These fungal diseases all show foliar symptoms of interveinal chlorosis and necrosis.

"When I get out into the field, the first thing I do is extract plants with foliar symptoms," says Nick Frederking, an AgriGold agronomist. "Then I'm evaluating that plant as a whole."

1. Stem canker is evident on the plant exterior. Sunken, dark brown lesions will climb the stem and hit at every node level.
2. Brown stem rot can be identified by splitting the soybean stem in half. Brown or decaying pith tissue will be evident.
3. Red crown rot can be seen at the soil line as a red streak or noticeable red fungal structures on the exterior base of the stem.
4. Sometimes, sudden death syndrome can be confirmed by a blue-green fungus on the soybean's root.

Distinguishing SDS from look-alikes is vital to making good management decisions.

"It's very easy to label those other diseases as SDS, and we have active ingredients to control SDS," Frederking says. "If it's not SDS, we don't have a lot of options for those other three. We have to manage those with variety resistance."

If SDS treatments are not controlling the disease, plant diagnostic labs can help confirm which pathogen is present.

Managing Outbreaks

Once the disease has been identified in a field, there's no in-season management that can prevent yield loss, but that doesn't mean there's nothing farmers can do.

"You don't get SDS every year," Mueller says. "When you do have a high SDS year, it's good to get out and take notes on what fields it's shown up in and what percentage of the field it's in."

Evaluate Varieties

High SDS years are also an opportune time to evaluate seed choice.

"There are good levels of resistance out there, but if there's a couple years without high SDS development, farmers might miss out on how resistant or susceptible the varieties are," Mueller says. "It's good to get out and check whatever ratings you had. Take advantage of the bad situation and get your homework done so you can make better decisions in future years."

SDS is caused by a soilborne pathogen, but other conditions can increase the risk of an SDS outbreak.

A wet spring and heavy rainfalls in August contributed to greater SDS pressure in 2022.

"A wet spring can help phase one — root rot," Mueller says. "Phase two is when that toxin moves, and you need heavy rainfalls for that to happen."

Planting early can also cause problems. Moderately cool, wet soils can boost fungal development.

"When we try to manage this disease, we're looking for varieties that have some genetic resistance against SDS first," Frederking says. "If we want to plant early and our varieties don't have that genetic resistance, then we may need to utilize some of the chemistries that have active ingredients against SDS."

Predicting SDS

To help farmers make better management decisions, start-up Agmatix is standardizing and analyzing years of data.

"There are so many different digital tools, but nobody speaks with one another. There is a lack of connectivity and a standardized way to curate data and translate it into real models that work," says Ron Baruchi, Agmatix CEO. "Our unique approach is this task of data standardization from experiments and trials because this is an area where data is high quality. We found that we can make lemonade out of lemons faster and much more scientifically sound."

In a recent SDS trial, the company derived data from 90 SDS field trials conducted over five years in six separate studies. The data was divided into three classes of susceptibil-ity: no disease, moderate disease, and severe disease. The final model was able to predict a field's class with 80% accuracy.

"We intend to increase that accuracy by getting more data in the environmental space," says Sagi Katz, Agmatix vice president of agronomy. That includes data on soil types, untreated seeds vs. treated seeds, and soils that had or didn't have SDS the previous year. Then that information is compared with the environmental conditions such as planting date and soil moisture to give a better prediction for future use.