Sparring with SDS:

Iowa State University’s three-pronged approach to combating Sudden Death Syndrome, or SDS, is bringing new hope in the war against Fusarium virguliforme. In-depth research on the behavior of the fungus, molecular-level study of the interaction between the pathogen and the soybean plant, and focused breeding efforts are yielding new insight and new tools for soybean farmers.

One of the biggest challenges of SDS is its unpredictability. The 2010 season marked Iowa’s worst outbreak of the disease, while in 2011 – another rainy spring that had growers fearing the worst – losses were low. What’s this year’s outlook? That’s anyone’s guess.

“We can’t really predict when SDS is going to show up or how severe it will be,” admits Leonor Leandro, assistant professor in the Department of Plant Pathology and Microbiology at Iowa State University. “And the disease may show up, but the impact on yield is going to depend on the soybean growth stage when it shows up and the environmental conditions during the growing season.”

Although SDS appears to ravage a field virtually overnight, the F. virguliforme pathogen actually engages the soybean plant in a running battle that can play out over weeks, notes Leandro.

It’s a race between the pathogen and the plant. If growing conditions favor root growth, the infected crop may stay ahead of the pathogen and outgrow the challenge. If cool, wet weather – which favors the fungus – compacted soils or crop stress put a damper on root growth, the pathogen can gain the upper hand. 

Wet June Portends Problems

Leandro and her colleagues have started to zero in on the environmental conditions that lead to the greatest amount of SDS damage. While cool, wet springs encourage infection of young plants, she says, a wet spell in June, when the crop is beginning to flower, appears to be particularly favorable to the disease. More generally, the more moisture there is throughout the entire season, the greater the risk of SDS.

The 2010 season was a proverbial “perfect storm” for SDS. Soybean roots were well-inoculated with the fungus that thrived in the sodden soils throughout the rainy planting season. As the crop shifted into reproductive mode, focusing its energy on flowers and pods instead of roots, the weather turned ideal for SDS – excessive rainfall that led to waterlogged soils. That encouraged the transport of the pathogen’s potent toxin through the plants’ xylem, to the leaves, scorching thousands of acres of soybeans across Iowa.

In 2011, the season again started wet, but turned dry before the fungus could kick into high gear, Leandro notes. That made all the difference.

Decoding Molecular Clues

While Leandro is studying the impact of environmental influences on SDS, Madan Bhattacharyya in ISU’s Department of Agronomy has been immersed in the very finest details of the pathogen’s anatomy, studying F. virguliforme at the molecular level.

Bhattacharyya and graduate student Hargeet Brar isolated FvTox1 a toxin produced by F. virguliforme that causes the devastating leaf symptoms of SDS. They stimulated the mice to produce an antibody to FvTox1. They then isolated the mice gene that makes the antibody to FvTox1. Hargeet, with the help of Iowa State University Plant Transformation Facility, inserted the antibody gene into a soybean plant.

“We’ve created a stable transgenic line,” he says. “As a result, we saw that symptom development was reduced. We saw very small disease spots instead of huge ones.”

Seed companies pointed out that starting with a mouse gene would likely doom the regulatory approval prospects for Bhattacharyya’s discovery, so he is returning to the lab to synthesize the antibody gene from scratch. He hopes to see soybean varieties capable of producing the antibody in the field in six to seven years. 

Resistant Varieties Expand

Meanwhile, Dr. Sylvia Cianzio, an ISU soybean breeder, has been steadily expanding the genetic base of SDS resistance and building a larger arsenal of breeding lines to give Iowa growers more options suited for Iowa’s growing season.

Cianzio is finding promising new options in four new populations that could serve as the foundation for a new generation of SDS-resistant soybeans.

“The encouraging part is that from one of those populations, they already found 20 highly resistant lines and 15 of them are also resistant to SCN,” says Leandro.

To Fight SDS, Fight SCN

Resistance to both SDS and SCN is important, Leandro notes. The nematode and the disease are thought to spread together, and it appears that the fungus can overwinter in SCN cysts. Feeding by cyst nematodes may provide an entry point for the pathogen, and also stress plants enough to make them more susceptible to the disease.

As a result, says Leandro, the best strategy to manage SDS is to keep SCN in mind, too. “If farmers know they have SCN, they should be managing for it – rotating crops, planting resistant varieties,” she says. 

The other key tactic is to watch planting conditions carefully to give your soybeans the best possible start. You can’t do anything about cool, wet June weather, but you can certainly try to avoid those problem conditions at planting.

“If you have warm temperatures in April but you know the temperature is going to drop quite a bit in a couple of days, or you know the soils are still very wet, avoid those cool, wet conditions if possible,” she advises. “If you have a field where you have seen SDS before, plant everything else first, then come back to that field. While we don’t recommend delaying planting to manage SDS, very cool, wet seedbed conditions should be avoided. Such conditions favor infection of germinating seedlings by the pathogen.”

Most important, don’t turn your back on SDS.

“Just because you don’t see symptoms doesn’t mean it’s not there,” Leandro warns. “In 2010, we had severe disease in fields that had never had a problem. The fungus was there. It’s a disease you should be looking out for and managing every year.”

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