Scientists are working at a tortoise-like pace against the hare-quick spread of Sudden Death Syndrome (SDS), a disease that can transform a green soybean field to a sickly yellow in days. Scientists can't yet stop SDS, but are working to slow the spread and understand the disease so they can develop truly resistant varieties.
Because SDS is such a huge problem, a coalition of 16 scientists are working on finding a solution. Funding is from the United Soybean Board and the North Central Soybean Research Board.
X.B. Yang, plant pathologist at Iowa State University, found that a plant can be infected early — even at germination. His research shows that the earlier a root is infected, the greater chance it has of exhibiting SDS symptoms later in the season.
“Roots have a differentiation stage. As the plant grows, the early root will form an outer layer and the inner layer will become the woody part of the root,” Yang says.
If the fungus attacks before the outer, protective layer forms around the woody part of the root, Yang says a larger amount of the fungal toxin will be found in the plant and cause SDS symptoms. Scientists are trying to pinpoint the chemical makeup of the toxin created by the fungus. They would like to use it to screen or enhance the selection of soybean germplasm in an effort to find or develop resistance through biotechnology.
“We don't have any magic bullets in this case because we don't have any highly resistant varieties available now,” says Glen Hartman, USDA-ARS plant pathologist at the University of Illinois. “We've been working the opposite way, in a sense, by telling growers that there are some super-susceptible varieties that should not be grown if there's a risk of SDS.”
That's where David Lightfoot, plant pathologist at Southern Illinois University, comes in. So far, he's found seven resistance genes and estimates there may only be eight or nine in the entire soybean germplasm. Those genes are bred into soybean lines to help reduce the occurrence of SDS.
Six of the genes he's found deal with reducing the symptoms of leaf scorch. One gene reduces the amount of fungus in the root.
Lightfoot is concerned that breeding is being focused on reducing the toxin's symptoms, not keeping it out of the roots.
“I call this the Sudafed effect. If you've got a bacterial flu, you can take Sudafed to deal with the symptoms (the leaf scorch) or you can take antibiotics to deal with the infection (which occurs at the root),” Lightfoot says. “We're breeding a lot of soybeans with Sudafed protection.”
Lightfoot says commercial varieties can have a stack of leaf scorch resistance genes that make them perform well even though they don't have root resistance, which does nothing to reduce the fusarium population in the soil. This makes SDS symptoms worse the next time around. There's currently no way for growers to tell if they have a root-resistant soybean or one with a “Sudafed-gene” stack.
Madan Bhattaacharyya, soybean molecular geneticist, and graduate student Junli Ji, Iowa State University, discovered that light is a component of SDS development.
Toxins released by fusarium fungus actually disrupt the photosynthesis process and turn the plant against itself, Bhattaacharyya says.
“When we expose the toxin-fed plants to light we can see free radical accumulation (yellowing and cell death) within minutes. The toxin alone is not sufficient to produce free radicals or the symptoms that will kill the plant.”
Bhattaacharyya estimates it may take another decade to introduce a transgenic soybean variety with a toxin-insensitive protein into breeding programs. So, scientists know they've got a long way to go, and it's a slow race. But they expect their tortoise-like determination to get them to the ultimate goal — highly-resistant varieties.