Seedling diseases and seed rot accounted for more than 20% of soybean establishment problems in the past five years, and farmers had to replant almost 20% of affected acres. (This comes from a March 2012 survey of Midwestern and Southeastern certified crop advisors in 12 major soybean-producing states as part of a soybean seedling disease study.)

From 2005 to 2007, the seedling disease yield cost is estimated at 37 million bushels/year, yet “many states don’t know a great deal about what causes seedling diseases in their area,” says Martin Chilvers, a Michigan State field crop pathologist.

Chilvers’ lab, together with university collaborators from 12 major producing states, is examining soybean seedling diseases to identify the range of soybean pathogens. Testing in the laboratory and greenhouse will follow to determine which ones cause disease and rule out organisms that are only “guilty by association.”

Identifying what’s actually causing disease, will lead to better seed treatments or genetic solutions, he notes.

“We might need to tailor seed treatments to different growing regions. So far, species tend to group by similar geography,” says Chilvers of the 55 species of Pythium and two species of Phytophthora identified in 2011 and the 40 species of Pythium and two Phytophthora from 2012.

“We noticed a big shift in species composition from 2011 to 2012 that is very likely associated with weather. Certain soil components also seem to drive what’s there. Temperature, soil texture and precipitation seem to be linked to this,” he explains.

“We’ve opened a can of worms,” says Alison Robertson, Iowa State University plant pathologist, speaking at a January Crop Advantage workshop in Ames. “We’re finding a range of species that will cause disease, and not all will respond to fungicide seed treatments.”

Disease triangle

Robertson cites a complex disease triangle as one of the challenges to control seedling disease.

“The three points on the triangle are a host, the pathogen and the right conditions; and all have to be present to have disease, but one field may have many different pathogens. Maybe if the temperature is 50 degrees, you get one pathogen, but at 63 degrees you get a different one, yet we can’t predict what the soil temperature will be a week after planting and which pathogen will be a threat.

With that complexity, she concludes, “When growers have problems, the failure may not be because the treatment failed, but because they are dealing with a pathogen population that is less sensitive to the treatment.”

With this information, “we now have a much better idea that it is a diversity of species causing seedling stand issues and that the common seed treatments were not controlling them,” says Anne Dorrance, University of Ohio professor of soybean pathology and host resistance. “There is no silver bullet to management – and it will take several approaches to eliminate the replant issues and one strategy will not work for all of the Pythium species. The seed treatment compounds have changed dramatically in the past decade – and more products are expected to be labeled in the next two to four years.

 “Producers will have a wide range of choices of seed treatments,” Dorrance adds. “There has also been a change in germplasm as companies have deployed new technologies.” (There's a big unknown regarding Pythium resistance in today's germplasm, whether it has increased or decreased.) “The next step is to focus on best management techniques.”

No single seed treatment “is effective against all seedling pathogens, ” says Jim Kurle, University of Minnesota associate professor, plant pathology. “This suggests a number of possibilities: Growers need to apply seed treatments based on the pathogen causing stand problems in their particular situation; no single compound is going to be effective in all situations, and applying an inappropriate fungicide may increase damage from seedling pathogens,” he says.

“Alison Robertson’s (Iowa) observations apply equally well for Minnesota,” says Minnesota’s Kurle. “We've had very erratic results with the seed treatments because the seedling pathogen most important in a particular year depends upon environmental conditions, moisture and temperature. For instance, Pythium is more likely to be important in saturated soils at low temperatures. And there are multiple Pythium species with differing temperature requirements. If saturated conditions occur later after soils have warmed, Phytophthora can be more important.”

Michigan’s Chilvers adds, “Right now it is hard to rapidly diagnose which species is which.” His work relies on DNA analysis in the laboratory.”

What is the bottom line for seed treatments?

“There is absolutely no doubt in my mind that seed treatments do protect stand in cold, wet soils,” Robertson says. “I have demonstration plots and growth chamber work that clearly show this – plus there are a few scientific publications that report this. In good seedbed conditions (moist and above 60 degrees F), the benefits of a seed treatment may not be evident. However when soil temps drop below 60 degrees and germination and emergence is retarded, seed treatments become vital.

“However, just because a seed treatment is used doesn't mean a farmer is "in the clear" - what we are learning from our research is that seed treatments are not the silver bullet. Farmers still need to scout their fields and assess stand. If they do come across problem fields, its not that seed treatment didn't work, its just the conditions were favorable for a pathogen that the seed treatment was not very effective against.”

The 12-state soybean seedling disease research effort was funded by USDA’s National Institute of Food and Agriculture with matching funds by the United Soybean Board and the North Central Soybean Research Program It will also develop diagnostics for seedling diseases.

“Eventually this work should lead to better diagnostic tools,” Robertson notes.

The ultimate goal is better technology to see what is causing problems at the root level and how it can be controlled.

“With seed getting more and more expensive, the question is how you make sure every seed you plant becomes productive,” Robertson says. “A single fungicide won’t control all types of pathogens.”

Get more information on resistant races of Phytophthora.