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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.”