Soybeans are a magnet for seedling diseases and seed rot. Together, those challenges accounted for more than 20% of soybean establishment problems in the five years leading up to a March 2012 survey of crop consultants across the Midwest and South. That makes seedling disease management a top-priority issue for many producers, especially when planting conditions are cool and soybean prices are hot.

Pathogens aplenty

It’s hard to know what you’ll be facing as planting-time weather goes through its cycles, notes Alison Robertson, Iowa State University crops pathologist, as she describes the components of what’s called the “disease triangle.” But you can almost bet there’s a pathogen waiting to take advantage of any opening.

“The three points on the triangle are a host, the pathogen and the right conditions; all have to be present to have disease, but one field may have many different pathogens,” Robertson explains. “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.”

Pythium: rotation problem

The pathogen most likely to take advantage of a wide span of soil conditions is pythium.

The broad genetic diversity of Pythium spp. means that nearly any planting temperature can encourage one pythium species or another to take hold, explains Anne Dorrance, Extension specialist, soybean disease, Ohio State University. Dozens of species of pythium affect soybeans as well as corn, making the disease a major challenge even in rotation.

Many pythium species are resistant to popular fungicides. In pythium’s case, some isolates are resistant to mefenoxam and metalaxyl, strobilurins, or both families, Dorrance says. Still, she notes, plant pathologists say growers with a long history of having to replant fields may benefit from a strobilurin in combination with mefenoxam or metalaxyl.