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.
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.
Phytophthora root and stem rot, caused by Phytophthora sojae, thrives in cool, wet conditions, as they allow spores to colonize young soybean roots. Though it’s primarily associated with cold springs, Phytophthora sojae is responsible for 8% to 10% of soybean crop losses nationwide — even in normal crop years.
Battling phytophthora got more complicated several years ago when it became clear that one of the industry’s top resistance genes, Rps1a, is no longer effective in many areas. Other resistance genes, Rps1c and Rps1k, may provide protection from the disease for now. A pair of newly discovered resistance genes from the soybean genome may add to the arsenal in years to come.
Meanwhile, strobilurin fungicides in seed treatments can add some suppression, and high rates of metalaxyl or mefenoxam can control the pathogen in highly susceptible and moderately susceptible varieties.
Not silver bullets
With a broad array of fungal species — some susceptible to popular fungicide seed treatments and some not — and dramatic changes in springtime weather, seed treatments are not a sure bet to return a profit, but they can make a big difference in cold conditions.
“In good seedbed conditions (moist and above 60 degrees F), the benefits of a seed treatment may not be evident,” acknowledges Robertson. “However, when soil temps drop below 60 degrees and germination and emergence is retarded, seed treatments become vital.
“What we are learning from our research is that seed treatments are not the silver bullet,” she adds. “Farmers still need to scout their fields and assess stand. If they do come across problem fields, it’s not that seed treatment didn’t work, it’s just the conditions were favorable for a pathogen that the seed treatment was not very effective against.”