Pythium caused several thousand corn acres in southeast Iowa to be replanted the past several years. Despite using seed treated for Pythium, they had trouble maintaining a good stand in some fields.

Last year and again this year, corn plants would emerge, but then die, says Alison Robertson, Iowa State University plant pathologist, whose investigative team documented this. In samples brought back to the lab, the mesocotyl tissue was rotten, she says.

Corn growth slows considerably in cold, wet soils with temperatures below 55 degrees, and seedlings become susceptible to Pythium, Robertson says.

“We saw corn at the V2 to V3 stage, that had been sitting in the ground 4 to 5 weeks. All corn seed treatments contain either metalaxyl or mefenoxam, both very effective against Pythium. We don’t know if the seed treatments didn’t last long enough or if the fungicide just wasn’t effective,” she explains.

“Resistance to this fungicide is real, so that’s always a concern. Last year, it appeared the species P. torulosum was a primary problem—80% of the isolates recovered from diseased corn seedlings belonged to this species. But this year we found a wider range of species of Pythium.”

Robertson says she heard of similar Pythium problems last year in Illinois and Indiana, and this year in Nebraska and Minnesota. Ohio researchers reported resistance several years ago. “It seems to be becoming more of an issue,” she says.

Knake says Pythium is in every field, and without a systemic fungicide seed treatment against it, growers would need to delay planting both corn and soybeans, or risk the need to replant too many acres each year.

We still use the original Pythium product introduced in the 1980s, Knake notes. “There’s no confirmed evidence of resistance, but it could develop. For Pythium, there’s only one mode of action. The industry is working on one more, but there’s really nothing a grower can do.”

Robertson and Knake agree that a “next-generation” seed treatment needs to be developed, tested, and made available. Robertson points to ethaboxam, a new fungicide that belongs to a different chemical group than metalaxyl, the primary ingredient that’s been used for years against Pythium and other pathogens.

 “It’s a new fungicide that will be combined with metalaxyl and marketed as the AP3 fungicide system. In our 2012 greenhouse evaluations, ethaboxam and metalaxyl in combination reduced root and mesocotyl rot and improved emergence,” Robertson says.