Rumors are we are in for another wet spring. "I suspect when a grower hears such a rumor, possible delays in planting "spring" to mind," says Alison Robertson, an Iowa State University Extension plant pathologist. "The first things I think of are the diseases that thrive in wet conditions at planting." Wet, saturated soil conditions at planting can increase the risk of damping-off caused by Pythium and Phytophthora. These fungal-like organisms survive in the soil or infested crop residues and produce hundreds of small, mobile spores called zoospores that swim in soil water and infect soybean seedling roots, causing damping-off and stand reduction. Since Phytophthora prefers warmer soil temperatures, it tends to cause more of a damping-off problem in later-planted beans, while damping off caused by Pythium is a problem in early planted beans, when soils are cooler.
Plant resistant bean varieties or use seed treatments?
Damping-off symptoms caused by these two pathogens are very alike. "Usually, we have to do some sort of lab test to distinguish which pathogen is present," she says. "Symptoms associated with damping-off caused by both Pythium and Phytophthora are wilted seedlings with tan to brown and soft-rotted stem tissue." Soybean varieties with resistance to Pythium are not available, but fungicide seed treatments can be effective at reducing disease within the first two weeks of planting, she says. Growers should consider using a seed treatment if particular fields have a history of damping off. "Soybeans are the only host of Phytophthora and soybean varieties with resistance genes to that pathogen are available," adds Robertson. "The population of Phytophthora in a field can be very complex, however. That's why a resistant gene may not be effective across a whole field. Even so, using a resistant soybean variety is recommended in fields with a history of Phytophthora. The same seed treatments that you can use to help control Pythium can reduce risk of early season Phytophthora damping off."
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