Since the third week of July, several diseases of soybeans started to show up in some Iowa soybean fields--white mold, sudden death syndrome (SDS) and downy mildew. "In most years, these diseases usually arenâ€™t seen until early August," says X.B. Yang, Iowa State University extension plant pathologist. "However, this summerâ€™s cool weather has promoted the unusually early occurrence."
So far, white mold has been found "here and there" in fields in the eastern half of Iowa. Some fields in early August had already had 10% to 20% dead plants. "For most fields, the level of pathogen should be low because we have had dry summers for several years in a row, prior to 2004," says Yang. "It will be a surprise if the disease turns out to be as severe this summer as we saw in 1996 or 1998. In those years, severe damage occurred in many soybean fields."
There is really nothing you can do to treat the disease now, says Palle Pedersen, Iowa State University extension soybean agronomist. He recommends you check your fields and write down the name of the bean variety you have planted this year.
If you find white mold in the field this summer, keep your notes. The next time you plant soybeans in the field again, youâ€™ll know youâ€™ll need to plant a resistant soybean variety.
"If you are finding white mold now, you know you are building up inoculum in that field and you need to select the right soybean variety the next time you plant soybeans thereâ€”a variety that is resistant to white mold," says Pedersen.
To view a photo showing a soybean plant with symptoms of white mold disease, click HERE.
Sudden death syndrome also occurring
SDS usually shows up in early August in Iowa, and like white mold, it showed up two weeks early this year. The ISU Plant Disease Clinic at Ames received early samples from Washington County in southeast Iowa, where the disease has been a problem for years. "In our experimental plot at Ames, some early planted beans had 50% defoliation by the third week of July," says Yang.
Because the 2004 growing season was cool early, it is likely that Iowa will have a number of fields with SDS this year, especially those fields planted before the first week of May. "Take good notes on the occurrence of this disease in your field and use this information for next yearâ€™s management," says Yang.
"SDS is manageable with resistant varieties and a planting plan," he says. "Unfortunately, there are no options to stop this disease for this season."
Downy mildew is showing up too
Cool weather and a moist foliar canopy in the evenings and nights are good for promoting several foliar diseases. Brown spot and bacterial blight are prevalent this year as is downy mildew.
Downy mildew is a fungal disease. This seedborne disease affects seed quality. It can be controlled by using fungicides that may be applied to seed bean if the disease risk is high.