The 2009 growing season had record cool weather in July that was ideal for soybean white mold occurrence. The disease was widespread in the north central region of the United States and agronomists even in southern Iowa observed this disease in many soybean fields.
X.B. Yang, Iowa State University plant pathologist, reports that In northern Iowa, patches of soybeans killed by white mold were obvious in many soybean fields along highways. Some farmers reported losses totaling more than $10,000 from white mold.
"Previously, white mold epidemics occurred mainly in even-numbered years because of the use of corn-soybean rotation plans," Yang says. "The outbreaks in 2009 were the first in an odd-numbered year since 1997 and they covered almost the entire north central region of the country except for areas west of the Missouri River.
"The 2009 epidemic suggests that there are plenty of white mold inoculums in the soil in the region and that it will be necessary to take precautions in 2010 to minimize the white mold risk for the 2010 season and following years."
If a soybean field had white mold in 2009 and the field will be in corn in 2010, Yang says you should no-till corn.
"Rotations with corn in no-till can reduce soybean white mold risk," Yang says. "Tillage will bury the sclerotia into soil, which increases the survival rate of white mold fungus. Sclerotia can survive in deep soil up to 7 years."
Yang says sclerotia within 2 inches of the soil surface germinate and produce spores even in corn fields. Under no-till, a large portion of the sclerotia germinate under the corn canopy, which reduces the amount of pathogen in the soil, he says.
He adds that the white mold risk in tilled fields was two to four times higher than in no-till fields, according to a study conducted 10 years ago.
"Rotation with seed corn fields will not be effective in white mold management," Yang says. "Sclerotia do not germinate in an open corn canopy field caused by detasseling. Don’t plant seed corn in a field that previously had white mold on soybeans, as a rotation with seed corn increases white mold risk."
Yang says fields that had white mold in 2009 should not be in soybeans in 2010. The risk for white mold can be high unless 2010 is a dry season.
He says you can also consider applying Contans to break the disease cycle. Contans is a biological control agent proven to be effective in white mold control in many high-value crops and is labeled for soybeans. Unfortunately, he says little research data from the public sector are available on the use of Contans to control soybean white mold.
If a field had a white mold outbreak in the last few years and will be in soybeans in 2010, Yang says you should avoid using susceptible varieties in 2010 and consider using white mold-tolerant varieties for your next soybean rotation.
"High-yielding, white mold-tolerant varieties are available," he says. "One thing to keep in mind is that the tolerance protection sometimes may not work due to a lack of consistency in disease pressure during the breeding of the mold-tolerant variety.
"Also, avoid planting soybean with narrow rows of 15 inches or less in fields that had white mold in the past."
Chemical control is an option if white mold risk is high in the coming season. There are several fungicides on the market for white mold control.
"Not all fungicides are effective for white mold and it's very important to use the right fungicides to control white mold," Yang says. "Read labels when you makie your plans this winter. Knowing the risk of white mold infection in July is the key to a good economical return in the control of white mold."