The wet and cool spring conditions in many areas of the Midwest have provided near-perfect conditions for soybean white mold to take hold. Below we examine the risk factors and conditions that favor its growth and recommendations for successful management.
What to Look For
According to the Soybean Research and Information Initiative, white mold is big problem in the North-Central part of the US. Cool and moist conditions when soybeans are flowering play the biggest part in determining its severity. It appears as a white, cottony, moldy growth on soybeans, and if present, can substantially impact yields. Not only that, but white mold can also affect seed quality, seed production, and even lead to plant death.
White mold in soybeans used to happen infrequently. Now the fungus is considerably more common. It’s been widespread since the 1990’s across much of the Great Lakes area, and by 1992 it was found in parts of the North-Central US.
Scientists aren’t sure why we’re seeing the fungus more often. It could be due to the greater canopy densities of soybean plants, changes in the genetic base of soybeans, or changes in the white mold pathogens.
Cool and Moist Spring Weather Ideal for White Mold Growth
Ryan Crossingham of Ag Update spoke with Jason Snell, an agronomy service representative at Syngenta back in May. Because of the extremely wet and cool spring across the Midwest, Snell predicted that white mold would be a concern this year. Prevalent delayed planting has moved growth stages back, and now they’ve lined up more with disease development timings.
“Cooler temperatures and slower growing conditions can lead to longer exposure during the timeframe of those stages that are most susceptible to disease pressure,” he said.
Of all the plant diseases out there, Snell is most concerned this season about white mold. He fears it might be as big of a problem this year as it was in 2017.
“Two years ago in 2017, there was a lot of white mold, so a lot of guys on a corn/soybean rotation had corn last year and now beans this year. The white mold inoculum can stay in the soil for 7-8 years, so if you had an issue two years ago or thought you might have, that inoculum is now in the soil.”
When it Strikes
It will attack the best looking fields. It will begin just as soybeans are beginning to flower, at the R1 stage. White mold needs dying material, so once the blooms begin to fall off and get caught in the branches of the beans. Spores can infect other blossoms and move on to the rest of the plant.
Iowa State University Extension says that symptoms of the disease can be seen between the R3 and R6 stage, and will appear as gray or white lesions at the nodes. Tissues between major veins can become grayish-green in color, and is often mistaken for other plant diseases. The lesions will progress, spreading above and below the nodes to the entire stem. White, fluffy growth will cover the infected areas, and eventually, black sclerotia will appear at later stages of the disease.
Snell says that the good news this year is that there are some new fungicides available to treat white mold. Syngenta has developed Miraves Neo, which is a combination of Dependent, Quadris and Tilt.
“Dependent is carrying the weight on the mold, Tilt is a curative for other diseases and Quadris is for overall plant health. You’re getting increased plant health, as well as white mold protection. We’re really excited about this product.”
Rhiannon Branch of Brownfield Ag News reports on an episode of Managing for Profit, which takes a look at management practices to prevent and treat white mold along with the financial and yield benefits of doing so. In the episode, Todd Thumma with Golden Harvest provides various product recommendations. You can listen to the episode here.
The Iowa State University Extension provides some bullet-point tips on management of white mold:
- Variety selection: There are no soybean varieties completely resistant, so varieties are rated based on degree of premature plant death. The rate of plant death is slower for some varieties and allows an acceptable yield even if the disease is present.
- Crop rotation: To reduce the amount of white mold sclerotia in the soil, it’s recommended to have two or three years of a non-host crop like corn, small grains and forage legumes. If a particular field is at risk for white mold, don’t rotate beans, peas, sunflowers or cole crops.
- Tillage: More sclerotia are found near the soil surface in no-till systems, but sclerotia numbers will decline if left undisturbed. Sclerotia will remain viable if they become buried 8 to10 inches in the soil.
- Canopy management: Before changing any practices that promote canopy closure, make sure your field isn’t at risk for white mold. More bushy soybean varieties and those with a dense, closed canopy are more conducive to white mold growth.
- Weed control: Many broadleaf weeds are hosts of the white mold pathogens. Certain herbicides (e.g., Cobra®) may suppress whit mold growth.
- Biocontrol: Some antagonistic fungi (found in Contans®) may be applied to the soil to colonize and reduce sclerotia numbers.
- Fungicides: Fungicides can manage white mold, but application timing is critical. They’re the most effective when applied immediately before infection.
Soybean white mold may be a big problem this year. Weather conditions and late planting have been ideal for the fungus to take hold. Know what to look for, when it’s likely to become a problem, and how to manage it effectively so it doesn’t impact your yields this season.