White mold, which can survive in soil eight or more years, has become “one of the most damaging pathogens of soybeans in the upper Midwest,” says Palle Pedersen, Iowa State University Extension soybean specialist. “It's also a tricky pathogen since yield losses don't occur every year.”
Soybean white mold, also known as Sclerotinia stem rot, is caused by a soil-borne fungus, Sclerotinia sclerotiorum. It infects the soybean primarily by entering the flower petals as well as through severe abrasions or wound areas on the plant — caused by hail, for example.
“The flowering period, spanning roughly 30 days, represents the window of susceptibility,” says Pedersen, “especially when environmental conditions are favorable for fungal growth.”
Rain, cool temperatures below 80ÞF, high relative humidity and moist soil are all conditions that promote white mold growth.
Once you get past the R1 to R3 growth stages, the chances of infection diminish, according to Craig Grau, University of Wisconsin-Madison plant pathologist.
“You may see some infection on side branches where flowers may still be forming. However, the infection usually remains benign and doesn't migrate into the main stem and kill the plant,” says Grau.
According to Emerson Nafziger, University of Illinois Extension agronomist, white mold was considered one of the biggest yield robbers in 2004. In some years with cool temperatures and wet conditions, yield losses up to 60% were reported.
Extreme yield losses of up to 75% were experienced in parts of northeastern and eastern Iowa, according to Pedersen. In Wisconsin, yield reductions averaged about 5% in 2004, but in some fields losses from 30% to 40% were reported, according to Grau.
While the clear-cut reasons for white mold's sudden spread in the north-central region are not fully understood, many agronomists point to changes in cultural practices and shorter rotations, as well as changes in the genetic base of current soybean varieties or the white mold pathogen itself.
“Management practices and environmental conditions that boost crop canopy density are also strong culprits in the spread of white mold,” says Grau. “Canopy management and the timing of its closure can make a difference in many cases as to whether or not white mold becomes prevalent, especially if wet conditions prevail.”
Grau says the growth and pathogenic activity of the white mold fungus are encouraged by dense soybean canopies created by planting in narrow row widths, high seeding densities, early planting, high soil fertility and other factors that promote plant health. It's unfortunate, Grau says, because in many ways white mold penalizes progressive soybean growers.
“However, it can be managed by variety selection,” says Iowa's Pedersen.
Since 1996, Grau and other researchers at the agricultural colleges in Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, North Dakota, Ohio, South Dakota and Wisconsin have received checkoff funding from the North Central Soybean Research Program to tackle the white mold problem. Some of that work has involved searching and identifying germplasm with increased resistance and developing more unified control recommendations to producers.
To help combat white mold, Pedersen and Grau recommend several basic strategies:
Do a thorough job of scouting fields for white mold and keep good field records on where the disease is occurring and under what types of conditions.
If you have problems with white mold, be sure to closely examine and stay up-to-date on variety trials so you can focus on particular lines exhibiting resistance or tolerance.
Good variety selection is still your best defense, according to the agronomists, and it can serve as the foundation of your white mold management plan. You can't compromise on variety selection if you have a history of white mold in a field.
Keep close tabs on your seeding rates and row spacings to maintain a canopy that isn't overly dense and promotes good airflow. Choosing the optimum seeding rate and not exceeding it will help somewhat. If white mold has been an ongoing problem, you might want to consider 30-in. row spacings.
“If producers are using varieties with a very solid record of low mortality from white mold, I wouldn't hesitate to continue drilling, if that's your plan,” says Grau. “And if you're using a 30-in. row system, I'd continue with that also. However, I would keep seeding rates below 150,000 — probably more in the 125,000-130,000 range.”
Reevaluate your tillage practices and/or crop rotations if you're experiencing white mold problems.
“Based on Wisconsin data and surveys of producers during the '90s in the North Central states, I think a blanket statement can be made that white mold tends to be less severe and prevalent in fields under no-till management,” says Grau. “We don't yet totally understand the full reasons for this, but that's what we're seeing from the data so far. The worse cases of white mold I've seen have been with moldboard plowing.”
Pedersen adds, “Since soybeans are a host plant of white mold, planting them more frequently in specific fields, year after year, can contribute to the problem. If white mold is present, planting continuous beans will only encourage the reproduction of the fungus that could lead to a high population in the soil and result in damaging outbreaks.”
The risk is even greater if other host crops are grown in rotation with soybeans. There are literally hundreds of host plants for white mold, according to Pedersen.
Besides soybeans, other broadleaf plants that are highly susceptible to white mold include dry beans, snap beans, lima beans, sunflowers, canola, carrots and cabbage.
Peas, potato, alfalfa and red clover, although considered hosts, are less susceptible to the white mold pathogen. Examples of non-host crops include corn, small grains and all forage grasses.
“Over the years, I've seen less white mold problems when comparing a corn-soybean-winter wheat rotation compared to a traditional corn-soybean rotation,” says Grau. “This has been especially true in the southeast part of Wisconsin where some producers have added a small grain into their rotation.”
Grau encourages producers who have white mold problems to seriously consider mixing up their crop rotation by adding in a non-host crop such as small grains, grasses or even a rye cover crop, for example.
“Using non-host crops like small grains tends to shade the ground more thoroughly and help deplete the white mold fungus,” he says.
According to the agronomists, it's also important to control broadleaf weeds that can also be hosts of the white mold pathogen, especially in crops grown in rotation with soybeans.