These recommendations are the cooperative effort of soybean researchers and Extension specialists in the North Central region. Click on image to view publication
Start with recordkeeping
Taking accurate notes about where and how much white mold occurs in each soybean field is important for future disease management planning. Tracking disease levels across years also will help determine the potential sclerotia (inoculum) load that may be present in a particular field.
Recording disease and yield performance for different varieties will help in future variety selection for
fields with a history of white mold
No soybean variety is completely resistant to white mold, but partially resistant varieties are available. A partially resistant variety has significantly less disease incidence than a susceptible variety, but some disease will occur when conditions are conducive. Avoid planting highly susceptible varieties in fields with a history of white mold.
Variety selection should be based on resistance ratings determined across multiple locations and multiple years. Check with seed dealers and local Extension for variety tests that ncludes varietal responses to white mold. Note that testing conditions and scoring methods vary within the seed industry.
Short crop rotations, such as a soybean-corn rotation, will eventually lead to a build up of sclerotia in the field. A minimum of two or three years of a non-host crop, such as corn or small grains (wheat, barley or oats) can reduce the number of sclerotia in the soil. Most sclerotia die over a three- to four-year period between soybean crops.
Forage legumes, such as alfalfa and clovers, are less susceptible to infection but still can be infected by S. sclerotiorum.
Soybean fields with a history of white mold should not be in two or three year rotations with other crops susceptible to white mold. These include edible beans, canola, cole crops (cabbage, broccoli) pulse crops (peas, chickpeas and lentils), sunflowers and potatoes.
The impact of tillage on white mold development is inconsistent, although several studies have indicated lower levels of disease in no-till.
Deep tillage may initially reduce white mold incidence by removing sclerotia from the upper profile which may reduce the number of apothecia produced. However, sclerotia can remain viable for more than three years if buried 8 to 10 inches in the soil, and may be returned to the soil surface in subsequent tillage operations.
Although more sclerotia are found near the soil surface in no-till systems, they may degrade faster in these soils compared to tilled soils.
15 inch row spacing. University of Wisconsin
Early planting, narrow row width, high plant populations, and high soil fertility all acclerate canopy closure and favor disease potential. However, maintain yield potential when modifying these practices.
High plant populations (175,000 plants or greater) have been associated with increased white mold incidence. Consider decreasing plant populations in fields with a history of white mold; however, be sure populations maintain yield potential.
Wider row spacing (more than 20 inches) can sometimes reduce white mold, but this does not always result in increased yield.
The application of manure should be avoided on fields with a history of white mold.
30 inch row spacing. University of Wisconsin
Many common weeds are also hosts of S. sclerotiorum., including Canada thistle, common vetch, redroot pigweed, curly dock, shepherd’s-purse, common burdock, dandelion, sow thistle, common chickweed, field pennycress, toothed spurge, common cocklebur, henbit, velvetleaf, common lambsquarters, common purslane, common ragweed, wild mustard and others. High weed populations of any kind in a soybean field also may contribute to the plant canopy, favoring disease development.
Avoid excessive irrigation until after flowering. Low moisture levels are critical for reducing the potential for apothecia formation and white mold development. Infrequent, heavy watering is better than light, frequent watering.
Some foliar-applied fungicides and herbicides have efficacy against S. sclerotiorum, although none offers complete control. Fungicides inhibit infection and growth of S. sclerotiorum, but how inhibition occurs depends on the specific fungicide. Currently, fungicides from three different chemistry classes are registered for white mold control in soybean (see "Fungicide Efficacy for Control of Soybean Foliar Diseases 2017). All of these fungicides have limited movement in plant tissues, and only upward movement is possible — none moves downward in the plant where infection by S. sclerotiorum often takes place.
Complete control of white mold using only chemical management strategies is generally not attainable, and, therefore, it should be considered only as one component of an integrated white mold management program. Reduction of white mold incidence achieved by fungicides in university field trials ranged from 0 to approximately 60 percent.
Sclerotium of Sclerotinia sclerotiorum colonized by the bacterium Coniothyrium minitans. Photo: A.J.Peltier
Biological control may be valuable as a long-term strategy to reduce sclerotia in a field. The fungus Coniothyrium minitans is the most widely available and tested biological control fungus for managing white mold. It is commercially available as Contans®. Application of C. minitans should occur a minimum of three months before white mold is likely to develop. This allows adequate time for the fungus to colonize and degrade sclerotia. Degraded sclerotia will not produce apothecia and, therefore, will not produce ascospores to initiate infection of soybean.
C. minitans should be incorporated as thoroughly as possible to a depth of 2 inches. Avoid additional tillage that can bring uncolonized sclerotia to the soil surface.
There are limited data available to document the efficacy of C. minitans for white mold management in soybean. The majority of studies published to date have focused on crops other than soybean. From this limited research, sclerotia numbers have been reduced by as much as 95 percent and white mold incidence has been reduced from 10 to almost 70 percent. More studies are underway.