UW Soybean Plant Health
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Over-wintering sclerotia survive the winter and germinate on or near the soil surface . |
Sclerotia sclerotiorum survives primarily as sclerotia in soil. Sclerotia are distributed throughout the plow layer of the soil and, similar to weed seeds, survive the winter in the soil and germinate if positioned near or at the soil surface when conditions are right in the spring and summer.
Reducing the number of sclerotia, just like depleting a weed seed bank, contributes to the control of white mold. However, sclerotia are long-lived and essentially impossible to totally eliminate from a field as long as a host crop is planted in intervals of 5 years or less.
Alfalfa is a host, but possibly not a good one, as S. sclerotiorum does not seem to multiply in alfalfa rotations as it does in rotations with other host crops. Corn, small grains and other grasses are not hosts and field research suggest that 2-3 years out of soybean (or other host crop) results in lower density of sclerotia in the soil, less white mold, and greater soybean yield potential (Tables 1 and 2).
Host crops of the white mold fungus |
Non-host crops |
| all beans, peas, sunflowers, cole crops | corn, small grains, all grass crops |
Planting continuous soybean or other host crop, especially in a field with a history of white mold, will greatly increase the risk of white mold. Sclerotinia sclerotiorum has a wide host range that includes snap bean, lima bean, dry beans, peas, sunflowers, canola, carrots, and cabbage.
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Table 1. Sclerotia density observed over three years in spring soil samples taken from crop rotation studies in Wisconsin 1995-1997. |
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| Location | No. of sclerotia/liter of soil |
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| 1995 | 1996 | 1997 | |
| Rock County | 4.9 | 2.5 | 1.3 |
| Walworth County | 4.4 | 1.5 | 0.7 |
| Dane County | 4.3 | 1.5 | 0.8 |
Less white mold has been observed in corn-soybean-small grain rotation sequences. Small grains form a dense canopy and promote germination of sclerotia without the presence of a host plant to infect. This situation could result in a drop in sclerotia numbers - provided the soil remains moist enough to encourage sclerotia germination.
Table 2. Rotation effect of two previous years of corn, small grain or soybean on incidence of white mold and soybean yield of moderately resistant and susceptible varieties at two locations in Wisconsin in 1997. |
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| Previous Crop | Moderately Resistant Variety | Susceptible Variety | ||||
| Yield (bu/acre) |
White Mold (% plants killed) |
No. sclerotia/ liter soil | Yield bu/acre |
White Mold (% plants killed) |
No. sclerotia/ liter soil |
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| Corn | 39.8 | 16 | 1.31 | 36.2 | 59 | 1.38 |
| Small grain | 51.1 | 12 | 1.57 | 38.7 | 57 | 1.57 |
| Soybean* | 45.6 | 0.49 | 0.49 | 39.8 | 53 | 1.05 |
| Average | 45.5 | 11 | 1.12 | 38.3 | 56 | 1.33 |
Choices in crop rotation and crop sequences with nonhosts can be made that contribute to the steady depletion of sclerotia in the soil. Choice of soybean variety and management practices that reduce the yearly incidence of white mold will also help to curb the build-up of the pathogen in the soil.
When planting back to soybean, do not use seed from harvested fields where white mold was observed. Sclerotia of the white mold fungus can survive in seed and as sclerotia mixed with seed unless it is professionally cleaned. Climatic conditions and crop canopy characteristics have more of an effect on apothecia numbers and disease development than sclerotia density. Climatic conditions and soil conditions influence canopy development by affecting emergence, plant population and plant growth. Crop canopy, once formed, can buffer the effects of weather changes and maintain conditions favorable for apothecia formation and longevity.
Multiple apothecia are formed from a single sclerotium which affects the relationship between sclerotia numbers and airborne inoculum production. An important but unanswered question: What are economic thresholds for sclerotia numbers? Can sclerotia ever be reduced below an economic threshold and, as a result, limit the severity of white mold in high yield potential soybeans?
Crop rotation is less effective as a control tactic once a field is highly infested with the white mold pathogen. If fields are highly infested, crop rotation will be modestly effective if a moderately resistant soybean variety is used in conjunction with crop rotation and possible other control tactics.
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
11/2008