White mold is a significant disease problem of soybean in the north-central region. The incidence and severity of white mold varies from year to year because of its sensitivity to weather conditions. Cool, moist conditions at the time of flowering is most conducive to the development of white mold.
White mold is caused by the soil fungus, Sclerotinia sclerotiorum. It is easily distinguished from most other soybean diseases by the presence of white cottony mycelia (moldy growth) and sclerotia on infected plant tissues.
White mold can substantially reduce yield, especially when climatic conditions and management practices favor high yield potential. Severe white mold infection weakens the plant and can result in wilting, lodging and plant death.
In addition to causing yield loss, white mold can affect seed quality and seed production.
White mold has progressed from a sporadic disease to an annual threat to soybean production
White mold was discovered in central Illinois in 1948. Although it eventually became a chronic problem in Michigan, Minnesota, and Wisconsin, outbreaks were localized, and occurred where soybeans were grown in rotation with other susceptible crops.
Beginning in 1990, however, the occurrence of white mold became more widespread in the Great Lake states, and by 1992 was prevalent throughout all the north-central states.
Reasons for the sudden increase of white mold are not fully understood, but are thought to be related to changes in cultural practices that promote a greater canopy density. The increase in white mold may also be due to changes in the genetic base of current soybean varieties, or changes in the white mold pathogen.
Developing a management plan based on knowledge of field history and best disease management practices can help reduce losses from white mold. An effective management plan should include adept recordkeeping, cultural practices, varietal resistance, and possibly chemical and biological control tactics.
(A) Sclerotinia sclerotiorum survives in the soil as sclerotia — hard, black structures that resemble mouse droppings.
(B) Sclerotia within the top two inches of soil can germinate to produce apothecia. Shaded, moist and cool soil favors germination. Apothecia are small, tan, cup-shaped mushrooms about 1/8 to 1/4 inch in diameter.
(C – D) Apothecia produce spores called ascospores that colonize senescing soybean flowers.
(E) Infection can spread into the stem at the node, and is favored by moisture and cool temperatures.
A dense canopy during flowering (growth stages R1 through R3) may provide an ideal microenvironment for spore production, dispersal, and white mold development.
(F) As the fungus grows from the young pods to the nodes and stem of the plant, it eventually girdles the stem and disrupts the transport of water and nutrients. This causes leaves and stems to turn brown and stand erect above the soybean canopy. Closer inspection of the stems may reveal thick white mold, and sclerotia of S. sclerotiorum, which can form on the inside and on the outside of infected stem and pods.
(A) The sclerotia fall to the soil during harvesting or remain in crop debris where they survive for many years. In the spring, sclerotia may germinate to form the apothecia, starting the disease cycle again.
White mold causes yield loss to soybean by reducing seed number and weight. Potential yield loss can be based on estimates of the disease incidence in several sections of the field.
To measure disease incidence, divide the number of plants with symptoms of white mold by the total number of plants assessed. For example, if 100 plants were examined and 45 plants had symptoms, the disease incidence would be 45%.
For every 10 percent increase in the incidence of white mold observed in the R7 soybean growth stage (beginning maturity), yield is reduced by 2 to 5 bushels per acre.
Sclerotia may be observed in harvested grain, which may result in a reduced grain value due to foreign material.
Sclerotinium sclerotiorum can infect soybean seed and be an important source of inoculum if planted into fields with no history of white mold.
Scouting for mushrooms (apothecia) at canopy closure
The best time to scout for apothecia is at canopy closure. This is generally in late June and early July in the north-central region, and will vary with row spacing.
Check the soil under plants growing in high soil moisture areas and closed canopies, especially during flowering. Check areas where moisture collects due to fogs, extended dew periods, and in pockets of poor air drainage.
Scouts should be aware of the difference between apothecia growing from sclerotia, and other harmless mushrooms, such as the one produced by the birds-nest fungus, growing on pieces of organic matter.
University research has indicated that the appearance of apothecia can be predicted using several variables including weather and the amount of soybean row closure in a field. Based on this research, Sporecaster models have been developed to forecast the risk of apothecia being present. You will need to scout your field to determine if the soybean crop meets thresholds such as canopy closure and the presence of flowers. For more information on how to use the app, including video tutorials, see “New smartphone app: Sporecaster, The Soybean White Mold Forecaster” from the University of Wisconsin.
Scouting for white mold-infested plants during pod set
Check near tree lines or other parts of a field that experience less wind disturbance, parts of the field with thick canopies, and fields with a history of white mold. White mold often occurs in patches within fields. Within these patches, look for scattered dead plants.
Infected plants appear wilted, which might be mistaken for drought. Inspect the stems for water-soaked lesions that progress above and below infected nodes, and eventually encircle the stem.
Look for white cottony mycelia (moldy growth) and sclerotia on infected plant tissues. These are signs of S. sclerotiorum that are diagnostic for white mold and allow it to be distinguished easily from most other soybean diseases.
The growth of the white mold fungus is strongly affected by the environmental conditions in the crop canopy, and is particularly favored by dense soybean canopies.
Rain events during flowering, and wet areas in fields due to fog and extended dew periods are especially favorable for white mold. Risk assessment for white mold is based on these factors, as well as the field history and soybean variety.
|Seasonal risk factors||Long-term risk factors|
Moderate temperatures, normal or above normal precipitation, and soil moisture at field capacity or above.Prolonged morning fog, and high canopy humidity (leaf wetness) at flowering into early pod development.White mold potential decreases if air temperatures are high (90 F) during flowering and early pod development.
Pathogen population (numbers of sclerotia) will gradually increase if
(1) other susceptible host crops are grown in rotation with soybean
(2) only 1- to 2-year intervals occur between soybean crops
(3) white mold susceptible varieties are grown.
|Early canopy closure
Early closure due to early planting, high plant population, narrow rows, and excessive plant nutrition, together with optimal climatic conditions creates a dense canopy and increased numbers of apothecia on the soil.
|Weed management systems
Pathogen population will increase if broadleaf weeds are not controlled because many weeds are hosts for S. sclerotiorum. Some herbicides used in rotation systems may be suppressive to white mold.
|History of white mold
Previous history of white mold is a major risk factor for future white mold development. It’s difficult to eradicate S.sclerotiorum from infested fields.The distribution of S. sclerotiorum and white mold in a field is usually not uniform but occurs in “hot spots”.
|Topography of field
Pockets of poor air drainage, tree lines, and other natural barriers that impede air movement will create a favorable microenvironment for white mold development.
|Soybean variety planted
Varieties range from partially resistant to highly susceptible. Plant structure and physiology govern variety reaction to infection by S. sclerotiorum.Variety reactions are variable if management practices and weather conditions promote excessively dense crop canopies.
(1) contaminated and infected seed
(2) movement of infested soil with equipment
(3) wind-borne spores from apothecia
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.
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.
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 2018). 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.
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.