White Mold of Soybean: Web Book

Management of White Mold

Management of White Mold

Multiple white mold management tools are available, and incorporation of multiple strategies is the best way to manage white mold. Additionally, management is informed by taking accurate field notes and using the available prediction tools.

Integrated Management of White Mold

Recordkeeping

Taking accurate notes about where and how much white mold occurs in each soybean field is important for future disease management planning. Sclerotia can survive for as many as eight years in soil. Tracking disease levels across years also will help to 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 variety selection for fields with a history of white mold. Farmers with precision planting capabilities may also find it useful to map specific locations within fields where white mold occurs. This enables targeted fungicide applications and planting population adjustments for these specific areas.

Cultural Control

Crop Rotation

A minimum of two to three years of a non-host crop, such as corn, flax, or small grains (for example, wheat, barley, or oat), can reduce the number of sclerotia in the soil. Forage legumes, such as alfalfa and clovers, are less susceptible to infection but are hosts for S. sclerotiorum. Soybean fields that have a history of white mold should not be in two- or three-year rotations with broadleaf hosts such as edible beans, canola, cole crops (cabbage, broccoli, etc.), pulse crops (peas, chickpeas, and lentils), sunflowers, and potatoes.

Tillage

The impact of tillage on white mold development has proven inconsistent. Deep tillage may initially reduce white mold incidence by removing sclerotia from the upper soil profile. However, sclerotia can remain viable for more than three years if buried 8-10 inches (20-25 cm) 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, sclerotia may degrade faster in no-till production systems.

Plant Populations

High plant populations contribute to dense, closed canopies. Greater plant populations have been associated with increased incidence of white mold. Consider decreasing plant populations while still maintaining populations required for good yield in your area. Use the lowest possible seeding rate that will achieve the recommended final plant density for your area. Local extension agronomists can help determine this number.

Row Spacing

Soybean planted into narrow rows may lead to faster and more complete canopy closure around the time of soybean flowering (Figure 1). Moving from 15-inch to 30-inch row spacing can sometimes reduce white mold severity by as much as 50 percent. However, moving to a wider row spacing can in some cases result in lower yield potential compared to narrow row spacings.

Figure 1. Narrow row spacings can promote white mold development.

Image: Adam Sisson

Planting Date and Relative Maturity

Early planting, late-maturing varieties, and varieties with a bushy architecture or that have a tendency to lodge can contribute to more closed canopies. However, direct impact of these factors on white mold incidence and yield varies, because disease development is highly dependent on weather conditions during the reproductive growth stages.

Fertility and Plant Nutrition

High soil fertility, especially the use of nitrogen-rich manures and fertilizers, favors white mold development by promoting lush plant growth and early canopy closure. The application of manure should be avoided on fields with a history of white mold.

Weed Control

Many common broadleaf weeds found in fields used for soybean production also are hosts of S. sclerotiorum (Table 1; Figure 2). High weed populations may also contribute to the plant canopy density, favoring disease development.

Table 1. Common host weeds of S. sclerotiorum. Adapted from Peltier et al. 2012.

Canada thistle Common vetch Redroot pigweed
Catchweed bedstraw Curly dock Shepard's purse
Common burdock Dandelion Sow thistle
Common chickweed Field pennycress Toothed spurge
Common cocklebur Henbit Velvetleaf
Common lambsquarters Hemp Venice mallow
Common purslane Jerusalem artichoke Wild carrot
Common ragweed Jimsonweed Wild mustard
Common sunflower Prickly lettuce Wild parsnip

Cover Crops

The use of small grain cover crops (like oat, wheat, or barley) grown with soybean can stimulate earlier emergence of apothecia compared to soybean grown alone. This can potentially lower white mold incidence. Consider first how cover crops may affect soil moisture, availability of soil nutrients, and shading before implementing. In organic systems, planting into roller-crimped rye cover crops can substantially reduce white mold incidence and severity. The mat of rye left after roller-crimping produces a dark environment at the soil surface that is not conducive for complete apothecial development. The thick rye mat may also function as a physical barrier limiting ascospore release and movement.

Irrigation Management

Avoid excessive and frequent irrigation during flowering. Low moisture levels within the soybean canopy are critical for reducing the potential for white mold development. Infrequent, heavy watering is better than frequent, light watering.

Variety Selection

Moderately resistant soybean varieties are available (Figure 3). Although resistant varieties contribute to lower disease severity, some disease development will occur when conditions favor white mold. Plant the least susceptible variety in fields with a history of white mold. Severity of white mold also depends on soybean genetics. Specific genetic traits can help plants avoid infection, or traits can contribute to physiological resistance where plants actively battle infections. New research has recently enabled quicker identification of resistant soybean plants and allows for comparisons across different soybean breeding programs at all institutions.

Figure 2. White mold on velvetleaf, a common broadleaf weed found in soybean fields.

Image: Daren Mueller

Figure 3. White mold-resistant soybean variety (upper middle) planted among disease-susceptible plants.

Image: Craig Grau

Chemical Control

Fungicides (and some PPO herbicides such as lactofen) can be a part of an integrated management system for white mold. Some foliar-applied fungicides and herbicides have efficacy against white mold, although none offer complete control.

Fungicides inhibit infection and growth of S. sclerotiorum, but how inhibition occurs depends on product selection. There are numerous products on the market that are labeled for white mold management. Table 2 includes products and programs that are most commonly used and have been evaluated in diverse locations. All of the effective products have limited upward movement in plant tissues, and none move downward in the plant where infection often occurs.

Herbicides containing lactofen as the active ingredient (Cobra® or PhoenixTM) do not directly inhibit S. sclerotiorum, but may reduce white mold incidence. 

Lactofen can modify the soybean canopy and delay or reduce flowering, which may alter the availability of potential infection sites for S. sclerotiorum.

Lactofen also can induce a systemic acquired resistance (SAR) response that increases production of antimicrobial chemicals known as phytoalexins (for example, glyceollin) by the soybean plant. Phytoalexins can inhibit the growth of S. sclerotiorum. Although these herbicides have potential benefits, their use also may result in crop damage that can reduce yields, particularly in years not conducive for disease.

The Crop Protection Network (CPN) publication Pesticide Impact on White Mold (Sclerotinia Stem Rot) and Soybean Yield (CPN-5001) details the efficacy and economics of using the pesticide programs listed in Table 2. It should be noted that some of the more efficacious programs are often more expensive. Thus, the economics of using a particular program should be considered relative to your soybean yield potential and grain sale price.

Table 2. The common active ingredients and associated treatment costs evaluated in white mold pesticide efficacy trials. Adapted from Willbur et al. 2019.

Active lngredient(s)  Trade Name (suggested application stage)  Typical Application Rates Active Ingredient Cost ($/A) Application Cost ($/A) Total Treatment Cost ($/A)¹
boscalid  Endura® (RI)  8.0 oz.  $38.76 $7.28 $46.05
boscalid + fluxapyroxad + pyraclostrobin Endura® (RI ) fb² Priaxor® (R3)  6.0 oz. fb 4.0 fl. oz. $46.94 $14.57 $61.51
fluazinam  Omega® (RI)  12.0 fl. oz.  $36.85 $7.28 $44.14
fluoxastrobin + flutriafol  Fortix® (RI)  5.0 fl. oz.  $16.33 $7.28 $23.61
lactofen  Cobra® (RI)  6.0 fl oz. $9.04 $7.28 $16.33
picoxystrobin  Aproach® (RI) fb Aproach® (R3)  9.0 fl. oz. fb 9.0 fl. oz. $39.94 $14.57 $54.51
prothioconazole  Proline® (RI) fb Proline® (R3)  5.0 fl. oz. fb 5.0 fl. oz.  $46.18 $14.57 $60.75
prothioconazole + trifloxystrobin  Proline® (RI) fb Stratego YLD® (R3)  3.0 fl. oz. fb 4.0 fl. oz.  $28.64 $14.57 $43.21
tetraconazole  Domark® (RI)  5.0 fl. oz.  $13.32 $7.28 $20.60
thiophanate-methyl  Topsin® (RI)  20 fl. oz.  $7.26 $7.28 $14.54
non-treated control  _ _ $0.00 $0.00 $0.00

¹Total Treatment Cost is the sum of the chemical list price and application cost;  ²fb = followed by. Several programs involve two fungicide applications. Those are indicated by fb in the trade name column.

Sporebuster

Recent university research was used to develop Sporebuster, a smartphone application for iPhone and Android platforms. The app was funded by soybean checkoff dollars and is freely available. Sporebuster helps farmers make economic decisions when they are selecting pesticides and deciding when to apply them to manage white mold. Sporebuster can run various scenarios – when disease pressure is low, medium, or high; when revenue potential is different – while adjusting pesticide application cost. This is a dynamic tool growers can use to tailor pesticide programs to specific farms and situations.

Sporebuster uses economic models and user inputs indicating soybean sale price, yield potential, and local pesticide program prices to estimate the likelihood of breaking even and average net gain when using the program. Sporebuster is available for iOS and Android.

For more information about fungicides available for white mold management, consult the CPN publication Fungicide Efficacy for Control of Soybean Foliar Diseases (CPN-1019).

How to use Sporebuster app for white mold fungicide decisions

Application Timing

Fungicide must be applied at the proper growth stage to maximize efficacy for white mold control. Fungicide applications at the R1 growth stage (beginning bloom) provide a greater level of control than applications made to soybean at the R3 growth stage (beginning pod). Efficacy of fungicides for white mold management declines greatly after symptom development. Use the Sporecaster smartphone app to assist with fungicide timing decisions. Sporecaster is available for iOS and Android.

Spray Coverage

Adequate plant coverage by a pesticide spray, deep in the soybean canopy where infections start, is important for managing white mold with foliar fungicides. Flat-fan spray nozzles that produce fine to medium droplets (approximately 200-400 microns) provide the best fungicide coverage on sprayed plants. Using higher carrier rates such as 20 GPA can also assist in providing good coverage of the plant by the pesticide spray, especially when the canopy is thick. Follow manufacturers’ recommendations for spray volume and be aware of environmental conditions (such as wind speed) that influence coverage.

Control Expectations

A chemical management strategy does not result in complete control of white mold and, therefore, should be considered only as one component of an integrated management program. Reduction of white mold incidence achieved by fungicides alone in university field trials ranged from zero to approximately 60 percent. Chemical control should be used in addition to using resistant varieties, changing the rotation, integrating cover crops, etc. Do not rely on chemical control alone for white mold management in soybean.

Biological Control

Biological control can also be part of an integrated white mold management program. The fungus Coniothyrium minitans is the most widely available and tested biological control fungus and is commercially available as Contans®. Application of C. minitans should occur a minimum of three months before white mold is likely to develop. Timely applications allow 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 two inches. Avoid additional tillage that can bring non-colonized sclerotia to the soil surface.

There are limited data available on the efficacy of C. minitans for white mold management in soybean. In a few studies, the sclerotia number was reduced by as much as 95 percent and the subsequent white mold incidence was reduced by 10-70 percent. 

Biological control products will not eliminate all sclerotia; fields heavily infested with sclerotia may continue to have disease development until the number of sclerotia in the soil is further reduced. More studies are needed to evaluate the efficacy of biological control products and their potential to reduce white mold of soybean, especially in fields with native populations of biological control fungi.

Image: Daren Mueller

Core white mold management considerations

Maintain records of field history and disease incidence of white mold.

Select soybean varieties carefully:

  • Use varieties with the best available levels of resistance.
  • Select the most appropriate maturity group for your region.
  • Use pathogen-free seed.

Follow good cultural practices:

  • Reduce plant populations and increase row width.
  • Rotate with non-host crops (especially small grains)
  • Consider reduced or no-till practices.
  • Use cover crops to reduce inoculum density.

Use fungicides properly. They may be warranted in fields with a history of white mold and where the risk of white mold is high. Fungicide application should occur between R1 and R3, before disease develops, for best results.

Consider biological control, which may be valuable as part of a long-term integrated management strategy to reduce sclerotia levels in a field.

Where irrigation is used, reduce frequency during flowering. Ensure irrigation is applied according to soil moisture requirements (i.e. avoid excessive irrigation events).


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Management of White Mold