In the North Central soybean producing areas of the United States and Ontario, Canada sudden death syndrome (SDS) is caused by the fungus Fusarium virguliforme. SDS is one of the most recognizable and important diseased in throughout the region. In some states, SDS may be present the majority of the fields and cause statewide yield reductions. In other, the disease is regional or rare. The symptoms of SDS are oftentimes similar to those caused by other soybean pathogens, making it difficult to diagnose in the field. It is known that the presence of other pathogens, such as soybean cyst nematode (SCN), can also exacerbate the disease.
The first observed SDS symptoms are often early leaf symptoms, including yellow spots between leaf veins, known as interveinal chlorosis. These lesions may expand and turn brown as the spots expand between veins, which is called interveinal necrosis. The leaf veins will remain green. As the disease progressed, leaves die and prematurely fall from the plant, while petioles remain attached to the stem. Pods and seeds may also be aborted.
Root and Stem Symptoms/Signs
SDS may cause discolored and decaying roots even if there are no noticeable foliar symptoms. The woody tissue in the taproot can become brown to grey, while the upper portion of the stem pith remains white. When soils are wet, the SDS fungus can reproduce on the root surface producing a mass of spores that are purple-blue. The foliar symptoms of SDS can appear very similar to several other soybean diseases, which make it important to closely examine the stem and root for an accurate diagnosis.
Favorable Environmental Conditions
Cool, wet conditions shortly after planting favor early root infections and disease establishment. Frequent or heavy rains midseason can favor early symptom expression. This puts irrigated fields at a higher risk for SDS. Hot and/or dry conditions in the spring or summer can halt of development of SDS. Some crop production practices may also increase the risk of disease development. Fields with poor soil nutrients, low pH, poor drainage, or moderate to severe soil compaction are at a higher risk for SDS.
Planting the most resistant varieties available is the foundation of an SDS management program. There are no soybean varieties completely resistant to SDS, but partially resistant varieties are available. However, weather and field conditions influence disease severity, even on partially resistant varieties. As there is a known link between SCN and SDS, planting SCN-resistant varieties and rotating crops may delay SDS onset and reduce disease severity. Most foliar fungicides and seed treatments are not effective in reducing SDS. However, fluopyram (ILeVO®, Bayer CropScience) has SDS severity and protected yield on susceptible varieties.
Minimizing soil movement may slow the introduction of the SDS fungus to new fields. Improving drainage and soil structure and reducing compaction can also help to alleviate SDS symptom severity. Tilling in known compacted areas may reduce disease symptoms by allowing the soil to warm more quickly during the spring. Rotating to corn does not typically reduce SDS in subsequent soybean crops. In fields with a severe history of the disease, delayed planting reduced conditions that favor the disease. Proper soil fertility levels is important in reducing plant stress and maintaining overall plant health, which may help reduce the occurrence of soybean diseases such as SDS. High soil moisture levels within a few days after planting and /or during the mid-late soybean reproductive stages may increase the risk of infection by the SDS pathogen.
Phytophthora root and stem rot is a common soilborne disease caused by a water mold, Phytophthora sojae. In poorly drained soils, Phytophthora can have a sever economic impact.
The most common early season symptoms of Phytophthora are areas of no stand and pre- and post- emergence damping off. Later in the season, Phytophthora symptoms on older plants include a brown lesion on the lower stem that extends from the root into the upper portions of the canopy. Symptoms may also include chlorosis of leaves, wilting, stunting, and death.
Favorable Environmental Conditions
Planting into saturated, warm (>60°F) soils with a history of Phytophthora root and stem rot poses the greatest risk for disease. Disease symptoms is most common in low-lying areas that are poorly drained and prone to saturation or flooding. Planting into no till also poses a risk for disease development due to increased water retention and higher inoculum density in the top 2 to 3 inches.
The best management tools for Phytophthora is the use of resistant varieties. There are two forms of resistance: race specific (Rps genes) and partial resistance. Rps genes will provide complete resistance to certain P. sojae populations. The most common Rps genes are Rps 1a, Rps 1c, Rps 1k, Rps 3a, and Rps 6. Partial resistance offers some resistance to all races of P. sojae. However, partial resistance is not expressed until the first true leaves have developed, leaving emerging seedlings still vulnerable. Other disease management options include some form of and/or tiling to improve drainage and seed treatments with active ingredients mefenoxam, metalaxyl, or ethaboxam.
White mold in soybean is caused by the pathogen S. sclerotiorum. S. sclerotiorum is a fungus that overwinters in the soil in the form of sclerotia which are small black survival structures, usually about the size of a grain of rice. Infection is highly influences by environmental conditions. The sclerotia germinate in the spring during cool and moist conditions producing ascospores which infect the plant through a natural opening or wound. The pathogen growths throughout the plant creating water soaked lesions that cause premature senescence of leaves and in extreme circumstances premature death of the whole plant. White mold can be found throughout the soybean growing region of the central United States and Canada.
Disease Signs and Symptoms
Plants will typically appear healthy until beginning reproductive stages. Early white mold symptoms include premature senescence of leaves and in extreme circumstances premature senescence of the the entire plant.
Description: Field displaying moderate white mold symptoms. Plant displaying typical white mold symptoms of wilting and premature senescence of leaves.
Occasionally apothecia, which are small tan cup shaped mushrooms, can be spotted in the field early in the growing season and are a sure sign that white mold is active in that field. After flowering occurs, the first sign of white mold infection is fluffy white mycelial growth on the main stem or petioles. Black sclerotia will begin to develop on the surface of the infected tissue as the fungus matures.
Description: Typical white mold signs include fluffy white mycelium growth on stem and petioles with sclerotia developing as the fungus matures.
Description: Sclerotia of various shapes and sizes collected from infected plant in northeast Indiana. Approximate size of sclerotia.
Favorable Environmental Conditions
Cool (50 to 68 degrees F) and wet (-0.01 to -0.3 MPa) conditions early in the growing season is favored for sclerotia germination. Canopy closure as well as the presence of morning dew or regular irrigation can further promote disease pressure. Consistent rain or irrigation after the canopy has closed will further promote disease pressure later in the growing season. Hot and dry conditions during the early season and into beginning reproductive stages is not conducive to white mold infection.
No soybean variety is completely resistant to white mold. Certain varieties have moderate tolerance and should be used in fields with a history of white mold. Other cultural practices can be used to further reduce white mold disease pressure.
- Variety selection
- Wide row spacing (30")
- Lower planting populations
- Proper irrigation rates
- Rotation to a non-host crop such as corn or wheat
- Timely fungicide or biofungicide application
*Historically it was believed that tillage would help to burry sclerotia in the top 2-inches of the soil reducing white mold disease pressure. It is now understood that tillage might create favorable conditions for the development of white mold. Dormant sclerotia buried deeper in the soil could potentially be brought to the surface and into favorable conditions for germination.
A timely fungicide or biofungicide applications at beginning reproductive stages might help to control disease pressure. Current research is underway to better understand white mold management practices in soybean with fungicides and biofungicides in Indiana.
- Crop Protection Network - White Mold
- Crop Protection Network - Scouting for White Mold
- Sporecaster App
The sporecaster app is a tool developed by the University of Wisconsin to assist in making management decision for white mold. Available for Apple and Android.
- Weather-Based Models for Assessing the Risk of Sclerotinia sclerotiorum Apothecial Presence in Soybean (Glycine max) Fields
- Validating Sclerotinia sclerotiorum Apothecial Models to Predict Sclerotinia Stem Rot in Soybean (Glycine max) Fields
- Meta-Analytic and Economic Approaches for Evaluation of Pesticide Impact on Sclerotinia Stem Rot Control and Soybean Yield in the North Central United States
- Performance and Profitability of Fungicides for Managing Soybean White Mold: A 10-Year Summary of Cooperative Trials
- Efficacy of Fungicides on Sclerotinia sclerotiorum and Their Potential for Control of Sclerotinia Stem Rot on Soybean
- Effects of Soil Temperature, Moisture, and Burial Depths on Carpogenic Germination of Sclerotinia sclerotiorum and S. minor