Managing White Mold of Soybeans
Crop Insights written by Steve Butzen
Crop Insights written by Steve Butzen
White mold (Sclerotinia sclerotiorum) is a fungal disease that can attack hundreds of plant species. Also known as Sclerotinia stem rot, it was once only a sporadic soybean disease in Minnesota, Wisconsin and Michigan. However, it has now progressed to become an annual threat to northern states from Minnesota to New York, as well as the northern areas of states bordering to the south. In some years, white mold is only eclipsed by Phytophthora and soybean cyst nematode in damage caused to soybeans in those states.
The spread of white mold is likely due to changes in soybean culture during the last 2 decades that have accelerated canopy development. This includes earlier planting, drilled or narrow rows and higher seeding rates. Ironically, these practices, along with improved genetics, have also been responsible for most of the yield and profitability gains over that timeframe. Maintaining those gains now depends partly on grower success in managing white mold, especially in years when disease incidence is high. This Crop Insights describes white mold of soybeans and current and future options for managing this disease.
White mold persists in soybean fields over time by production of survival structures called sclerotia. Sclerotia are dark, irregularly shaped bodies 1/4 to 3/4 inches long formed within the white, cottony growth on the inside and outside of the stem during the fall. These compact masses of hardened mycelia contain food reserves and as such function much like seeds, surviving for years in the soil and eventually germinating.
In the most common form of germination, a sclerotium produces 1 or more dark germ tubes or stipes that grow upward from a depth of 2 inches or less in the soil. When it reaches the soil surface, the germ tube is triggered by light to produce a small, flesh-colored structure much like a mushroom, called an apothecium. One sclerotium can produce numerous apothecia simultaneously or sequentially throughout the growing season. Each apothecium produces millions of spores beneath the plant canopy, which are periodically released and spread to the plants.
Spores are not able to invade plants directly, but rather, must colonize dead plant tissue before moving into the plant. Senescing flowers provide a ready source of dead tissue for preliminary colonization. From these senescing flowers in the branch axils or stuck to developing pods, the fungus spreads to healthy tissue.
The first symptom of white mold infection appears as a water-soaked stem lesion originating from a node. If the lesion remains wet, it becomes overgrown with white mold. The disease can then spread directly from plant to plant by contact with this moldy tissue. Sclerotia are formed within the moldy growth and inside the stem to complete the disease cycle.
Plant damage is incurred as tissue rot and formation of sclerotia inside the stem result in rapid wilting and death of the upper part of the plant. As the disease progresses, premature death of the entire plant can occur.
Wet, cool conditions are required throughout the white mold disease cycle, including germination of the sclerotia in the soil, spore release, infection of soybean flowers by spores and spread of white mold from plant to plant.
Early establishment of a dense soybean canopy increases the likelihood that the cool, high-humidity conditions required for white mold development will occur. Many common soybean management practices lead to early, dense canopy formation, including early planting, drilled or narrow rows and high plant populations. In fact, researchers and agronomists often suggest that achieving canopy closure before flowering is a key to maximizing yields. Consequently, early canopy closure is a goal for many soybean producers, especially in northern locations and growing environments where solar radiation may be limited. Unfortunately, this practice also encourages white mold development.
The North Central Plant Health Initiative has developed the following list of risk factors for white mold.2
Seasonal Risk Factors for White Mold Development
Long-Term Risk Factors for White Mold Development
White mold is a disease of high yield potential soybeans. Often, the better the establishment and growth of the crop, the more likely it will be damaged by white mold. However, a strategy to control the disease by abandoning high-yield management practices is counter-productive. Instead, growers should employ other practices to reduce white mold damage, except perhaps where disease pressure is extreme, and plant damage and yield loss are common most years.
No one practice alone will be effective in controlling white mold, but several options are available to help reduce disease pressure. Current options include disease avoidance, variety selection, changes in cropping systems including tillage and rotation, and adjusting production methods such as planting practices, chemical applications and weed control.
White mold spreads either by movement of spores or sclerotia from field to field. Spores are airborne and may originate from any field that has had white mold in the past. Spores are thought to move only short distances - about 150 feet according to some studies - so infection by spores would progress from field edges. There is little known about stopping the spread of spores.
Sclerotia move from field to field in harvest equipment or in contaminated seed. Harvest equipment should be thoroughly cleaned when moving from infected to non-infected fields. Harvesting infected fields last provides additional safety.
Because sclerotia are roughly the size of soybean seed, they can't be easily separated by the combine. Soybeans harvested from infected fields are most likely loaded with sclerotia. Planting these soybeans would place them at the ideal depth for germination and infection of that crop and field. Growers should absolutely not save seed from infected fields.
Pioneer avoids growing seed beans in fields with a history of white mold. In addition, seed is thoroughly cleaned and inspected to ensure that it is disease-free. Seed cleaning with a gravity table or centrifugal tower is essential to remove sclerotia. Fungicide seed treatments can help en-sure that no disease is transmitted by mycelia present on seed.
At this time there is no known genetic resistance to white mold - all varieties are capable of developing white mold symptoms under severe infestations. However, there are clear varietal differences in degree of tolerance. Pioneer provides white mold tolerance scores for all its varieties adapted to areas where white mold occurs. These scores reflect varietal differences in the rate at which the infection develops and the extent of damage it causes. Scores are based on Pioneer research observations of comparative white mold tolerance among the varieties across multiple locations and years. Growers should consult their Pioneer sales professional to select local white mold tolerant varieties.
Future solutions to white mold control will likely come from development of varieties with higher tolerance and resistance. Pioneer researchers have targeted improvement of varieties for white mold tolerance as a key research objective. To accomplish this goal, soybean breeders use new lab and field techniques as well as conventional selection in white mold environments. Pioneer scientists also continue to screen novel, exotic and alternative germplasm sources with native tolerance to white mold. Future possibilities include transgenic approaches - transferring resistance genes from other crops or organisms into soybeans.
It is well-established that many current practices that increase soybean yields also increase white mold. Whether growers should abandon their yield-enhancing practices to help control white mold is debatable. In areas with lower white mold levels or drier climate, production practices which increase yield but also increase white mold levels may still be highest yielding. However, in areas with higher white mold levels and a cool, wet climate, some change in production practices may be necessary to limit early, dense canopy development.
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Grau, C.R. and J.E. Kurle. 2001. White mold in soybean. Wisconsin Extension Bulletin A3695. University of Wisconsin, Madison.
Wessel, J. and S. Butzen, 2013. Integrated management of white mold in soybean production. Crop Insights Vol. 23, no. 13. Pioneer, Johnston, Iowa.
1Image courtesy of the Plant Disease Clinic, Extension Plant Pathology, University of Minnesota.
2Plant Health Initiative. North Central Soybean Research Program (content modified).
3Fungicide performance is variable and subject to a variety of environmental and disease pressures. Individual results may vary.
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