Understanding White Mold Disease in Soybeans
White mold, also known as Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum and can be a significant problem in the northern corn-growing region. White mold can substantially reduce soybean yields, especially when climatic conditions and management practices favor high yield potential.
In addition to causing yield loss, white mold can affect seed quality and seed production. Sclerotia may be observed in harvested grain, which may cause price discounts for foreign material delivered at the elevator. Soybean seed can also be infected, resulting in reduced germination and provide a source of inoculum if planted into fields with no history of white mold.
White Mold Identification and Disease Cycle
White mold is a relatively easy disease to identify. It is so named because the fungal disease produces white, fluffy, cottony growth on the outside of the stem and on the pods (Figure 1). Other symptoms also include wilted leaves, stems that appear “bleached” and shredding of stem tissue. Sclerotia, small black structures that resemble mouse or rat droppings, can be found on and inside plants that have been affected by white mold.
The disease cycle of white mold is complicated. Favorable environmental conditions and soybean growth stages must intersect for the disease to occur. The fungus overwinters in the soil as sclerotia that can survive in the soil for many years. Under wet conditions, sclerotia within the top two inches of soil germinate and form small mushroom-like structures called apothecia. Airborne spores (ascospores) are discharged from the apothecia and carried by the wind to soybean plants. Ascospores that land on the senescing petals of soybean flowers are the most likely to cause infection. The ascospores germinate, grow, and infect the stems of the soybean plant. If cool and wet conditions continue, the disease can develop throughout the plant. Eventually, sclerotia will form on and inside the affected soybean plants. Sclerotia are similar in size and density to soybean seed and can be found mixed with the soybean seed after harvest. Sclerotia can also be blown out the back of the combine during harvest, adding more inoculum to the field.
White Mold Management
White mold management is challenging when environmental conditions favor the disease. Management plans should be based on field history and integrate several management tactics that include soybean product tolerance, cultural practices and chemical control options.
White mold has a wide host range and sclerotia have the ability to survive in the soil for several years. However, most sclerotia die over a three- to four-year period between soybean crops, so rotation to non-host crops like small grains and cereals can help reduce the sclerotia load in the soil over the long term.
In a growing season, only sclerotia in the top two inches of the soil surface germinate and release spores. Deep tillage to bury infected residue can prevent germination of sclerotia, but additional tillage brings sclerotia to the surface where they can germinate. In no-till fields, sclerotia remain on the surface and a large number germinate during the corn or other rotational crop years. This reduces the amount of viable sclerotia left to germinate when soybeans are again planted. Tillage may spread sclerotia within the field. Therefore, in no-till fields sclerotia may remain confined to hot spots.
If white mold occurs for the first time in fields, tillage can be used to bury the sclerotia. Tillage in subsequent years should be avoided. Reduced tillage and no-till are preferable for fields with a history of white mold infestation.
No soybean products are completely resistant to white mold, but tolerant products can be effective in managing white mold. For fields with a history of white mold, partially resistant products should be planted. Planting susceptible products should be avoided in fields with a history of white mold, in low-lying areas, or with natural barriers to wind movement, such as tree lines.
In low to moderate disease pressure environments, white mold increases as row spacing narrows. Under high disease pressure, white mold severity is similar between wide and narrow rows. Increased row spacing generally results in a decrease in the amount of white mold, but does not necessarily correspond with an increase in yield.
High plant populations contribute to dense, closed canopies. Higher populations have been associated with increased white mold incidence. In fields with a history of white mold, consider decreasing plant populations; however, be sure populations maintain yield potential.
Chemical Control Options
Especially in fields where white mold has been an issue previously, it is critical to use management options such as product selection, crop rotation and reduced tillage. An in-season fungicide application, such as Delaro® Complete Fungicide, can help manage white mold. In-season applications of fungicides are recommended at growth stage R1 (flowering). This is the stage when the disease inoculum can infect plants through dying blossoms; therefore, fungicides can be applied to help reduce the number of infections. There may be a benefit of reducing the severity of the disease when applications are made up to growth stage R3 (early pod). Fungicide effectiveness depends on spray penetration into the canopy. Fungicides need to be placed as deep into the canopy as possible. Sprayer reconfiguration may be necessary to obtain good coverage and canopy penetration. Nozzle type, spray pressure, application volume and speed will determine the uniformity of spray deposition and penetration into the canopy. Proper nozzle orientation and overlap is also critical to achieve good spray coverage. Bayer offers a unique solution for the prevention and control of diseases Delaro® Complete Fungicide.
Once symptoms of white mold are evident, fungicide applications may no longer manage the disease. Soybean fields with high levels of white mold should be harvested last to reduce the spread of sclerotia.
What is Delaro® Complete Fungicide and how does it work?
Delaro® Complete Fungicide is a premium fungicide for unmatched, broad-spectrum control of white mold and other damaging diseases. Unlike other soybean fungicides, Delaro Complete Fungicide delivers a best-in-class residual efficacy from all three active ingredients.
Delaro Complete contains three active ingredients. The first, fluopyram, is a group 7, or SDHI, which has activity on important soybean diseases including white mold, brown spot and offers plant health benefits. The second, prothioconazole, is a group 3, or triazole, which has a very broad and deep spectrum of disease control and activity. The third, trifloxystrobin, is a group 11, or strobilurin, that shows preventative activity on many diseases, and also offers benefits to plant health. These powerful ingredients work together to provide consistent control of most major corn and soybean diseases and increased plant health all season long. Delaro Complete Fungicide diffuses in and on the waxy surface of the plant. It then expands protection across the plant's exterior by moving via surface moisture, such as dew. Within the plant, Delaro Complete Fungicide is transported systemically throughout plant tissue for enhanced coverage.
Follow all individual product label instructions for proper application timing, application volume, application equipment, and environmental and harvest interval precautions.
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
Not all products are registered in all states and may be subject to use restrictions. The distribution, sale, or use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. Check with your local dealer or representative for the product registration status in your state. Bayer, Bayer Cross and Delaro® are registered trademarks of Bayer Group. All other trademarks are the property of their respective owners. ©2021 Bayer Group. All rights reserved.
Mueller D., Bradley C., Chilver M., Esker P., Malvick D., Peltier A., Sisson A., and Wise K. June 2015. Soybean disease management: white mold. CPN-1005. Crop Protection Network. http://cropprotectionnetwork.org.
Mueller D. July 2, 2014. Managing white mold in soybean. Integrated Crop Management. Iowa State University Extension. http://crops.extension.iastate.edu
Smith D. White mold of soybean (Sclerotinia stem rot). Wisconsin Field Crops Pathology. University of Wisconsin-Madison. http://fyi.extension.wisc.edu