White mold disease, also called Sclerotinia stem rot, often shows up during the reproductive growth stages of soybean plants in years that are cool and wet, much like 2009. The warm weather in the summer of 2010 and so far during the summer of 2011 has minimized the development of white mold throughout much of Iowa for these last two growing seasons. Even so, farmers are encouraged to scout their soybean fields and identify those fields that have white mold.
The following recommendations and observations are provided by Daren Mueller and Alison Robertson, two Iowa State University Extension plant pathologists.
You need to scout and identify bean fields with white mold this year
Why is it important to identify white mold this late in the season? In general, fungicides are not effective after symptoms have developed. Therefore, it is not generally recommended that you apply fungicides to soybeans after pod set (growth stage R3). Fungicides are more effective if applied before disease gets established in a field. While little can be done to control the disease once it can be seen in the field, there still is value in scouting.
Locating "hot spots" of white mold disease in a bean field may trigger management strategies to reduce the number of sclerotia for subsequent years. For example, if a particular field has "hot spots," you may want to consider designing a management plan that includes a biological control product.
Such a product, called Contans, is the most widely available and tested biological control product for managing white mold. Contans is a commercial biocontrol agent sold as a powder formulation that contains the live fungus Coniothyrium minitans. This fungus will colonize and degrade the small, black survival structures called sclerotia. Degraded sclerotia will not produce inoculum to initiate infection of soybeans.
Should you use biocontrol product to try to control white mold disease?
Going over your field records and scouting your soybean fields this summer will help you identify areas of a field to treat with this biological control product, so it can provide control of white mold the next time you plant soybeans in that field.
Contans should be applied a minimum of three months before the time white mold is likely to develop. Applications may also be made in the fall, after the crop (either soybeans or corn) is harvested. After soil application, the Contans product should be incorporated as thoroughly as possible in the soil to a depth of two inches. Additional tillage should be avoided after incorporation, since tillage can bring uncolonized sclerotia back to the soil surface.
Scouting fields this August will also help you evaluate soybean varieties
Other reasons for scouting your bean fields this August and recording where white mold shows up are to evaluate soybean varieties or identify fields that are prone to having white mold infestation. Such information may influence what soybean variety you plant the next time that field is in soybeans.
Remember, sclerotia can survive more than two years in the soil. You can also take extra precautions to keep your combine clean of soybean stems and crop residue after harvesting fields that have white mold. This "cleaning of your combine" will help prevent spreading the fungus to new fields, since the soybean stem and crop residue debris often contains sclerotia.