Select resistant soybean varieties to manage disease
In addition to specific resistance, soybean varieties are also rated for “tolerance” to Phytophthora by many companies. Research has shown that a high tolerance score with a resistance gene is the best combination for managing this disease with genetics.
Producers should keep records on which fields they identify with Phytophthora so that they can select soybean varieties with resistance and/or tolerance to the disease for those fields. This disease will not affect corn. Fields with a long-term history of Phytophthora may require a different resistance gene if you start seeing Phytophthora-killed plants even with resistant varieties.
For more information see the UNL Extension NebGuide, Management of Phytophthora Root and Stem Rot of Soybeans (G1785).
Sclerotinia Stem Rot (White Mold)
White mold or Sclerotinia stem rot is a disease that starts early in the season. The actual infection occurs on the senescing flower, which the fungus feeds on and infects. All infections in soybean will typically start at a node. You can even tell when the infection occurred based on how high up the plant the stem lesions and fungal growth are. This year’s cool, wet conditions during flowering were favorable for infection. Even at harvest you can identify Sclerotinia stem rot in the field as the stems will be very light (bleached) in color. Upon close inspection you will see dark black bodies (sclerotia) of the fungus on the stems. When dead stems are split, you often will see the sclerotia of the fungus inside.
Management of Sclerotinia stem rot integrates variety selection and cultural management. Longer term rotations can help reduce sclerotia numbers. High plant populations and narrow rows can increase white mold potential. Varieties vary in their level of partial resistance, and no varieties currently have complete resistance. Select varieties with good scores for Sclerotinia in fields with a history of this disease over several years. This is not a disease we see every year in Nebraska, but some locations —typically in river bottom areas in the northern half of the state—have it more consistently.
For more information on Sclerotinia, see the related resource on the North Central Soybean Research Program Plant Health Initiative website.
Brown Stem Rot and Sudden Death Syndrome
In 2013, many fields were affected by either brown stem rot (BSR) or sudden death syndrome (SDS). While foliar symptoms of these diseases are similar, there are key diagnostic differences in other symptoms. Management differs for the two diseases and requires a different variety selection process. Both can result in patches of soybean plants dying in fields. Foliar symptoms are not always present for BSR, therefore any pockets of stressed or dying plants can be suspect for the disease. Foliar symptoms start with interveinal necrosis; then the spots coalesce to form brown streaks between the leaf veins with yellow margins.