As I drive through southeastern Illinois, I see very few soybean fields that have started to turn. This is due in part to the late planting season and in part to the soybean maturities most common to this region. I have, however, seen a few fields with the characteristic yellowing of Sudden Death Syndrome (SDS). Questions were raised early in the season as to whether or not this disease would be prevalent in 2019 and it turns out we are starting to see it show up in many soybean fields across the state.
Causes and Development
SDS is a unique disease because it infects the soybean plant early in the growing season, but does not develop visible symptoms until late in the growing season. It is caused by the fungus Fusarium virguliforme, which overwinters as spores in crop residue. Infections occur as soybeans germinate and emerge. Even though a soybean plant may be infected with the disease early, the growing conditions during the remainder of the season will determine whether or not visible symptoms develop. Conditions favoring disease infection and development include planting into cool, wet soils followed by wet conditions during flowering and hot and dry weather thereafter. For much of the state we’ve seen exactly these conditions.
SDS symptoms have a distinct look when examined. Leaves will demonstrate yellow, interveinal chlorosis and eventual necrosis. Leaflets will fall off leaving the petioles attached. When stems are split, the pith will look normal and be white in color. Brown stem rot exhibits the same interveinal chlorosis on the leaves, but leaves typically remain attached and the pith of the stem will be brown and rotten looking.
The economic impact of SDS can vary depending upon the severity of the disease and the timing of symptom development. Yield losses can range from slight to greater than 50%. The earlier symptoms develop the greater the opportunity for yield loss. Plants that are severely infected will abort flowers and pods, have decreased seed size and senesce prematurely.
Unfortunately, due to the life cycle of SDS, there are no effective management options once symptoms are visible. Foliar fungicides are not effective in controlling this disease. The best way to manage for SDS occurs prior to and at planting. Rotating away from soybeans will help reduce the amount of pathogen in the soil. This may require more than one year of alternative crops to be highly effective.
Be sure not to plant into cool, wet soils. Although there has been a focus on earlier planting of soybeans, use caution if the field has had a history of SDS. Do what you can to help reduce excess soil moisture and compaction if planting early. Another key management tactic is to plant soybeans with a high level of resistance to SDS. Most varieties have a rating scale showing their resistance level to SDS, so be sure to pick varieties that are highly resistant. There are also a few seed treatments that are effective in managing for SDS. These are not your typical fungicide seed treatments. For control of SDS utilize products like ILeVO® or Saltro®. Add one of these in addition to your normal fungicide seed treatment since they target different pathogens.
If your soybeans seem to be maturing a little too early, stop and examine them because it’s possible they are infected with SDS. Although there isn’t much you can do to help the current crop against this disease, proper identification of its presence can help you make management decisions for next year to protect against this pathogen.