Last August, it was common to drive by fields that were "smoked" with sudden death syndrome (SDS). It seemed every field of soybeans in a large area of Iowa was ravaged by the disease.
The start to the 2011 growing season has been characterized by alternating periods of cool, very wet weather and hot, dry weather. Research has shown cool, wet conditions favor infection by the SDS fungus and disease development.
So, are we going to have another epidemic of SDS in 2011? "Unfortunately, we just don't know," says Alison Robertson, an Iowa State University Extension plant pathologist. "One thing we do know, however, is scouting is important to identify fields with SDS so management practices for subsequent soybean crops can be implemented."
Symptoms usually occur in early August; last year SDS showed up earlier
SDS symptoms usually occur after flowering, around the first week of August, although in 2010 numerous reports of SDS occurred in mid-July. Individual soybean plants in compacted areas of the field usually develop symptoms first, she says. Scattered yellow spots occur in the tissue between the leaf veins. The spots enlarge to form yellow blotches which then die. The leaves eventually drop, but the petiole remains attached to the stem.
Diseased plants can be pulled easily from the soil because the lateral roots have often rotted away. The tap root of affected plants is also rotted. When the soil is wet, blue fungal growth may be seen growing on the outer surface of the tap root. If the tap root is split longitudinally, the woody tissue is discolored gray-brown.
There is no rescue treatment for SDS, fungicides won't control it
There are no in-season rescue treatments for SDS; however, good records should be kept to ensure proper management practices are followed in subsequent years.
Management of SDS is difficult but should start with soybean variety selection, says Robertson. Although there are no varieties that are completely resistant to SDS, some varieties are affected less by the disease. They have what is known as partial resistance.
If you have fields with SDS symptoms, make a careful note to plant a soybean variety with partial resistance when you return beans to that field again in the future, she advises. Be sure to visit seed company demonstration plots in your area to get an idea of what varieties might work for you. Another thing to do is to check for soybean cyst nematode (SCN) populations in affected fields. SDS may be more severe in fields where SCN populations are high. Controlling SCN in your field can help control SDS, too.