"Both the incidence and severity of SDS vary greatly from year to year in Missouri," says Laura Sweets, extension associate professor with the MU Commercial Agriculture Program. "SDS is a problem in river bottom fields in the central and eastern portions of the state. In years when environmental conditions are favorable for infection and symptom development, SDS may be found in almost every soybean-producing area of the state and may be quite severe in all of those areas."
The symptoms of SDS usually develop from pod set to early pod fill, she says, "but infection occurs earlier in the season when plants are in the vegetative stages of growth. Furthermore, infection by the fungus that causes SDS is favored by cool soil temperatures and high soil moisture," she says. "Fields that are planted early, so young soybean plants are subjected to periods of cool, wet weather conditions, are more likely to have SDS than fields that are planted later, when soils tend to be warmer and drier."
The 2002 growing season started with a short period of warm, dry weather, Sweets notes, and about 10 percent of the Missouri soybean crop had been planted by the first week in May. "An extended period of cool, wet weather then blanketed most of the state. Soybean planting did not resume in earnest until the end of May,” when there was "a virtual halt in precipitation."
Most of the SDS-infected fields in 2002 had been planted early, when "weather conditions were quite favorable for infection by the SDS pathogen," she says.
In 2001 and 2002, Sweets and former MU weed scientist Bill Johnson conducted SDS trials at two locations in production fields with a history of the disease, hoping to evaluate the effects of variety, herbicide treatment and seed treatment.
Because of rainy weather in 2001, planting was delayed until May 10 and May 16, Sweets says. "Weather conditions after planting were warm and dry, and little SDS developed in either trial." In 2002, both sites were planted on April 14 and were subjected to the subsequent period of cool, wet weather. "SDS developed at both sites, and was quite severe at the site near Petersburg, MO."
Even with the limited data, "there was a striking difference between the two years," she says. Similar varieties and treatments were used during both years, "but the planting date in 2002 was almost a month earlier than in 2001. In 2002, soybean plants were in early vegetative stages for almost a month while environmental conditions were very conducive to disease development."
While the incidence of SDS in the 2001 trials was too low to give valid results, the 2002 trials demonstrated the importance of other management techniques, especially variety selection, Sweets says.
"Management options are still somewhat limited," she says, "but last year's observations and my experiences with SDS trials in production fields have led me to believe that variety selection and planting date are extremely important."
Other factors affecting SDS include poor drainage and compacted soils, continuous cropping of soybeans, soybean cyst nematode pressure and timeliness of harvest, she says. "Also, when planning field operations for the coming season, try to plant fields with histories of SDS last, when soils have had an opportunity to warm up and dry out."