First detection of SDS in South Dakota
During the 2012 soybean growing season, samples from eight fields in five counties in South Dakota tested positive for the sudden death syndrome pathogen. This is the first detection of sudden death syndrome of soybean in South Dakota, says Emmanuel Byamukama, South Dakota State University Extension plant pathologist.
Although SDS is a relatively new disease in the Midwest, Byamukama said this disease has been occurring in the southern states for almost 25 years.
"SDS has been found in our neighboring states; Minnesota, Iowa, and Nebraska. The pathogen survives in crop residue or freely in the soil as thick-walled structures called chlamydospores," he said.
Byamukama explained that Sudden Death Syndrome (SDS) of soybean is a soil inhabiting fungal pathogen called Fusarium virguliforme that attacks soybeans early in the growing season but symptoms suddenly appear later in the growing season—during the flowering/reproductive growth stages through pod fill.
Byamukama explained that the chlamydospores can withstand freezing temperatures and resist desiccation for several years. When the soil starts to warm in the spring, developing soybean roots stimulate the chlamidospores to germinate and then infect young soybean roots. Chlamidospsores can be moved around with flowing water and through any practices that move soil (e.g. farm machinery).
"Research shows that the fungus also survives well on corn kernels left on the soil during harvesting or shattered by hail," Byamukama said.
The SDS pathogen infects soybean seedlings just as the seeds germinate, but symptoms may not be seen until flowering. The fungus colonizes the root cortical tissue in the early growth stages of the plant (V1 through V6). At flowering, the fungus penetrates into the vascular tissue of the plant. The fungus then produces toxins that are translocated to the leaves. It is these toxins that scorch the leaves, eventually killing them. The fungus itself does not invade leaves.
Because SDS causes premature leaf drop and flower/ pod abortion, yield losses can range from minimal, with only a few plants infected, to 100 percent depending on the cultivar and the stage of development when symptoms first appear. However, because SDS spreads in soil, usually only patches within the field may be infected. Over the years, inoculum can build up and spread to larger patches or even the entire field.
"The plants that looked perfectly normal turn yellow and die in a very sudden and short time frame, which is one to two weeks," he said. SDS causes symptoms on both roots and foliage.
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