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.
Signs and Symptoms of Sudden Death Syndrome
On foliage, symptoms of SDS first appear as small, pale green, circular spots on leaves during the early reproductive growth stages. These spots enlarge into striking/flashy yellow irregular blotches between veins while the veins remain green. The yellowed blotches turn brown and die.
"In severe cases, the leaves drop prematurely leaving the petioles attached to the stem. Infected plants may not always show foliar symptoms," Byamukama said.
Roots of a soybean plant infected with SDS are rotted and discolored. Diseased plants can easily be pulled out of the ground because of rotted lateral roots. If the plants are pulled when the soil is moist; small, light-blue patches can be seen on the surface of the taproot. When the tap root of the infected plant is split lengthwise, the internal tissue will be gray to brown, as opposed to the normal cream white color of a healthy plant.
Scout fields and send suspect plants in for testing
SDS, being a soil-borne pathogen, is difficult to manage and by the time symptoms are seen, there is little that can be done to manage the disease, Byamukama said.
"Seed treatments have not been found effective and foliar fungicides do not protect soybean from SDS infection. It is therefore important that growers scout their fields," he said.
If SDS issue is suspected, Byamukama said South Dakota growers should send samples to the Plant Diagnostic Clinic at SDSU at no grower cost, thanks to a grant from South Dakota Soybean Research and Promotion Council. Other state’s growers should also confirm SDS with clinical testing in their state, and ag retailers can be a big help to assist these growers in identification and sampling for testing. Byamukama also encourages growers to keep notes on the history of SDS in their fields.
Management Practices for SDS
Fortunately there are a number of management strategies in place that can lessen the impact of SDS on soybean yield.
"If SDS is confirmed in the field, use soybean cultivars that are SDS resistant or SDS tolerant," Byamukama said.
Seed companies provide disease ratings for SDS, growers should check for SDS rating, once SDS has been confirmed in their fields. Planting should be in warm and well drained soils. Wet and cool soils promote SDS pathogen infection. SDS is commonly found in plants that are also infected with the soybean cyst nematode. Therefore, managing the soybean cyst nematode may reduce chances of SDS infection. Because the SDS pathogen can survive on corn kernels, clean corn harvesting is encouraged.