By Jennifer Bradley
Sudden death syndrome is relatively new to the state, according to Shawn Conley, soybean and wheat Extension Specialist at the University of Wisconsin-Madison, but it is becoming a growing concern for soybean growers each year.
The first documented case of SDS in Wisconsin was in 2005, and soybean specialists have been tracking it since.
"I would not call it ubiquitous yet, but it is definitely present," Conley says. "It's also a bigger issue than most growers think because the symptomology is the same as brown stem rot."
Because of this, many growers are assuming it's the latter and not running diagnostics to confirm otherwise. The Wisconsin Soybean Marketing Board is now providing an SDS test along with any samples sent in for soybean cyst nematode evaluation. If SDS is found, the farmer will be notified.
SDS is a fungus called fusarium virguliforme and is found in the soil around soybean plants. Planting soybeans into cool, wet soils enhances a field's SDS challenge, as do midsummer rains which drench the ground.
SDS symptoms appear mid- to late- season. The first symptoms to appear are yellow blotches that eventually become large irregular patches on a soybean plant's leaves. The veins will remain green, but the other areas turn brown or white and begin to die. The leaves then drop, but the petioles remain on the plant's stem. Sometimes symptoms are also misdiagnosed as plant stress, due to drought, since the area in the field may look scorched.
A notable difference between BSR and SDS is that the latter causes root rot. Another distinguishing factor can be seen within a soybean's internal stem. In those plants infected with SDS, the inner core of the root and stem remains white, and only the cortical cells are tinted gray or brown. With BSR, a brown discoloration is found in the inner core.
The average yield loss documented for SDS over the years has been in the 20% to 30% range, and SDS is resilient. It has been proven to remain in soil for long periods of time as thick-walled spores, and even with crop rotation, survive for several years after planting corn in a field.
The biggest challenge, however, is that there is not true genetic resistance available, says Conley.
"BSR and SDS are equally detrimental to a crop, but there is resistance to stem rot," he explains.
Launching an assault on SDS
"Detection is the first thing," Conley recommends for farmers. "Don't assume it is BSR. Know what pathogen is in the field." He says they should be out at the end of July, scouting for symptoms of SDS on soybean plants.
Conley explains the Wisconsin Soybean Marketing Board is in its second year of a grant program to study SDS and develop solid management strategies to help those facing this disease in their soybean fields. Based on samples that researchers have received, the disease is most prevalent in the southeast and northeast areas of the state.
"The conventional wisdom up to this point has been to delay planting," he explains. The challenge with that is an automatic yield loss, and if a farmer battles SDS, they're having a yield loss anyway. Conley says preliminary data is showing that the yield loss from SDS is still lower than what would occur if a farmer decided to plant later than normal.
If SDS is found, Conley says growers should submit soil samples to their county Extension agent or agronomist, but then begin working with their seed companies to find the most tolerable variety. Seed companies are screening for SDS now and will have tolerance ratings for their products.
"These will be decisions for 2015," Conley remarks, and adds that another decision for next year will be seed treatments. He doesn't know of any with a label yet, but says there are some promising ones on the horizon, and suspects these will be available in 2015.
Bradley lives in Chilton.