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How to Stop SDS From Slicing Soybean Yields
Sudden death syndrome (SDS) is an aptly named sneaky disease. All season, soybeans seem to have bountiful yield potential. In late summer, though, right around the R3 and R4 stages (beginning and full pod), soybeans suddenly exhibit crinkly and chlorotic leaves and die.
The symptoms and soybean death that farmers see in August and September are rooted earlier in the season, when the soilborne fungus Fusarium solani f. sp.glycines can infect soybean roots as early as one week after crop emergence in cool and wet soils. “The infection stays in the roots,” says Jessie Alt, a Corteva Agriscience soybean breeder.
Early in the season, infected roots can reduce yield potential. “It’s not a foliar disease; it’s a root disease that starts after planting,” says Daren Mueller, Iowa State University (ISU) Extension plant pathologist. “This creates a smaller root system with lateral roots rotted, as in a fusarium root rot.”
Cool and wet soils are conducive to this fungus causing early-season root disease in soybeans. “Even without late-season symptoms, root rot from early-season infections can reduce yields by 2 to 4 bushels per acre. SDS can cause destruction to the root mass, and then the plant cannot take up water and nutrients effectively,” says Jennifer Riggs, a BASF seed treatment product development manager.
Weather conditions can influence whether SDS occurs. Later in the summer, wet and humid weather can prompt toxins to migrate from roots into leaves and cause the chlorotic and crinkly leaves that mark SDS. However, dry weather during reproductive stages can nix infections that occurred during cool and wet early-season conditions.
What to do
Unfortunately, nothing can be done to halt SDS when farmers notice symptoms in August and September. Tools exist, though, for farmers to manage it. Seed treatments, such as BASF’s Ilevo and Syngenta’s Mertect 340-F, are planting-time options that can help reduce early-season infections.
Delayed planting that may enable soybean farmers to dodge wet and cold soils is an option. However, it comes with a catch.
“Delayed planting can reduce symptoms, but it can cut yield potential, too,” says Shawn Conley, University of Wisconsin (UW) Extension agronomist.
A better bet is to plant soybeans with SDS seed treatments. Even with a slight to moderate infestation, Conley has seen a benefit to SDS seed treatments. UW data showed there is an 87% chance of a return on investment when soybean seed was treated with Ilevo in a planting rate of 140,000 plants per acre. The economic optimal seeding rate ($16 per acre) for Ilevo-treated soybeans is 103,250 seeds per acre, he says.
There’s no bulletproof soybean variety for SDS. Still, there are varieties that tolerate SDS more than other varieties.
Seed companies rate SDS tolerance on a numerical scale. Scale rates may differ between companies. Seed companies may vary in varietal SDS ratings. In Corteva Agriscience’s case, SDS ratings range from 1 (no tolerance) to 9 (top tolerance). Corteva Agriscience’s top SDS-tolerant rating for a Pioneer variety is 8. Corteva Agriscience charges no premium to buy SCN-tolerant soybeans.
Under heavy SDS pressure, even an SDS-tolerant variety from Pioneer (Corteva Agriscience is the parent company) will show some signs of SDS. “It can still impact seed size, but not seed number,” says Alt.
Soybean farmers should also balance fields prone to SDS with those that have also been infested by soybean cyst nematode (SCN).
“It is complex,” says Alt. “We understand more every year. There has been good research showing a link between SCN and SDS. SCN will open wounds in the roots, clearing the way for SDS. The SDS fungus can also overwinter in cyst nematode bodies, too. Iowa State University research suggests that SCN feeding promotes lateral root growth, which allows for additional SDS infection opportunities.”
A complicating factor is that SCN has been plagued by SCN races resisting SCN-resistant varieties. It’s the same principle that exists in herbicide weed resistance. Since over 95% of SCN-resistant soybean varieties use the PI 88788 source of resistance, repeated use results in races resisting the SCN resistance.
One option is to plant soybeans with a new source of resistance, such as the Peking source. Historically, soybean varieties with the Peking source of resistance have been plagued by yield drag.
No more. Alt says in varieties offered by Corteva Agriscience, breeders using tools like molecular breeding in Maturity Groups 2 and 3 have eliminated yield drag.
“They just yield phenomenally,” she says.
Chill in the Spring
Soybeans are tougher than you think. BASF data shows early planted soybeans set up higher yields. In Illinois, March-planted soybeans that snow fell on several times before emergence yielded well, says A.J. Woodyard, a BASF technical crop production specialist based in Ivesdale, Illinois.
“Soybeans can handle cold temperatures prior to emergence just fine,” he says.
Tillage is a different story. Tilling excessively wet soils can haunt you during the entire growing season.
“I can’t tell you how many times that tillage passes made while the soils were wet have led to horizontal compaction layers,” says Woodyard.
It’s understandable why this mistake is made. If you’re behind the eight ball and the 10-day forecast shows continual rain and you need to till the field prior to planting, fieldwork often wins out. Still, congested and shallow roots that can’t penetrate subsoil due to compacted hardpan are signs of fields tilled under excessively wet conditions, says Woodyard.
“Once this occurs, you can’t recover the rest of the year,” he says.
Where to Start?
A number of alligators are out there in your fields – sudden death syndrome (SDS), soybean cyst nematode (SCN), brown stem rot (BSR) – all waiting to gobble up soybean yield potential. What to do?
“Manage your most yield-limiting factor,” says Jessie Alt, Corteva Agriscience soybean breeder. “It may be SDS in some cases, brown stem rot in others.”
By Gil Gullickson