Successful Farming - - YIELD QUEST - By Gil Gul­lick­son, Crops Tech­nol­ogy Edi­tor

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 earl­y­sea­son 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 prod- uct 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 Ilevotreated soy­beans is 103,250 seeds per acre, he says.

Va­ri­ety Se­lec­tion

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 SCN­tol­er­ant 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.

Chlorotic and crin­kled soy­beans are the hall­mark symp­toms of SDS.

SDS symp­toms show up in late sum­mer, but in­fec­tions start in the spring, says ISU’s Daren Mueller.

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