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Sudden Death Syndrome Popping Up in Midwest Soybeans

The ample moisture in the last week or so in parts of the Corn Belt has been a boon for corn and soybean crops rounding the final turn and going into the homestretch toward fall harvest.

For soybeans, it’s also causing some disease pressures to tighten, namely sudden death syndrome (SDS). Some farmers say they’re seeing the disease pop up and that yield potential could start to get trimmed by SDS.

“Everywhere. Probably less than 10% of the fields don't have at least a few spots showing. A few fields are a week from it taking the whole field,” says Marketing Talk veteran adviser Mizzou_Tiger. “Definitely bad and came in early enough to thump yield. We even have it in two varieties that have almost perfect SDS scores. Granted, the area affected is much smaller in those fields compared to two other numbers. But it’s still there, which is scary.”

Adds Marketing Talk senior contributor hanktbd: “Nearly every field has some here, and a few are getting pretty severe. Not sure how much of a yield hit, as it has never been more than a minor problem in this area until now.”

The disease’s flare-up has been fueled by ample rainfall, and it’s especially problematic in areas where rain has been accompanied by generally overcast conditions and a lack of sunshine – conditions that are perfect for SDS, experts say. And it goes back well before hot, soggy conditions developed in the Corn Belt in recent weeks.

“Unlike last year, most of the soybean crop was planted before the bulk of the rains started, which further increases the risk of SDS. The early wet weather we have experienced helps increase the root rot phase of the disease,” says Iowa State University Extension plant pathologist Daren Mueller. “One of the driving factors for late-season SDS development is significant rainfall during the late-vegetative and early reproductive stages.”

If you suspect SDS in your fields, look for a few key characteristics. First and foremost, SDS causes defoliation and yellowing of upper leaves. If your fields have been infected, but you’re just seeing spotty symptoms, it’s a sign you’re early in the disease cycle, according to a Purdue University report by a team of plant pathologists led by Andreas Westphal.

“When symptoms first appear in a field, they may be confined to a few small areas or strips in the field, often in wetter or compacted areas, such as turn rows. Over the following two or three weeks, affected areas may enlarge, and other areas in the field may show symptoms,” according to the university report. “Because the SDS fungus can overwinter in soil, areas of a field that show symptoms of the disease often grow larger with each growing season until most of the field is affected.”

There’s one kicker with SDS, though: Once a field is infected, symptoms can sometimes resemble those from other similar diseases, making it important to scout even after you know you’ve got a disease infection on your hands.

“Early symptoms are mottling and mosaic of the leaves. Later, leaf tissue between the major veins turns yellow, then dies and turns brown. Soon after, the leaflets die and shrivel. In severe cases, the leaflets will drop off, leaving the petioles attached. Examining the interior of the lower stem and taproot provides a more diagnostic symptom. When split, the lower stem and taproot of a plant infected with SDS will exhibit a slightly tan to light-brown discoloration compared to a healthy plant. The pith will remain white or slightly cream-colored,” according to the university report by Westphal and other Purdue plant pathologists. “It is important to look for these stem symptoms because other diseases (such as brown stem rot) or chemical burn can produce foliar symptoms similar to SDS (although in other diseases dead leaflets tend to remain attached to the petioles). For example, brown stem rot darkens the pith, but there is little discoloration of the cortex. If a plant with advanced foliar symptoms of SDS is dug up when soil is moist, there may be small, light blue patches on the taproot’s surface near the soil line. These patches are blue spore masses of the fungus that causes SDS. As the plant dries, the blue color will fade, but these blue spore masses, seen in conjunction with the other symptoms mentioned above, are strong indicators of SDS.”

The bad news is if you’ve got SDS at this point in the season, you’re not likely to find an easy solution. Most SDS planning happens earlier on in the season, namely with the selection of resistant soybean varieties. Fungicides are sometimes applied early in the season, but they typically aren’t effective. 

“Although soybean varieties less sensitive to SDS have been developed, there are no highly resistant varieties. Fungicides applied in furrow during planting or as seed treatments have only limited effects, and fungicides applied to foliage have no effect on SDS, presumably because the fungal infection is restricted to root systems and fungicides typically do not move downward in the plant to reach the site of infection,” according to Westphal’s report. 

Other SDS control options include adjusting planting dates to avoid planting soybeans in cooler, damp soils and eliminating compaction through adjusting tillage systems. The latter strategy has worked well for Marketing Talk senior contributor tree fmr.

“Talked with my brother yesterday; he said some soybeans have patches turning color, but does not know if it is SDS. Says all of ours are still green and look good,” he says. “The only thing we do different is all no-till. We never have had any SDS to speak of.”

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