Sudden death syndrome vs. brown stem rot: telling them apart

Symptoms side by side

The foliar symptoms are a toxin effect in SDS: the fungus stays in the roots and sends a toxin to the leaves. In BSR the fungus grows up the xylem and plugs it. That is why the stem tells the story and the leaf does not.

The split-stem test

Dig, do not pull, so the taproot comes up intact. Shake off the soil and look at the taproot for rot and for blue-grey fungal growth, which is common but not universal in SDS and never present in BSR. Then split the stem with a knife from the root crown to above the first few nodes.

White pith with a brown or gray outer layer at the base is SDS. Brown pith, darkest at the nodes and running up the stem, is BSR. If the pith is white, the roots are not rotted and there is a brown lesion on the outside of the stem at a node, think stem canker instead. If the vascular tissue in both root and stem is brown but the leaves died without interveinal yellowing and stayed attached, consider Fusarium wilt. White mold shows a bleached external lesion with black sclerotia, and Phytophthora shows a chocolate-brown lesion running up from the soil line.

Check several plants from the patch. Both diseases can occur in the same field and occasionally in the same plant.

Conditions that favour each disease

SDS infection happens early, within days of planting, when soil is cool and wet. Early planting into cold wet ground, poor drainage, compaction and low pH all raise risk, as do high-yield environments and irrigation. Heavy rain during the reproductive stages then triggers the foliar symptoms, which is why a dry July can hide infected roots and a wet one can light up a whole field. Soybean cyst nematode (SCN) increases SDS severity; fields with both lose more yield than fields with either.

BSR develops when temperatures are cool, between about 60 and 75°F (16 and 24°C), with adequate soil moisture early in the season; it is suppressed above 80°F (27°C). Foliar symptoms are strongest when weather is cool during the reproductive stages and may not appear at all if R3 to R4 is hot. Moisture stress later in the season increases the damage. SCN also raises BSR risk and breaks the BSR resistance in most resistant varieties.

Both diseases follow soybean-on-soybean and short rotations, but the inoculum behaves differently. C. gregata has no long-term survival structure and depends on soybean residue, so rotation starves it. F. virguliforme survives for years in soil and residue, and corn does not reduce it; the fungus can maintain or increase on corn residue.

Scouting and when it matters for yield

Scout from R3 onward, especially after a wet spell, concentrating on low, wet and compacted areas and known SCN patches. Record the stage at which symptoms appear. For SDS, foliar symptoms that start at or after R6 cause little loss because seed fill is nearly done; symptoms at R3 to R5 can cause substantial loss in a susceptible variety. Losses are rarely field-wide because both diseases come in patches.

Map the patches. The map is the input for next year's variety placement and for an SCN soil sample this fall, because an SDS diagnosis is a strong reason to find out your SCN egg count.

Management

There is no in-season rescue for either disease. Foliar fungicides have no effect on SDS; in the Crop Protection Network's multi-state trials no foliar product reduced SDS foliar symptoms. The same is true for BSR, where the fungus is inside the stem.

Variety. Partially resistant varieties are the foundation for SDS; no variety is immune, but ratings differ widely and the better ones protect yield under pressure. For BSR, resistant varieties are the most efficient control. Many varieties with PI 88788 SCN resistance also carry BSR resistance, while varieties with Peking or Hartwig SCN resistance may be BSR-susceptible, so if you are rotating SCN resistance sources into Peking in a BSR field, check the BSR rating specifically.

Seed treatment for SDS. Fluopyram (FRAC 7, e.g., ILEVO) is the seed treatment with the trial record. In a meta-analysis of more than 200 trials across 12 states and Ontario from 2013 to 2015, fluopyram reduced the SDS foliar disease index by 35 percent and raised yield by 4.4 bu/ac (0.30 t/ha), or 7.6 percent, over the base seed treatment. Where SDS was severe, fluopyram protected yield more than 80 percent of the time and the probability of a positive return was 89 percent at a $13 per acre cost and $10 per bushel; where SDS symptoms were low or absent, the probability of return was very low. Pydiflumetofen (FRAC 7, e.g., Saltro) was registered for SDS in 2019 and has the same mode of action, so alternating the two is not a resistance strategy. As of October 2026, CPN monitoring has found no fluopyram resistance in F. virguliforme. Use these treatments where the field has an SDS history, not as insurance everywhere. There is no seed treatment for BSR.

Rotation. For BSR, keep soybeans out of severely infested fields for at least three years, longer in no-till, and consider shredding soybean straw to speed residue breakdown. For SDS, rotation to corn does not help; in Iowa research a third year of alfalfa or an alfalfa-red clover mix reduced SDS in the following soybean crop, and small grains may also help.

SCN management. Managing SCN helps both diseases. Sample this fall, use SCN-resistant varieties, rotate the resistance source where the BSR rating allows, and rotate to non-hosts.

Soil and planting. For SDS, improve drainage, reduce compaction and keep pH and fertility in range. Delaying planting reduces SDS risk only in fields with a severe history, and it costs yield potential, so reserve it for those fields.

Canada note

The seed treatments and fungicides discussed here are US registrations. Ontario growers should check the Health Canada PMRA label and PCP number for the product and the SDS claim; a US label does not apply in Canada.