Core principles
- Know your field history. Most soybean diseases recur where they've appeared before.
- Pick resistance first. Genetic resistance or tolerance is built into the seed — check the ratings in our variety database.
- Rotate crops and resistance sources to slow pathogen adaptation.
- Confirm before you spray. Many symptoms look alike; a plant diagnostic lab can save money.
Soybean cyst nematode (SCN)
The #1 yield robber in North America Soil test
- Signs
- Often no visible symptoms — yield losses can occur with healthy-looking plants. Look for small white-to-yellow cysts on roots 5–8 weeks after planting; stunting in patches when severe.
- Conditions
- Spreads with any soil movement; worse in dry years and on lighter soils.
- Manage
- Soil sample to know egg counts; plant SCN-resistant varieties; rotate with non-host crops such as corn; rotate resistance sources (PI 88788 ↔ Peking) because many populations have adapted to PI 88788; consider nematode-protectant seed treatments.
The large majority of SCN-resistant varieties use PI 88788 resistance. Where an HG type test shows SCN reproducing well on PI 88788, Peking-source varieties add diversity. The SCN Coalition has excellent sampling guidance.
Sudden death syndrome (SDS)
Root infection, leaf symptoms R3–R6
- Signs
- Yellow blotches between leaf veins turning brown, with veins staying green; leaflets drop but petioles stay attached. Split roots show healthy white pith (unlike brown stem rot).
- Conditions
- Infection happens early in cool, wet soils; symptoms show later after heavy rains in mid-summer. Compaction and SCN make it worse.
- Manage
- SDS-tolerant varieties; seed treatments labeled for SDS; improve drainage and reduce compaction; manage SCN.
White mold (Sclerotinia stem rot)
Cool, humid canopies at flowering R1–R3
- Signs
- Fluffy white growth on stems, with black sclerotia (resting bodies) inside and on stems; wilting, gray-white dead plants.
- Conditions
- Cool temperatures and wet, humid canopies during flowering; most common in the northern U.S. and in narrow rows with heavy canopies or irrigation.
- Manage
- Tolerant varieties; wider rows or lower populations in high-risk fields; rotation away from susceptible broadleaf crops; fungicide or biological control timed at early flowering when risk is high.
Phytophthora root and stem rot (PRR)
Wet, poorly drained soils Any stage
- Signs
- Seed and seedling rot early; later, dark brown lesions moving up the stem from the soil line and wilting plants.
- Conditions
- Saturated, heavy clay soils; flooding or ponding.
- Manage
- Varieties with an effective Rps gene plus strong partial resistance (field tolerance); seed treatments with oomycete activity; drainage improvement.
Frogeye leaf spot
Warm, humid weather R1–R5
- Signs
- Small circular lesions with gray-tan centers and dark reddish-purple borders on leaves.
- Conditions
- Warm, humid conditions; residue from previous soybeans; more common in the South but has moved north.
- Manage
- Resistant varieties; rotation and residue management; foliar fungicide near R3 when warranted. Strobilurin (QoI, FRAC 11) resistance is widespread — use products with multiple effective modes of action.
Brown stem rot, charcoal rot and IDC
- Brown stem rot: brown discoloration inside the stem pith; can mimic SDS leaf symptoms. Manage with resistant varieties and rotation.
- Charcoal rot: favored by hot, dry stress; gray-black specks inside the lower stem and taproot. Reduce stress (irrigation, avoid compaction); variety tolerance varies.
- Iron deficiency chlorosis (IDC): not a disease but a major yield limiter on high-pH, calcareous soils — yellow leaves with green veins early in the season. Choose IDC-tolerant varieties; in-furrow iron chelate (FeEDDHA) can help on problem fields.
Scouting calendar
| Timing | What to look for |
|---|---|
| Emergence (VE–V2) | Stand counts, seedling rots, damping-off, crusting |
| V3–V6 | IDC yellowing, early PRR, weeds, bean leaf beetle |
| R1–R3 | White mold risk, frogeye, SCN cysts on roots, fungicide decisions |
| R3–R6 | SDS and BSR leaf symptoms, stink bugs, defoliators, charcoal rot in dry spells |
| After harvest | Soil sample for SCN; map problem areas for next year's variety choices |
For photos and detailed fact sheets, see the Crop Protection Network and your state extension plant pathologist. Always follow pesticide labels.