After pathogens infect plants and begin to colonize the host tissue, the plants will often exhibit distinct symptoms, i.e., a stem canker, lesions with a specific color, and distinct leaf spots. Also, after infection there are signs of the pathogen, i.e., soybean cyst nematode females on roots, bluish cast of the mycelium of downy mildew on leaves or white fluffy mycelium of Sclerotinia white mold on stems. Each soybean disease can best be diagnosed shortly after symptom development. This is also the best time to confirm infections through laboratory analysis before the plants have completely died and are invaded by other non-pathogens. Table 2 lists the best times of year and soybean growth stages for scouting specific soybean pathogens.
| Table 2. Best Times to Diagnose Soybean Diseases in Ohio. | ||
| Time of Year | Soybean Growth Stage | Soybean Disease |
| April to May | Seed to V3 (2 to 3 weeks after planting) |
Phytophthora seedling damping off
Pythium seedling damping off Rhizoctonia damping off |
| July and early August | R1-R6 (flowering stages to beginning pod fill) |
Phytophthora stem rot
Sclerotinia stem rot (in wet years) Sudden death syndrome Soybean cyst nematode |
| Mid-August to September | R6-R7 (1 to 2 weeks before maturity) |
Sclerotinia stem rot
Brown stem rot Soybean cyst nematode Diaporthe stem canker |
| September to October | At harvest | Phomopsis seed decay
Diaporthe complex on stems Purple seed stain Seed coat mottling |
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| Figure 8-1. Soybean Growth Stages, from seed to seedling, two to three weeks after planting. |
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| Figure 8-2. Soybean Growth Stages, flowering stages V2-V6. |
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| Figure 8-3. Soybean Growth Stages, R4-R8. |
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