By August, many soybean farmers feel that the major decisions are behind them. The crop has established a canopy, herbicide applications have mostly ended and farmers have completed or started their fungicide plans.
The crop, however, still has work to do. Soybeans are setting pods, filling seed and drawing heavily on available water and nutrients. Plants whose roots were infected by diseases months ago may finally show symptoms, while insects can build as pods develop. Heat and drying soils can also create nutrient symptoms that look like fertility problems. 
Missouri enters August with a wide range of crop stages and field conditions. USDA’s July 27 Crop Progress report showed that 72% of Missouri soybeans had reached bloom and 37% had set pods as of July 26. USDA rated 62% of the crop good or excellent and 31% fair.
Statewide moisture levels remained generally adequate, but averages can hide major differences among fields. Poorly drained areas may still hold excess moisture, while lighter soils and compacted areas can dry quickly under intense heat.
That contrast became clear by the end of July, when extreme heat warnings covered much of Missouri. Forecast highs approached 100 degrees in several regions, while heat index values reached 110 to 115 in some areas. Short-term forecasts called for scattered storms and somewhat cooler temperatures later in the week, but NOAA’s July 26 outlook still favored above-normal temperatures and near-normal precipitation across Missouri during the first part of August.
That combination makes August scouting especially important. Similar symptoms may appear across several fields, but they may point to very different problems.
Wet Beginnings Can Produce Late Disease
University of Missouri Extension says cool, wet soils early in the season favor the root infections that cause sudden death syndrome, although plants often hide leaf symptoms until reproductive growth begins.
The fungi that cause SDS infect roots early, sometimes soon after germination. Farmers may not see the familiar yellowing and browning between leaf veins until plants begin setting and filling pods.
By the time those symptoms appear, farmers cannot reverse the infection with a foliar treatment. They should examine the entire plant before making a diagnosis. SDS damages roots but usually leaves the center of the upper stem white.
Red crown rot can create similar leaf symptoms and has become a growing concern in Missouri. MU Extension reported that researchers confirmed the disease in nine Missouri counties during the 2025 growing season after finding the state’s first case in Marion County in 2024.
The disease often appears after the R3 growth stage, especially in low-lying or poorly drained areas. Farmers may find red discoloration near the soil line, white fungal growth and small red fruiting structures around the crown and roots.
Hot, dry weather can bring charcoal rot into the picture. MU Extension says charcoal rot favors warm, dry soils and often becomes more severe when plants face moisture stress. Fields with lighter soils, compaction or weak root systems may show symptoms first when extreme heat dries the root zone.
Farmers should look beyond yellow leaves. The location of symptoms, their pattern across the field and the condition of the roots and lower stems can help separate one disease from another.
A diagnosis may not save affected plants this season, but it can guide variety selection, seed treatment, drainage work and soybean cyst nematode management next year.
Insects Gain Ground as Seeds Develop
August also increases the value of soybean plants to insects. A statewide University of Missouri study sampled 316 commercial soybean fields during 2024 and 2025. Researchers found that stink bug numbers increased during reproductive growth and peaked near R6, or full seed. Green stink bugs accounted for more than 80% of the insects collected.
Stink bugs feed directly on pods and developing seeds. Their feeding can reduce yield, damage seed quality and delay maturity, but farmers should not treat simply because they find insects in the field.
MU Extension recommends a treatment threshold of 0.4 stink bugs per sweep, or 40 bugs in 100 sweeps, for grain soybeans. The threshold drops to 0.2 bugs per sweep for seed production fields. Farmers should scout field edges first but also check field interiors before making a treatment decision.
Spider mites respond differently to weather. Rain and high humidity can suppress populations, while hot, dry conditions help them build quickly. Infestations often begin along field borders, waterways and other stressed areas.
Damage starts as small yellow spots or stippling before leaves turn bronze or brown. Rain may temporarily slow an infestation, but populations can rebound if heat and dry weather return. MU Extension’s published guidance recommends treatment when mites are present and leaf yellowing reaches 20% before pod set or 10% after pod set. Farmers should also consider crop stage, the size of the affected area and the weather forecast before spraying.
Tissue Tests Need More Context
Late-season yellowing often pushes farmers toward tissue testing, but August limits what those samples can reveal and what farmers can still correct. MU Extension identifies first bloom as the ideal time to sample soybean tissue and advises against sampling after the first pods begin filling.
Later samples can still help diagnose a field problem, but standard sufficiency ranges become harder to interpret. Weather adds another complication. Dry soil slows nutrient movement toward roots, while root disease, compaction, poor drainage and high soil temperatures can restrict uptake. A tissue test may show low nutrient levels without explaining why the plant cannot access those nutrients.
That distinction matters before farmers invest in a foliar application. The soil may contain adequate fertility while damaged or stressed roots struggle to take it up. Adding more fertilizer will not necessarily correct the underlying problem or produce an economic return.
MU Extension recommends pairing tissue samples with soil samples. Farmers should also sample healthy and affected areas separately and note differences in soil type, drainage and field history.
By August, tissue analysis may offer more value as a diagnostic and planning tool for next season than as a reason to make a rescue application.
Decide What Still Deserves Action
August scouting should help farmers answer two questions: Can they still protect yield this season, or is the field showing a problem that should shape next year’s decisions? An insect population above threshold may justify treatment. A growing spider mite infestation may require action before leaf loss spreads. Tissue and soil tests may identify a correctable nutrient problem, but farmers need enough growing season left for the crop to respond.
Disease symptoms tell a different story. Farmers have no foliar rescue treatment that can reverse established SDS, red crown rot or charcoal rot after symptoms appear. They can still document affected areas, submit diagnostic samples and use those findings to guide future management.
Missouri’s mix of heat, humidity, scattered rainfall and uneven field conditions could bring several problems to the surface at once. Farmers who scout carefully can separate what still warrants an investment from what would only add cost to a crop that cannot respond.
