If your soybeans are flowering in cool, wet July weather, White mold could be your biggest threat this season. While fungicides are often the go-to method for controlling White mold, there’s only a short critical window of time when they are most effective.
If you spray too early, you may miss when the fungus is most vulnerable to treatment. Too late, and the pathogen may already be established, reducing the efficacy of the fungicide.
That’s why timing is everything. Let’s break down when and why your soybeans might be at risk for White mold and the best way to protect them.
Is my crop at risk for soybean White mold?
White mold thrives in temperatures below 85 F with frequent rain.1 As the canopy closes, White mold starts to rear its ugly head. The shaded, humid environment created by canopy closure provides the perfect conditions for White mold to infect plants. Soybeans are most vulnerable to White mold during the flowering stages (R1-R3).2
Here are some key factors to consider when assessing your risk for a White mold outbreak:
History of soybean White mold in the field
The dormant White mold sclerotia, which often look like rat droppings, can live in the soil for five to seven years, so fields with past outbreaks are at high risk.3 Sclerotia germinate produce fruiting bodies which release spores that infect senescing flowers. Proactive fungicide sprays will be the best defense when White mold conditions are present.
Canopy and row density
Tightly spaced rows and high plant density encourage earlier canopy closure, providing the shady, humid conditions White mold spores are waiting for.4 Recent recommendations suggest a wide row spacing of 30 inches5 and a seeding rate of less than 140,000 seeds/acre to help prevent White mold infections.6
High humidity and ambient air temperature
Topsoil that remains cool and moist for 7-10 days can trigger White mold sporing, and ambient temperatures below 85 F with over 40 hours of continuous moisture on the leaves at R1 create ideal conditions for infection.7
Additional risk factors could include:
- High soil fertility – Excess nitrogen promotes lush growth, trapping moisture and limiting airflow.
- Protected, slow-drying fields – Areas sheltered from wind or with poor drainage stay humid longer.
- Poor air circulation – Limited airflow under the canopy prolongs leaf and soil wetness.
- Early planting – Early-planted soybeans will flower during cooler, wet conditions, an ideal time for White mold development.
Continually tracking these conditions can be challenging. Fortunately, the free Sporecaster app helps farmers make management decisions regarding White mold. The app uses GPS coordinates to monitor the optimal weather conditions for White mold development.8
The best fungicide application timing for soybean White mold
First fungicide application — R1 to R2 (begin flowering to full flowering)
For best control results, apply fungicide between R1 and early R2 when soybeans are flowering and before the canopy is 60-85% closed. This ensures you treat any White mold before canopy closure promotes ideal growing conditions for the fungus.9
Current fungicides don’t travel very far systemically through the plant, and none travel downward.10 White mold infections typically start below the soybean canopy, and if a fungicide doesn’t cover this area, it will have little to no effect on infections. Essentially, if your spraying covers only the top of the canopy, then your soybeans will remain susceptible to White mold. Adjustments should be made to spraying equipment to ensure coverage deep within the canopy.
If the canopy is fully closed in mid-to-late R1, it can be beneficial to spray at this time before infection risk peaks. However, delaying fungicide applications to early R2 (80-99% canopy closed) can significantly improve White mold control.11
Endura® fungicide has been shown to be one the most effective foliar fungicides for white mold control when applied just prior to canopy closure.
Second fungicide application — R2 to R3 (full flowering to early pod set)
If the weather is still cool and wet ten days after your initial treatment, a second pass may be needed between R2 and R3.12 This will help ensure White mold is suppressed and doesn’t become established as the crop matures.
Why late applications of White mold soybean fungicides (post-R3) are less effective
Fungicides will control White mold before it develops on soybeans but won’t cure it after it’s infected your crop. After the pathogen has colonized plant tissue, it will kill the plant. If you reactively spray once you start to see symptoms, you are too late.13
Not all fungicides provide equal protection at these vulnerable stages of soybean growth. For the best control against White mold, apply Endura between R1 and R3 flowering stages.14 It has been shown to be the most effective product to control White mold.
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
