Fungicide use for managing white mold in soybeans
Consider these factors when deciding if and when fungicides should be applied to manage white mold in soybeans, as well as selecting effective fungicides and equipping and operating sprayers.
White mold is one of the most damaging soybean diseases in Michigan. However, disease development is challenging to predict as it is highly dependent on the following factors: a susceptible host, presence of the pathogen, and favorable environmental conditions. The current environmental conditions are favoring white mold development in some areas of the state.
Foliar fungicides can be an effective tool for managing white mold. However, they are only one of the tactics available for managing white mold and should be used in combination with other management practices such as partially resistant varieties, wide rows, reduced planting rates, irrigation water management and appropriate tillage method. Also, consider the following factors before purchasing and applying a fungicide to manage white mold in soybeans.
History of white mold? White mold sclerotia (survival structures resembling rat droppings shown in the photo) can survive in the soil for five to seven years when buried more than 2 inches deep.
How dense is the soybean canopy? Factors such as narrow rows, high planting populations, early planting dates and high fertility levels all contribute to a dense soybean canopy which promotes white mold development.
Can you apply the fungicide at the optimum time? Application must be made between R1 and R3 growth stages to protect flowers from infection. Historically, application timings have been recommended at the R1 growth stage (one open flower per plant on 50% of the plants). However, applications made at R2 (one open blossom on one of the upper two nodes on the main stem having unrolled leaves) have performed well in recent research conducted in North Dakota. A second fungicide application made approximately 10 days after the first application may improve control if the weather is predicted to remain cool and wet or humid. Sporecaster, a relatively new phone app, was created to suggest if a fungicide should be applied and when to make the application. Using Sporecaster is highly recommended.
Has the topsoil remained cool and continuously moist for seven to 10 days prior to the R1 growth stage? These conditions induce sclerotia germination, apothecia formation and spore dispersal.
Is the ambient air temperature predicted to be cool (less than 85 degrees Fahrenheit) and the foliage to remain continuously moist or wet for at least 40 hours at the beginning of the R1 growth stage? These conditions favor infection.
Is the fungicide effective in managing white mold? The following fungicides have been rated as providing good to very good management of white mold when properly applied:
- Approach 2.08SC (good to very good, based on two applications – R1 and R3)
- Endura 0.7DF (very good)
- Lektivar 40SC (good)
- Omega 500DF (good)
- Propulse 3.34SC (good)
Is your sprayer equipped and operated to maximize droplet penetration into the crop canopy and plant coverage? Detailed information on equipping and operating sprayers to control insects and diseases in soybeans is available. A summary of the key factors is provided below:
- Apply 15 to 20 gallons of water per acre.
- Adjust nozzle pressure to around 40 psi.
- Maintain ground speed at 10 mph or less.
- Equip the boom with nozzles that produce a single flat fan spray pattern directed straight down into the canopy in large, dense canopies.
- Select nozzles that generate fine to medium droplets under the parameters listed above. Fine to medium droplets have volume median diameters (VMDs) ranging from 200 to 350 microns.
- Use the mid-point in the crop canopy as your target in shorter plants and 5 inches below the top of the canopy in tall plants and adjust the boom height from this point as recommended by the nozzle manufacturer for the spacing and spray angle of your nozzles.
You have realistic expectations? Fungicides are not a silver bullet for managing white mold with the level of control ranging from 10 to 80 percent in research trials.
For additional information, please view the MSU Extension Field Crops Virtual Breakfast Series recording on white mold management.
This article was previously published in the 2021 Michigan Soybean On-Farm Research In-season Management Guide published by the Michigan Soybean Committee.