Applying fungicides for white mold management in soybeans

Factors to consider before applying a fungicide for white mold management in soybeans.

White mold was prevalent in 2014, causing significant yield reductions in many soybean fields. Last year was particularly severe due to the conducive weather conditions for plant infection by the white mold fungus at the time of flowering. Weather will play a significant role in the outbreak or lack thereof again this year. Foliar fungicides are one of the tools in the white mold management tool box. However, they are not the only tool and should be used in combination with other white mold management strategies such as partially resistant varieties, wide rows, reduced planting populations, irrigation water management and appropriate tillage method.

Michigan State University Extension recommends producers consider the following factors before purchasing and applying a fungicide to manage white mold in soybeans.

  • Does the field have a history of white mold? White mold sclerotia (survival structures resembling rat droppings) can survive in the soil for five to seven years when buried more than 2 inches deep.
  • How dense is the soybean canopy at flower initiation? 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. While dense soybean canopies favor white mold development, significant yield losses due to white mold can also occur in open crop canopies when overcast, cool and humid conditions prevail during flowering.
  • Will you be able to 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 R1 growth stage (one open flower per plant on 50 percent of 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. Last year’s trials at Montcalm, Michigan, indicated R3 applications may have performed slightly better than R1 applications. A second fungicide application made approximately 10 days after the first application may improve control if weather is predicted to remain cool and wet or humid.
  • Has the top 2 inches of soil remained cool (40 to 60 degrees Fahrenheit) and continuously moist for seven to 10 days prior to R1 growth stage? These conditions induce sclerotia germination, apothecia formation and spore dispersal.
  • Is the temperature predicted to be cool (less than 85 F) and foliage to remain continuously moist or wet for at least 40 hours at the beginning of R1 growth stage? These conditions favor infection.
  • Is the fungicide effective in managing white mold? The following fungicides have been rated as providing fair to very good management of white mold when properly applied:
    • Topguard 1.04 SC (Fair)
    • Proline 480 SC (Fair)
    • Domark 230 ME (Fair)
    • Topsin-M (Fair)
    • Endura 0.7 DF (Very Good)
    • Aproach 2.08 SC (Good)
  • Is your sprayer equipped and operated to maximize droplet penetration into the crop canopy and plant coverage? Fungicide performance is significantly affected by spray coverage. The following recommendations will improve spray coverage and fungicide performance:
    • Apply 15 to 20 gallons of water per acre.
    • Adjust nozzle pressure to around 40 pounds per square inch.
    • Maintain ground speed at 10 miles per hour or less.
    • Equip the boom with nozzles that produce a single, flat fan spray pattern directed straight down into the canopy.
    • 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 and adjust the boom height from this point as recommended by the nozzle manufacturer for spacing and spray angle of your nozzles.
  • Are you willing to accept white mold control of 80 percent or less? Fungicides are not a silver bullet for white mold management. In 2013 trials, we obtained 80 percent control of white mold; however, in 2014 with the severe weather conditions and heavy disease pressure, we only managed to control 9 percent of the disease.

Dr. Chilver’s work is funded in part by MSU’s AgBioResearch.

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