Last year presented a number of growing season challenges. One of particular concern was the increase of white mold in soybeans, which affected large portions of Iowa, South Dakota, Minnesota, Wisconsin, North Dakota and the Great Lakes region. We’ve witnessed and heard about a lot of white mold damage throughout this area over the past few years, and for those with a history of white mold issues in their fields, it will likely be a problem again in 2020. Let’s take a look at the make-up of white mold and management strategies growers can implement to help prevent the disease from robbing yields this year.
What is white mold?
White mold, or sclerotinia stem rot, is a fungal pathogen that starts on soybean stems. Tissue infected with sclerotinia forms a white fungus called mycelium, which gradually turns into a dark, hardened structure referred to as sclerotia. Sclerotia is known to overwinter and can come back year after year. From sclerotia, apothecium can form, producing small mushroom-like structures on infected plants. This often occurs at the same time that flowers are present on the plant, from the R1 to R3 growth stage, especially when moderate temps and high humidity persist.
What conditions favor the development of white mold?
White mold thrives in moderate temperatures that range below 68 degrees within a 30-day window. This encourages the apothecium to grow during bloom to early pod set. Other favorable conditions include higher humidity (greater than 60 percent), wet weather and fields with a past history of white mold. Irrigation has also been known to encourage white mold, as it cools the crop canopy. If warmer temps are in the 30-day forecast (e.g., multiple days measuring 86 degrees Fahrenheit or above), irrigated acres may be susceptible to apothecium.
Other factors that promote white mold include no-till or minimum tillage fields, narrow-row spacing, higher planting populations, high manure level in soil, high nitrogen level in soil and areas of lodging. Not rotating crops and choosing a susceptible variety for your field can also play a factor in encouraging white mold growth. Also, watch for fields with increased weed pressure. Pigweed and ragweed species are hosts to white mold.
What are some key management strategies to combat white mold?
Growers need to consider a systematic approach and keep an open mind to combat white mold. A field-by-field approach may be necessary.
First, growers should look for soybean varieties that have good tolerance to white mold and a plant structure suitable for the environment they are planting into (e.g., no-till, narrow rows). In some instances, growers may need to switch to 30-inch rows and push back their final population to around 100,000. While studies in the North have shown around a 3 bu/acre yield advantage with 15-inch rows, narrow rows are more susceptible to white mold because there is less air movement between rows. Growers with heavy white mold pressure need to consider if the advantages of narrow rows outweigh the higher likelihood of white mold.
A bio fungicide is a parasitic fungal organism that lives and feeds off sclerotia. Bio fungicides should be incorporated into the soil approximately two inches deep for three months prior to soybean blooming. The biggest concern with using bio fungicides is if they’re economical. These products can be costly (around $35/acre) and are sometimes a multi-year commitment.
Stine XP soybean seed treatments can offer growers extra protection from white mold early in the growing season, when seed is at its most vulnerable. Our Stine XP Complete option combines fungicide with Heads Up® plant protectant. Seed treatments can help kill fungal pathogens like sclerotia that weaken a soybean plant at the very beginning of its lifecycle.
Growers interested in fungicide options for white mold should consult their local Stine agronomist or university extension office. There are a some really great new foliar fungicides available that feature multiple modes of action to help control diseases like white mold; however, it’s important to know the appropriate timing of application during the reproductive stage of the soybean plant before proceeding. Remember that fungicides need to be applied down into the canopy to be properly effective and to follow specific boom height recommendations before spraying. While fungicides are a great option for white mold in corn, not all products are labeled for use in soybeans.
When there’s a history of heavy white mold pressure, growers can also consider certain burner herbicides to burn back the canopy of the soybeans. We’ve seen some good response with this method for white mold control. While a bit riskier, if you have favorable growing conditions, this method can reduce the risk of yield loss caused by white mold.
I also recommend that growers who are concerned about white mold in their soybean fields consult the University of Wisconsin’s Sporecaster. This app was developed to assist growers in making management decisions for white mold. Their research indicates that the appearance of apothecia can be predicted using several variables, such as weather and the amount of row closure in a field. Sporecaster uses GPS coordinates to see if weather has been favorable for apothecia in a specific field, factoring in max temps, relative humidity, max wind speed and risk predictions, to name a few. Learn more here.
The most important thing you can do to prevent white mold from hitting your fields this growing season is to plan ahead. Evaluate your fields and make the necessary preparations to proactively respond to the potential threat of white mold. For more information on white mold management, contact your local Stine agronomist.