In Minnesota, 2019 was the third consecutive year of high soybean white mold pressure.

Although it isn’t clear if white mold will be a big problem this year, there are precautions you can take to help manage it.

Here are seven tips to help prepare and control soybean white mold:

1. Pick a variety with high white mold tolerance. The No. 1 way to manage white mold is through variety selection. If your fields have a history of white mold, consider the pros and cons of using the latest trait varieties.

If you have a soybean variety you’ve planted for several years that you know does well on white mold, use that in your most vulnerable fields. Keep newer traits on ground that does not have a history of white mold, at least until that trait’s performance is proven.

2. Pay attention to crop rotation. If you had fields planted in soybeans last year, you’ll probably rotate them to corn or wheat this year so white mold won’t be a problem. But if you had soybean fields with white mold in 2018 that went to corn in 2019 and are returning to soybeans now, the sclerotia bodies that caused the white mold are still in the soil.

If we get cool, cloudy, wet, high-humidity weather in July and August, then the disease will thrive.

3. Watch fields with high fertility. Tall, bushy, canopied, high-yielding soybean plants attract more moisture, which is an invitation for white mold.

If you have fields with a lot of manure history and high fertility levels in your soil tests — specifically phosphorus — incorporate another management strategy to combat white mold, such as a fungicide application. Don’t cut back on fertilizer.

4. Time your fungicide application right. Sclerotia bodies sprout apothecia, which look like tiny mushrooms and release spores. In nearly all instances, white mold is caused by these spores infecting the soybean plant through a dead flower that has recently pollinated but has not yet formed a pod.

When you start seeing flowers on your soybean plants, and certainly before those flowers start to die, that’s the optimal time to apply a fungicide. This occurs about midway through the R1 growth stage (initiation of flowering) and before R2 (full flowering) — usually in early July.

5. Be proactive. The tough thing about white mold is you must make management decisions about controlling it a month to two months before you see any signs or symptoms. Scouting for apothecia is nearly impossible. They’re very small and short-lived.

You must go by the percentage of flowering plants or the calendar date to stay ahead of the disease. By the time you see white mold growing on soybean plant stems, it’s too late to control.

6. Do not expect a seed treatment to control your white mold problem. While certain seed treatments may provide some control of white mold, they are not nearly as effective at controlling the disease as a foliar-applied fungicide application. Use both as part of a program of white mold control.

7. Know the potential of herbicide. Applying certain Group 14 herbicides that are also protoporphyrinogen oxidase (PPO) inhibitors can stimulate the systemic acquired resistance (SAR) mode of action, which can help minimize white mold damage if applied at initiation of flowering.

Remember, a Group 14 herbicide “burns” the soybean plant to stimulate that white mold response. Although you may lose some top-end yield, it’s an effective treatment if you do get white mold in your fields.

Talk with your trusted adviser about the best white mold management strategy for your soybeans.  

Glady is a regional agronomist with WinField United in west-central Minnesota. Contact him at [email protected].