Research Highlights

Shifting fungicide applications for white mold can improve yield

By Carol Brown

Fungicide applications for soybeans can be tricky. North Dakota researcher Michael Wunsch has been trying to outsmart white mold for some time.

Wunsch is a plant pathologist at the North Dakota State University (NDSU) Research Extension Center at Carrington. He has been researching diseases in soybeans, dry edible beans and other crops for several years and has studied white mold in soybeans for nearly a decade.

“White mold is a sporadic problem. It doesn’t happen every single year and workable management recommendations haven’t been developed to combat it,” Wunsch said.

Wunsch is making headway to change this. His research projects are exploring the effectiveness of soybean fungicide application timing and droplet size. The research projects are supported by the North Dakota Soybean Council.

“Ultimately, we’re chasing yield,” he said. “We want to manage the disease and maximize profitability. Sometimes a little disease is okay, if you have higher yields and acceptable soybean quality. We’re trying to give people recommendations for how they can optimize one fungicide application, as multiple applications aren’t always economically feasible.”

A shift in the fungicide application recommendations has emerged from this multi-year research. In North Dakota and other states affected by white mold, many farmers have traditionally applied fungicide at the R1 growth stage with fine droplets, Wunsch said. But his findings demonstrate that fungicide efficacy is often sharply improved by delaying application until the R2 stage, unless the canopy is at or near closure earlier.

“The vast majority of the time it’s optimal if you wait until the R2 growth stage to spray, like we have found in our timing studies, because the canopy usually doesn’t close until then,” he said.

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When conditions are favorable for white mold, Wunsch has guidelines for fungicide application:

• Spray fungicide if the canopy is closed at late R1
• Wait to spray if the canopy is still open at late R1
• Spray at early R2 if the canopy is at or near closure
• Wait to spray if the crop is at early R2 and the canopy has not closed
• Apply fungicide when 100 percent of the plants have reached R2 growth stage, even if the canopy has not yet closed.

Wunsch said that if conditions favor white mold, it is important to apply fungicide at the R2 growth stage even if the canopy has not yet closed.

“When soybeans reach the R2 growth stage, we have seen significant yield penalty when delaying fungicide applications, even if the canopy does not close for another week or more,” he commented. “There are enough dead blossoms on the plants at R2 that significant levels of white mold infection can occur even when the canopy is still open.”

Droplet size makes a difference
Wunsch is also studying white mold fungicide spray droplet size efficacy on soybean yield.

Modern fungicides exhibit upward movement in plant tissues, unlike herbicides which go up and down the plant. The Sclerotinia sclerotiorum fungus attacks dead tissue, usually the first dead soybean blossoms. To be effective, the fungicide needs to be applied to the interior of the canopy where the blossoms are located.

“It’s all about canopy,” he said. “In North Dakota at the R2 stage, most soybean plants are about 2-feet tall,” he said. “The fungicide performs best when it is deposited to the bottom 8- or 9-inches of the crop canopy. The spray droplet size that optimizes fungicide deposition to the lower canopy is determined by canopy closure.”

Wunsch’s research indicates that the traditional recommendation of applying fungicide for white mold with fine to medium droplets is optimal only when the canopy is open or averages less than 80 percent. When canopy closure averages 80-90 percent, medium droplets have optimized fungicide performance. When canopy closure averages more than 90 percent, spraying coarse droplets of fungicide works better to penetrate dense canopy.

This is when Wunsch uses the analogy of a ping pong ball versus a lead ball. Fine droplets of fungicide have no weight to them and bounce like ping pong balls off the leaves. Like a lead ball that has weight to it, larger fungicide droplets have velocity to get inside the canopy. Droplets should be just coarse enough to get inside to the target. If they’re coarser than necessary, coverage is lost, he said.

Research results showed when fungicide was applied in coarse droplets when the canopy was near closure, there was an average 8-bushel gain in yield. When fungicide was applied in fine to medium droplets, Wunsch saw an average yield gain of 3- to 4-bushels. The study was conducted using several soybean varieties, each with different susceptibility to white mold.

White mold 101
White mold needs two things for development: cool, wet conditions and dead plant tissue as a target for infection. The Sclerotinia sclerotiorum fungus produces hard, black resting structures called sclerotia, which are found in the soil deposited from previously infected crops. The sclerotia can persist in the soil for many years. These structures germinate in moist soils and develop mushroom-like structures that produce spores that can infect dead tissue, which for soybeans are often the first dead blossoms. Once the pathogen colonizes in the dead tissue, it invades living tissue.

As temperatures rise, more moisture is needed for white mold to develop. Wunsch said that when daytime highs only reach the low 70s, heavy dew and elevated relative humidity is sufficient for spores to infect and cause disease. As temperatures rise into the 80s, recurrent rainfall or overhead irrigation are generally needed to cause significant infection. When daytime highs are in the 90s during bloom, regardless of how wet it is, white mold doesn’t usually appear, no matter the field history.

There is a lag time before outward symptoms develop. Most farmers don’t know they have white mold until it has progressed significantly and crops begin to die. Fungicides applied at this stage are not effective. All fungicides should be applied during bloom based on a perceived disease risk, Wunsch said.

Farmers need to be aware of environmental conditions as the crop develops.  Growth stages R2, R3, and R4 are at highest risk for white mold. If the plant is entering the R2 growth stage and there is enough soil moisture to facilitate the production of apothecia under the canopy, the risk for white mold spores is greater. Wunsch says to watch the weather at this point. The cooler it is, the less moisture will be needed for spore infection. The warmer it gets, the wetter it needs to be for white mold to develop.

Farmers can fight the white mold battle by adjusting the combination of fungicide timing and droplet size. With these two components farmers can make tremendous headway, Wunsch said. Getting the timing and droplet size right could translate into two to three times the yield response to fungicide.

For more information about these research projects, visit Wunsch’s research summaries: https://www.ag.ndsu.edu/CarringtonREC/plant-pathology-1

Also see the National Soybean Checkoff Research Database for more information about Wunsch’s white mold research: https://www.soybeanresearchdata.com/Project.aspx?id=53154 and https://www.soybeanresearchdata.com/Project.aspx?id=52337

Published: Aug 11, 2020