Sclerotinia stem rot, also known as white mold, is a problem in both dry edible beans and soybeans.
Michael Wunsch, North Dakota State University plant pathologist at the Carrington Research Extension Center, has worked on finding new solutions to controlling the disease in both crops and is finding similar solutions.
Wunsch, speaking at Bean Day in Fargo on Jan. 19, 2023, said there’s no one solution for white mold in dry edible beans. But after three years of row spacing and seeding rate work on pinto and kidney beans, he has figured out the simplest management technique.
“The easiest thing that you can do from our research to manage white mold is, on your problem fields, be at the lower end of that normal seeding rate,” he said.
But the research Wunsch has done, along with co-principal investigator NDSU agronomist Kelly Cooper at the NDSU Robert Titus Irrigation Research Site, also has looked at both the timing of fungicide and the types of droplets used in spraying.
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The droplet size has been the “real game changer” for soybeans and dry beans, Wunsch said. Using a fine mist had been the established way to apply fungicides, but that doesn't work for white mold, which does not appear at the top of the canopy like other fungal infections. Fine droplets don’t get through to where the disease lives when the canopy is significantly closed.
The researchers conducted field trials in Carrington and Oakes in 2020 and 2022 and found white mold management and dry bean yield were optimized by increasing spray droplet size between application one and application two as the canopy became denser.
In four of eight pinto bean studies, fine droplets in the first application and coarse droplets at the second application provided the best management, while in the other four studies, medium droplets in the first application followed by coarse application in the second was best. Additional research is needed to identify the canopy characteristics when fine droplets versus medium droplets are optimal in the first application.
When the canopy was moderately open to nearing closure (80-95% of the ground covered) at the first fungicide application and at or near closure (88-99% of the ground covered) at the second fungicide application, kidney bean yield under white mold pressure was optimized by applying fungicides with medium droplets in the first application and coarse droplets in the second application.
Wunsch said using the least coarse droplet possible is important, as coverage area decreases the coarser the droplets get. However, increased canopy closure requires increased size of droplets.
“You have to get a coarser droplet as your canopy gets denser,” he said.
A surprising piece of the research was that increasing spray volume from 10 gallons per acre to 25 gallons per acre had no impact on white mold severity or dry bean yield. Wunsch said follow-up research is planned.
When it comes to timing of spraying, Wunsch said the mantra always has been to spray before infection. But spraying too early also may have a penalty, he said.
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“What we have learned is, when you apply too early before there is any significant risk of infection, you can actually really hurt yourself,” he told the crowd at Bean Day.
The reason, he explained, is that fungicides do not “translocate” into new growth, which happens fast at early bloom.
“Remember that any additional vegetative growth after you apply your fungicide is not protected,” he said.
Researchers conducted field studies under overhead irrigation in Carrington and Oakes in 2017, 2020, 2021 and 2022 to evaluate the growth stage at which fungicides should be applied when weather conditions favor white mold. Four application timings were evaluated. The first application was made at early bloom (target 10-50% of plants with an open blossom) and subsequent timings were spaced two to four days apart. Testing was conducted with a single fungicide application of Topsin (30 or 40 fluid ounces per acre) or with sequential applications of Topsin followed by Endura (8 ounces per acre) 10-14 days apart. Irrigation was managed to create conditions favorable for white mold.
The percent of plants with initial pin-shaped pods was utilized to characterize fungicide application timing. Dry beans produce small pin-shaped pods as soon as blossoms deteriorate with age. Nearly all white mold infections are initiated on dead blossoms, and the percent of plants with dead blossoms is a better predictor of susceptibility to white mold than the percent of plants with open blossoms, research has found.
In pinto beans, optimum fungicide application timing differed by canopy closure and daytime temperatures. When the canopy was open and temperatures reached the mid- to upper 80F range many days, fungicide performance was maximized by delaying applications until 50-85% of plants had pin-pods (when two sequential fungicide applications were made 10-14 days apart) or until 70-85% of plants had pin-pods (single application). When the canopy was at or near closure and daytime highs were predominantly in the mid-70 to low 80F range, fungicide performance was maximized with applications made when 10-20% of plants had pin-pods (two sequential applications) or 60-85% of plants had pin-pods (single application).
In black beans, optimum fungicide application timing differed by canopy closure. When the canopy was open, fungicide performance was maximized when 1-20% of plants had initial pin-pods and two sequential fungicide applications were made 10-12 days apart. When a single application was made, disease management was poor, and no difference was observed across application timings. When the canopy was at or near closure, fungicide performance was maximized with applications made when 30-50% of plants had pin-pods (two sequential applications) or 60-100% of plants had pin-pods (single application).
In navy beans, optimum fungicide application timing was influenced by daytime high temperatures. When daytime high temperatures were predominantly in the mid- 80 to low 90F range, fungicide performance was maximized when the first application of a two-application sequence was made when 21-50% of plants had initial pin-pods. When a single application was made, fungicide performance was optimized when 65-96% of plants had pin-pods. When daytime high temperatures were predominantly in the mid- 70 to low 80F range, fungicide performance was maximized when the first application of a two-application sequence was made when 3-20% of plants had initial pin-pods. When a single application was made in cool weather, white mold overwhelmed the fungicide, and no difference was observed across application timings.
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In kidney beans, when a single fungicide application was made, fungicide performance was maximized by delaying applications until 55-80% of plants had initial pin-shaped pods. A variable response to application timing was observed when two sequential applications were made, and additional studies are needed to reach conclusions. Only four field studies were conducted with kidney beans (versus six to eight studies with the other market classes).
Optimum fungicide application timing was generally two to three days earlier for the first application of a two-application sequence (10-14 days apart) versus a single application. A single fungicide application does not confer sufficient residual to protect dry beans for the entire bloom period. The results indicate that when a single fungicide application is made, it is best to delay the application slightly in order to extend protection through more of the critical full bloom when dry beans are at their height of susceptibility to white mold.
The results suggest that when rainfall or irrigation patterns are favorable for white mold as dry beans enter bloom, fungicide application timing can be optimized on the basis of forecasted daytime high temperatures, average canopy closure, and the percent of plants with initial pin-shaped pods.
“We have to be very very good producers and we have to be very timely with our applications,” Wunsch said.
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