Research Highlights

Timing is Crucial for Fungicide Application to Reduce White Mold in Soybeans

By Carol Brown

White mold in soybeans is difficult to manage. The fungus that causes it, Sclerotinia sclerotiorum, has a broad host range, infecting most broadleaf crops, and it persists for many years in the soil. It is sensitive to weather conditions and is most severe in cool, moist conditions.

Michael Wunsch, a plant pathologist at the North Dakota State University Research Extension Center in Carrington, has been studying white mold management in soybeans, sunflowers and dry edible beans for more than a decade. Through several research projects backed by the North Dakota Soybean Council, Wunsch keeps fine-tuning management practices to reduce losses to this disease. His most recent study is looking at fungicide application frequency and the number of days between applications for best disease management.

“This current study is part of a broader research effort that we’ve embarked on to reduce economic losses to white mold and improve profitability of soybean production when conditions favor white mold,” says Wunsch. “North Dakota farmers also grow dry edible beans, which also have white mold issues. When I started this work a decade ago, producers reported a fairly good response to fungicides in dry edible beans — but not so with soybeans.”

Wunsch has researched white mold mitigation through several avenues, including fungicide application timing, soybean row spacing, seeding rates and fungicide droplet size, and has developed recommendations for improving white mold management in these areas. The latest study, which is wrapping up after the 2022 harvest, seeks to develop recommendations to optimize the number of fungicide applications and interval length between applications for a range of soybean maturity groups.

“The first part of this research project is looking at whether one or two fungicide applications are needed,” Wunsch explains. “And the second part looks at if two applications are needed, when should that second application be done?” 

He and his team evaluated the response to one versus two applications of the fungicide Endura on a large number of soybean varieties (42 to 48 per year), spanning the full range of soybean maturities grown in North Dakota. They conducted the tests at two locations for two years on maturity groups ranging from 00.5 to 1.3.

“Within our range of soybean maturities, the response to a second fungicide application increased with soybean maturity,” he says. “In the mid-zeros — 0.4 or 0.5 maturity — it became profitable to make the second application. It makes logical sense because the soybean bloom period lengthens with longer maturity varieties, creating a longer window of susceptibility for white mold to infect the plant.”

Wunsch says that the second fungicide application is needed to protect yields in longer maturing soybeans since the residual from the first application wears off before the window of susceptibility is over. He cautions these recommendations apply only when conditions are favorable for white mold. 

Fungicide Application Intervals

Farmers may wonder if that second fungicide application would be helpful for their soybean yields, when should it be applied? Wunsch says the industry standard answer has been 10 to 14 days apart, but rigorous data is lacking for this recommendation. In Brazil, where white mold is also a major constraint on soybean production, fungicides are typically applied 7 to 10 days apart. To determine the optimum interval, he tested applications 7, 10, 12 or 14 days apart. 

“In the first two years of this study, the data showed something that I didn’t expect, but actually made sense,” he remarks. “As the soybean maturity increased, the optimum number of days between fungicide applications increased.”

The same logic applies here due to the differences between maturity groups. With a shorter maturity soybean, the bloom period is shorter. When the second fungicide application occurs 7 days after the first, residual activity extends through the end of the bloom in shorter maturity soybeans. In longer maturity soybeans, a longer interval is needed between applications in order for residual from the second application to provide protection through the end of bloom. 

In preliminary results from the first two years of this study, applying fungicides 7 days apart optimized white mold management in 0.6 maturity soybeans; applying fungicides 7 to 10 days apart optimized white mold management in 0.9 maturity soybeans; and applying fungicides 14 days apart optimized white mold management in 1.1 maturity soybeans (Figure 1).

“With the higher maturities such as 1.1, you are better off delaying the second fungicide application in order for residual activity to reach the end of the reproductive period,” he says. “Sacrificing a little bit of white mold infection on the new growth that occurs after the first application is better than sacrificing a lot of unprotected foliage at the end of the bloom.”

Wunsch is in the process of analyzing the data from the 2022 crop, the third year of this study. View Wunsch’s research updates on this study as well as his work with white mold in dry beans at the North Dakota State University Carrington Research Center Plant Pathology website.

Published: Jan 9, 2023