Research Highlights

Soybean Fungicide Evaluations Validate When to Treat

By Laura Temple

Maryland, along with the entire Atlantic Coast region, often has high humidity during much of the growing season. Because of that, disease pressure tends to be high, according to Andy Kness, an agricultural science agent in Harford County for University of Maryland Extension, with a plant pathology background.

When companies release new fungicides for in-season soybean applications, Kness says farmers want to know how they compare to other commonly used products under real-world conditions. To provide this information, he manages annual comparison trials with soy checkoff support from the Maryland Soybean Board and product donations from suppliers.

“For example, Miravis fungicides, released by Syngenta a few years ago, contains two modes of action,” he says. “They were also labeled for two consecutive applications 14 days apart. Farmers wanted to know how that compared to other commonly used fungicides in soybeans.”

The Western Maryland Research & Education Center in Keedyseville and the Wye Research and Education Center in Queenstown host his comparison trials. The two locations have allowed him to observe fungicide performance relating to yield, plant health and late-season greenness under different growing conditions. 

“The trials consistently show that fungicides make a difference when there is disease pressure,” Kness summarizes. “It doesn’t make a difference when there is no disease pressure or when another major factor limits yield.”

He observed an example of this in the 2020 trials that compared Miravis Top, a relatively new fungicide, Priaxor, an industry standard for the region, and untreated soybeans. Soybean yields at WMREC showed no statistical differences between treatments and the untreated check. However, soybean yields at WYE improved with one fungicide treatment, and they increased even more with a second fungicide treatment 14 days later.

“Western Maryland was very dry that year, receiving less than 20 inches of rain from May to November, so the yield-limiting factor was moisture much more than disease pressure,” Kness says. “The other trial location received 47 inches of rain and a severe infestation of herbicide-resistant marestail, both of which impacted the severity of frogeye leaf spot infestations.” 

He notes that frogeye leaf spot attacks soybean fields most years, and that fungicide treatments effectively protect and improve yield. However, in 2021, stem canker also appeared in the WMREC trials. This soil-borne disease can cause soybeans to shrink or be discolored within pods, but all the fungicide treatments effectively protected soybean quality.

Overall, when disease pressure is present in trials, the untreated check suffers most. A single fungicide treatment steps yield up noticeably, and treatments with two applications of a fungicide 14 days apart attain the highest yield. However, the fungicide treatments, especially two applications, keep plants greener longer. While that can improve yield, green stems can also be challenging to harvest.

“I did look at return on investment with a second application of fungicide, assuming a flat rate for application costs,” Kness says. “My calculations showed that a 4 to 9 bushel-per-acre yield increase was needed to pay for an application. Farmers will need to pencil out for their operation if two applications of a fungicide would pay.”

He offers recommendations as farmers evaluate disease pressure and fungicide options.

• Consider soybean varieties at planting, as genetic resistance or tolerance to specific diseases is the most cost-effective management option.
• Apply fungicides to more susceptible soybean varieties between the reproductive growth stages R1 and R3 when weather conditions are conducive to disease development. In the Mid-Atlantic region, that usually means hot, humid or wet weather.
• Fungicides with two to three modes of action tend to give more response than products with a single mode of action.
• Do the math and consider disease pressure to decide if a second fungicide application 14 days later is warranted.

“These trials provide information about the efficacy of new fungicides,” he says. “In the trials focused on Miravis Top shortly after it was released, it stood out as a good fungicide option when there is disease pressure. I hope to continue these trials, especially when new products become available, to help famers determine if those fungicides are worthwhile investments.”

Published: Feb 20, 2023