FARGO, N.D. – Northern Great Plains wheat growers in low-disease environments should think twice before applying foliar fungicides to their crops, according to a new study.

“Growers can’t treat every situation the same way,” said Andrew Friskop, a North Dakota State University assistant professor and Extension plant pathologist. “We’re trying to manage diseases that behave differently in different conditions. It’s a much more dynamic system than a simple input every time.”

Friskop led the study in conjunction with colleagues from the NDSU Department of Plant Pathology; Montana State University’s Kate Binzen Fuller, assistant professor and Extension specialist in the Department of Agricultural Economics and Economics, and Mary Burrows, a professor in the Montana State University Department of Plant Sciences and Plant Pathology.

The study analyzed yield data from 46 fungicide trials conducted in low-disease environments from 2007 to 2014 on hard red spring wheat and hard red winter wheat in Montana and North Dakota. The study was published in the journal Plant Health Progress in September, 2018.

“The data we pulled gives us hard numbers and we can ask growers, ‘What do you gain if you spray?’” Friskop said. “It’s good for growers to see if they spray, they may not get that money back.”

In the late 2000s, relatively high wheat prices and low fungicide costs meant growers in the northern Great Plains routinely applied fungicides, even in the absence of diseases. Fungicides were marketed to growers as a tool to benefit plant health and boost yields, which made sense given that foliar fungal diseases – such as tan spot, Septoria and Stagnospora leaf spot, powdery mildew and leaf and stripe rust – are among the limiting factors for wheat production.

However, using a fungicide in the absence of disease doesn’t always pencil out. Fungicides are simply not economical until moderate to high levels of disease are present.

“We’re telling growers, ‘Scout before you spray,’ especially when prices are what they are,’” Friskop said. “We think it’s important to assess the level of disease risk because the expectation on the return on your investment is marginal.”

A wheat crop is at low foliar-disease risk when it’s a resistant variety, when pathogen-infested residue is not apparent and when conditions are dry. A crop is at high foliar-disease risk when it’s a susceptible variety, when infested residue is apparent, when disease has been detected and when conditions are wet.

“We’ve been treating all varieties the same,” Friskop said. “We’ve been spraying resistant varieties with fungicide because we’ve been using the same production practices for several years.”

The study focused on fungicide application to address foliar diseases during the two times early in the season a grower might be applying it: during an herbicide pass, thrown in as a preventive measure; and during flag leaf emergence.

“Our trials showed fungicide is best used when disease is one of the limiting factors in the field,” Friskop said. “When you apply at flag leaf at low disease risk, there is a one-and-a-half-bushel higher yield, but it only covers your return on investment 30 percent of the time.”

Using a fungicide in low wheat disease environments can contribute to the development of fungicide resistance, reducing the efficacy of chemistries.

“Using foliar fungicide as a routine input, we always think of economics, but we also need to think about fungicide stewardship,” Friskop said. “We’ve seen situations where the overuse of fungicides in Europe and elsewhere leads to a pathogen resistant population and then you lose a tool in your toolbox.”

The data from this study have been used to help create an economic decision tool: http://msuextension.org/econtools/fungicide/graph.html. Users can adjust the costs of application, predicted yield changes from a fungicide application and the price of wheat to help the decision-making process.