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Sunflower Rust: Primed for a Comeback?
Saturday, March 1, 2025
filed under: Disease
By Sam Markell*
A sunflower leaf with rust at the 1% severity threshold level.
Photo credit: Andrew Friskop
It’s been years since sunflower rust last caused widespread yield loss in the Northern Great Plains. But could that change in 2025?
Over the last two years, NSA-funded NDSU graduate student Zach Ittel has surveyed fields in Minnesota, North and South Dakota for rust. “We surveyed 51 fields in 2023, and found rust in 49 of them”, Ittel reports. “In 2024, all of the 51 locations we visited had rust.”
This high prevalence of rust is very unusual. Typically, we expect to find rust in about half the fields we visit at the end of the season. But Ittel has been surveying in mid-August, and he is finding it in nearly every field.
Unlike rust diseases that occur on wheat, barley, corn or other crops, sunflower rust can survive the winter in the Northern Great Plains. The pathogen causing rust, Puccinia helianthi, produces five different spore types. At the end of the season, the dusty-characteristic cinnamon-brown pustules of sunflower rust turn into coal-black “telia.” This spore stage is extremely hardy and allows the pathogen to survive the northern winters.
With all the rust appearing in 2023 and 2024, the safe bet is that sunflower rust will have survived this winter.
But, surviving the winter doesn’t necessarily mean a rust epidemic is imminent.
Rust is a sporadic disease, and multiple factors determine whether a field, a farm or a region may see yield loss. Free moisture is the most important environmental condition that rust needs to reproduce. Leaves must stay wet for many hours for spores to germinate and infect the plant. Consequently, heavy dew and fog are much more likely to give rust a foothold than would a thunderstorm.
The effectiveness of genetic resistance in sunflower hybrids also influences whether yield loss will occur. Breeders have long been incorporating resistance genes into hybrids to manage rust; but rust pathogens are notorious for developing new races that cause disease on otherwise resistant hybrids.
The worst rust epidemics I personally have witnessed occurred in 2008 and 2009. Those seasons were cool and wet, fog was common, and dew stayed on the leaves until noon. Genetic resistance in the majority of oilseed and confection hybrids was not holding up against new rust races at that time, and initially, fungicides were not available to manage the outbreak.
Further south, University of Nebraska plant pathologist Bob Harveson had never worked on rust until the late 2000s. “I’ve never seen anything quite like it,” he says. “Rust was on sunflower in the ditches, ornamentals in gardens, and in every field I checked.” It was a perfect storm for sunflower rust, and it reduced yield across the Northern Great Plains.
The rust epidemic in 2008 and 2009 prompted the National Sunflower Association to invest in research to manage sunflower rust by supporting (then) NDSU graduate student Andrew Friskop. His research identified the new pathogen races that overcame the genetic resistance in the 2000s, and it laid the foundation for fungicide recommendations for managing the disease.
Several years later, NSA-supported (then) NDSU graduate student Brandt Berghuis would refine Friskop’s research into the management recommendations we use today. (NSA investment is coming full circle, as (now) Dr. Friskop (NDSU) and Dr. Berghuis (University of Wisconsin at River Falls) are supporting Ittel and Markell’s ongoing efforts to help growers understand and manage sunflower rust.)
The high rust prevalence in 2023 and 2024 makes it critical to scout for rust this season. Unless conditions are extremely favorable, scouting isn’t necessarily recommended until the canopy nears closure (when lower leaves stay wet longer and provide the optimum conditions for infection).
In a typical field, rust first appears on a field edge near last year’s sunflower (such as an adjacent field) and/or along shelterbelts. Once rust is found, it can spread rapidly throughout fields if conditions stay favorable.
Fortunately, rust management has come a long way since the late 2000s.
At last check, a dozen fungicides are labeled for sunflower rust management. Years of efficacy trials have demonstrated the triazole (FRAC 3) fungicides (such as tebuconazole and generics), strobilurin (FRAC 11) fungicides (such as Headline, Quadris and generics) and premixes that contain those FRAC groups are among the most efficacious fungicides against rust.
Application timing is critical. A fungicide application is recommended if 1% rust severity or more is observed on the upper four fully expanded sunflower leaves at growth stage R5 or earlier. While 1% does not sound like much, the disease is very explosive when conditions are favorable. Research done by Brandt Berghuis in the late 2010s demonstrated that even trace levels of rust on the upper leaves at R5 would rob yield if conditions were favorable. “In a couple trials, we saw approximately 300- to 500-lb yield increases with fungicide applications made at R5 when only trace levels of rust were observed on the upper leaves.” Berghuis recounts. “This isn’t necessarily typical; but we were growing susceptible confection hybrids in a high-yield environment, and conditions for rust were excellent. The epidemic exploded like wildfire.”
In addition to advances in fungicide management, sunflower hybrids may be more resistant to rust than those grown 15 years ago. Rust resistance genes identified by scientists in the Sunflower and Plant Biology Research Unit at the USDA-ARS in Fargo over the last two decades are likely incorporated into hybrids now being grown across the Northern Great Plains.
More resistant hybrids being grown means epidemics may be less likely to occur. The tricky question then becomes, “Have new rust races emerged?” Zach Ittel hopes to answer that question later this year.
Ittel collected and isolated dozens of pathogen samples from Minnesota and the Dakotas during his 2023 and 2024 field surveys and received samples from Nebraska and Wisconsin via Harveson and Berghuis, respectively. Over the next several months, Ittel will determine which rust races are occurring in our region and better understand the threat they present to our hybrids.
In summary, as of this mid-March writing, it’s still too early to say whether rust will make a comeback in 2025 — but it is reloading. Remember that despite several dry years, better hybrids and effective fungicides, rust was still found in nearly every surveyed field last year. We have great tools to manage rust, but it is important to scout on a timely basis in 2025.
* Sam Markell is extension plant pathologist with North Dakota State University.