Combination of control methods needed to protect sugarbeets from cercospora leaf spot disease

Sugarbeet varieties with the CR+ trait are not a panacea so farmers still should use fungicides to control cercospora leaf spot disease in their fields, a sugarbeet researcher advises.

Research shows that the disease adapts to management practices, which means that if farmers don’t use fungicides in conjunction with sugarbeet seed that has CR+ trait, cercospora leaf spot outbreaks could occur, said Nathan Wyatt, U.S. Agriculture Department Agricultural Research Service plant pathologist.

“You’ve got to use all of your tools,” Wyatt said.

Cercosopora leaf spot turns the plant’s leaves brown, reducing yield potential and the storability of the crop. Former NDSU Extension plant pathologist Mohamed Khan has called the disease the greatest problem facing sugarbeet growers.

In 2016, cercospora resulted in more than $140 million in sugarbeet crop losses in North Dakota and Minnesota, according to NDSU. Infested fields can easily lose 40% of their yield and about 2 to 3 percentage points of sugar — a loss of millions of pounds of sugar and millions of dollars throughout the growing regions.

That can result in losses of $300 to $500 per acre, said Khan, now NDSU Extension assistant director of agriculture and natural resources.

KWS Saat, parent company of Beta Seed, commercialized the CR+ trait for sugarbeet seed for some farmers in the southern Red River Valley in North Dakota and Minnesota in 2021.

The seed varieties with the trait were welcomed with enthusiasm. All of the farmers who grow sugarbeets for Minn-Dak Farmers Co-op in Wahpeton, North Dakota, planted seed with the CR+ trait in 2023 and half of the growers who raise the crop for American Crystal Sugar Co. in Moorhead, Minnesota, and for Southern Minnesota Beet Sugar Co-op in Renville, Minnesota, planted the variety.

Wyatt’s research focuses on studying the potential for cercospora leaf spot to develop to management practices including planting sugarbeets with the CR+ trait.

“In all of history the history of sugarbeets this is the first good resistance we’ve had," Wyatt said. “Preserving that long term is extremely important.”

He surveyed and sampled sugarbeets with the CR+ trait in sugarbeets grown by Southern Minnesota Beet Sugar and Minn-Dak Farmer Co-op farmers in 2021, '22 and '23, and also surveyed and sampled American Crystal in '22 and '23.

During the research, DNA is extracted from cercospora collected from sugarbeets with the CR+ trait and the isolates are compared to varieties that do not have the trait.

Preliminary results of the research showed that adaptation to the CR+ trait occurred, and Wyatt expects that to become exacerbated, especially if growing season weather conditions turn wet in the future.

He and other sugarbeet researchers and agronomists stress the need for vigilance and the importance of utilizing all the management tools available, including fungicides for cercospora leaf spot disease control.

“If you were just to use CR+ as the only management tool, on an annual basis, it will spur pathogens to go around that resistance,” Wayatt said.

Instead, farmers should use integrated pest management practices to reduce the development of resistance.

That means that besides planting the CR+ sugarbeet trait, farmers should continue to use fungicides to control cercospora leaf spot.

It is essential that all farmers get on board with a program made up of both planting sugarbeets with the CR+ trait and spraying for fungicides, because the cercospora leaf spot pathogen is spread by water splashing on leaves and by wind, so it can spread to other sugarbeet growers’ fields.

“With the wind, it seems to disperse the longest distance,” Wyatt said.

He is researching to determine why there is similar genetic makeup of cercospora leaf spot disease in the northern Red River Valley and the southern Red River Valley.

“It could be as crazy as birds landing in fields, picking up spores and landing on other fields,” Wyatt said.

He and sugarbeet cooperative agronomists are stressing to farmers that it only takes a few growers to rely solely on planting sugarbeets with the CR+ trait for the disease to spread and result in widespread damage.

“If you’re not helping out by applying fungicides, you’re really allowing that adaptation to occur,” Wyatt said. “If you continue to expose nature to one thing, nature adapts around it.”

If an adaption to cercospora leaf spot results in resistance to sugarbeets with the CR+ trait, it will jeopardize the efficacy of the varieties, Wyatt said.

“If you don’t protect this, it will go away,” he said.

The sugarbeet industry would be hard hit by the loss of sugarbeet seed with the CR+ trait because replacing it is a lengthy process.

While breeding of small grains, such as wheat and barley, can be put on a fast track and several generations of the crops can be bred within a year, it takes one and a half years to breed sugarbeets. That means that there is about a 10-year period between finding the resistance and releasing a variety with the trait, Wyatt said.

“It’s extremely important to protect these varieties,” he said.