U of I researchers target sugar beet disease

By John O’Connell

University of Idaho

PARMA – University of Idaho researchers have promising leads on management practices and new pesticide options to help the state’s sugar beet farmers get a handle on a fungal disease that’s posing an increasing threat in their fields. 

Cercospora beticola thrives in moist, warm conditions and creates necrotic spots on sugar beet leaves. Plants expend their energy regrowing dead or damaged leaves at the expense of sugar production, resulting in yield losses of up to 40% in susceptible cultivars.

James Woodhall, an associate professor of plant pathology based at the U of I Parma Research and Extension Center, and his graduate student, Hayden Woods, obtained an $18,000 grant through the U.S. Department of Agriculture’s IR-4 Project to evaluate seven different fungicide programs for controlling the disease.

IR-4 develops data for the registration of safe and effective pest management solutions for specialty crops with the U.S. Environmental Protection Agency.

Amalgamated Sugar Co. is conducting separate trials assessing the same fungicide programs.

“This is a sugar beet disease that is gaining importance in Idaho. It just seems to be getting more and more severe,” Woodhall said. “It was first found in Idaho in the 1960s, but it’s slowly getting worse.”

Woodhall believes a combination of factors have contributed to mounting grower headaches from Cercospora beticola.

Changes in irrigation likely play a role, as most farmers have switched from in-furrow irrigation to overhead sprinklers, which moisten leaves and create favorable conditions for spores.

He also suspects the disease is entering the state on growers’ sugar beet seed and is likely over-wintering in infected sugar beet tissue in fields.

Large Cercospora spores don’t travel far but can take hold in adjacent fields and spread slowly from one field to the next.

Perhaps the greatest challenge growers face in managing Cercospora is that it quickly develops resistance to pesticides. Woodhall and Woods have sought to identify new modes of action to include in pesticide programs to avoid the onset of resistance to commonly used products.

“The long-term approach is we need to have resistant varieties,” Woodhall said. “Our near-term approach is we need cultural management and we need additional chemical management options.”

Woodhall and Woods enjoyed good results with a treatment regime that included an application of a fungicide that’s already labeled for sugar beets but not widely used, containing the active ingredient thiophanate-methyl.

The addition of that product contributed to a 67% reduction in disease pressure, compared with a 35% reduction resulting from a comparable program that didn’t include thiophanate-methyl.

They also found two fungicides that aren’t currently labeled for sugar beets that provided strong control against Cercospora beticola.

When applied four times throughout the season, one product reduced disease pressure by 44%. Four applications of the other promising product reduced pressure by 31%.

Furthermore, a promising new biological product reduced disease pressure by 40%.

It would likely take three to four years to get special-needs labels approved for any of the products that aren’t currently labeled for sugar beets.

“These are screening trials just to see if it works and how it performs. Once we get all of this data back, if it looks good it will be tested for crop residues,” said Ronda Hirnyck, UI Extension pesticide specialist and principal investigator for U of I’s IR4 Project. “We like to have products we know perform in the field in the real world before we spend money on residue trials.”

Through their research, Woodhall and Woods also found that irrigation methods play an important role in disease pressure. To investigate this, they used a large plot trial over a whole field as opposed to the small plot trials typically done in field work.

The work on the large plots was more representative and showed the potential difference irrigation type can have on disease development. Where in-furrow irrigation was used, Cercospora pressure was reduced by 35% compared with plots irrigated with overhead sprinklers. 

Tilling soil and burying debris from the previous crop can reduce soil-borne Cercospora inoculum.

Woodhall and Woods are also working to develop a new diagnostic laboratory test for Cercospora in soil.

They have spore traps set out near an infected field to capture Cercospora beticola spores, which should help them establish a primer for a soil test. Currently, they must rely on plant tissue or culture to accurately test for the pathogen.

“We’re starting to get more recognition of the seriousness of this disease – 2022 and 2023, those are the worst years we’ve had in the state,” Woods said.