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Research agronomist Mike Metzger of Minn-Dak Farmers Cooperative of Wahpeton, N.D., demonstrates how a field microscope is placed on a sugar beet leaf lesion to study it for evidence of cercospora leafspot disease. Photo taken Aug. 4, 2017, at Rothsay, Minn. (Forum News Service/Agweek/Mikkel Pates)

Beet farmers head off cercospora

ROTHSAY, Minn. — After cercospora leaf spot robbed sugar beet yields last year, farmers this year have largely kept ahead of it.

Cercospora is the most devastating foliar leaf disease that the region's sugar beets encounter. Farmers deal with it every year, losing sugar, root quality and storability, as well as processing efficiency.

In the 2016 crop, environmental factors lined up to create a "massive cercospora pressure, with beets that should have looked green like 'a field of tobacco,'" says Mike Metzger, research agronomist for Minn-Dak Farmers Cooperative in Wahpeton, N.D. "Coupled with that we had the strobulurin fungicides (brand names include Headline, Priaxor and Gem) fail on us because the disease has become resistant to them," Metzger says, adding, that Minn-Dak no longer recommends the products because "they are essentially non-effective."

Beet growers this year have shifted to Super Tins (triphenyl tin hydroxides) and the triozoles (Eminent, Spire, Proline) "to protect those two chemistries," he says. Minn-Dak recommends tank mix partners with every one of them.

"We have to," Metzger says. "And so we alternate with tank mix partners throughout the season."

Minn-Dak introduced a five-time spraying regimen in 2017 to counter the disease, while Southern Minnesota Beet Sugar Cooperative at Renville, Minn., has a seven-spray program. American Crystal Sugar Co. in Moorhead, Minn., started with a four- to six-spray regimen, but favorable weather allowed them to pull back to three- to five-spray programs this year, with most spraying four times.

Cory Skauge, an agriculturist with American Crystal Sugar Co. in the company's Moorhead factory district, says his growers have been doing a good job of keeping ahead of cercospora this year.

Despite the cercospora impacts last year, both co-ops had large yields anyway, and both have cut back acres in 2017 to right-size their crops with factory capacities. Minn-Dak cut acreage by 17 percent after having to leave 12 percent of last year's crop in the field and processing beets into June.

Using this year's cercospora-fighting recipe is like using "Triple A ballplayers in a major league baseball game," Metzger says, looking at a beet field on the Haugrud Farms Inc., near Rothsay, Minn., on Aug. 4. "If you coach them right, you can win the game. But the other team is going to get a few runs. That's why we still see some lesions out here, but it's well below the economic threshold."

Metzger and field agronomist Cody Wahlstrom explained the finer points of verifying to Lance Johnson of Haugrud Farms Inc. It's not always easy to differentiate cercospora from other bacterial diseases. Cercospora is a "necrotroph," meaning that it is a parasitic organism that produces a toxin that kills living cells of the host and then feeds on the dead matter, Metzger says.

Wahlstrom and Johnson held up the leaves to the sun to see a tell-tale "red halo," which indicates the toxin at work. The agronomists gave Johnson tips on how to use a relatively inexpensive hand-held microscope to look at the lesions.

"You're actually looking for little white spikes up there — that's the mycelium," Metzger says, referring to the vegetative part of the fungus. "At the end of those are little round dots that look like little peppercorns. That's the little cercospora spores, which, with any kind of wind, or rain or brushing again the plant, will move within the same leaf, from leaf-to-leaf and even plant to plant."