Cercospora threat remains

FARGO, N.D. -- With leaf canopies closing on a potentially record sugar beet crop, farmers have a potentially large and valuable crop to protect, and should be scouting actively for Cercospora leaf spot disease, says Mohamed Khan, North Dakota State University extension beet specialist.

Cercospora is the biggest leaf disease problem for beets, cutting tonnage and extractable sugar content. If first control is late, control will be difficult all season. Khan recommends against cutting fungicide rates.

Al Cattanach, an agronomist with American Crystal Sugar Co. in Moorhead, Minn., says that as of July 14 he's only heard of two incidents of Cercospora reports in the company's five factory districts. One of Crystal's staff found a "one-plant, one-field" situation south of Moorhead, and a Centrol consult had a similar report from the Glyndon, Minn., area -- both in the first week of July.

Cattanach says periods of dry and cool conditions have lessened the incidence of the disease, but the co-op is advising shareholder-growers to keep vigilant.

"We'll find Cercospora in our beet growing area every year,' he says. "It's always here, but it varies how early it comes and how severe. That's dictated a great deal by weather."

In the NDSU weekly Crop & Pest Report in July, Khan notes that past fungicide control has been good so the general inoculums level is low, but the threat returns when day temperatures in the 80- to 90-degree area. Humid condition favor for the disease, Khan says.

Be prepared

Growers should be prepared for fungicide applications earlier than previous years and "budget for multiple applications, should this become necessary," Khan says.

Cattanach says he expects growers may have to spray two or three times this year, compared with last year, when conditions were cooler and the co-op averaged 1.5 sprays last year.

Cattanach says after the first application, the beet rower can wait for a couple of weeks to thoroughly monitor again for a potential subsequent application. As of July 14, Cattanach says he knew of no one who had sprayed a first time.

Khan suggests growers look for circular spots about one-eighth inch in diameter with ash gray centers and dark brown or reddish-purple borders.

"Only one application of a Topsin M in combination with a protectant fungicide (triphenyltin hydroxide) should be used in Hillsboro, East Grand Forks, Crookston and Drayton factory districts," Khan advises. He adds growers shouldn't use the fungicides from the same class "back-to-back" and avoid using a particular class as a "stand-alone" where there is known resistance to that chemistry. Spray pressure of 100 psi and higher offers better control.

These "fungicides are labeled and the most widely used on sugar beet for Cercospora leaf spot control with re-entry interval (REI) and post harvest interval (PHI)," Khan says, listing these Product REI (hr) PHI (day):

- SuperTin 80WP and 4L 48 7.

- AgriTin 80WP and 4L 48 7.

- Headline 4 7.

- Gem 12 21.

- Inspire XT 12 21.

- Proline 48 7.

- Eminent 48 14.

- Topsin WSB and 4.5F 12 21.

"Please note that manufactures of glyphosate do not recommend mixing glyphosate with fungicides," Khan says., "Growers make these applications at their own risk."

NDSU trials indicated that some fungicides mixed with glyphsate and AMS at labeled rates "did not result in any phytotoxicity, and weed and leaf spot control were effective."