Leaf diseases complicate tobacco production

When Joey Scott of Kenly, N.C., heard that blue mold had been found on tobacco in his neighborhood, he knew he had to take action.

“We decided to spray twice, first with Quadris and then a week later with Forum and Dithane,” he says. “The control was good.”

But in hindsight, he thinks he should have put out the Forum/Dithane first and then come back with the Quadris, which is preventative but not curative.

“There weren’t any signs of blue mold at the time of the first spray,” he says. That made him think he didn’t need to start with a curative.

“But a week after we sprayed, we started to see lesions,” he says.

The first discovery of blue mold this year had been found a few days earlier in a field about two miles away from Scott’s farm.

There is a non-chemical management practice that can help reduce loss to blue mold, he learned. It is early topping.

“When you begin to top, your tobacco goes from the reproductive stage to more of a vegetative stage,” says Scott. “There is a chemical change that is not conducive to the spread of blue mold.”

He started topping his tobacco in mid-June, ahead of schedule for him, and it helped.

But don’t expect miracles from early topping, says Mina Mila, North Carolina Extension tobacco specialist. It can lead to some yield loss, and the blue mold control it provides may not offset it. “We don’t recommend disrupting your management plan to top early if you don’t have blue mold yet,” she says.

But once blue mold arrives in your area, early topping becomes a more attractive option. “This disease just does not attack the tobacco plant as effectively after it has been topped,” she says.

The county Extension tobacco agents in the Wilson area got out in front of this infection, says Mila, and she suggested control strategies for farmers in the infected areas.

“That was important to keep damage from this epidemic low,” says Mila.

“Joey Scott’s tobacco agent, Norman Harrell of Wilson, made frequent visits, including some on Sundays, to make sure the disease was kept in check.”

The North Carolina growers got a break from the weather. Active blue mold sporulation continued from initial discovery on May 28 until Friday, June 10. “But the temperature got up to 97 degrees over that weekend,” said Scott. “By Monday (June 14), I could find no more active spores.”

There were later incidences in Duplin, Lee and Moore counties in North Carolina and in Pennsylvania and Connecticut.

Continued success will depend on good recognition of blue mold in the field. It can be confused with the fungal disease target spot.

“It appears first on the top of the leaf as little yellow dots about the size of a dime,” says Scott. “Then a couple of days after that, you can see a blue-gray mold if you look under the leaf.”

A preventative spray of Quadris (azoxystrobin) at 8 ounces per acre is recommended where blue mold is expected. Complete coverage of the plant is important, so apply Quadris under high pressure.

Once you have blue mold, consider using Forum (dimethomorph), mixed with another fungicide such as Dithane (mancozeb). Complete coverage, including the underside of the leaf, is crucial for good control. So use a high-pressure sprayer with drop nozzles between rows, and hollow-cone nozzles. The materials need to dry on the leaf surface to be rainfast. Do not use surfactants.

Another control option is Actigard, but it must be applied five days prior to the onset of infection to achieve control, says Darrell Hensley Tennessee Extension plant pathologist.

However, Actigard offers some advantages over other control chemicals. “It may be applied with flat fan nozzles, and coverage is not as critical as with protectant-type fungicides,” he says.

Actigard should be applied to burley tobacco no smaller than 18 inches in height. “Use it only on actively growing plants that are not experiencing a stressful growing environment due to drought or herbicide injury,” says Hensley.

There is another disease that could enter the picture now that growers have applied Quadris for blue mold. If target spot — which like blue mold attacks the tobacco leaf — appears later in the season, some farmers could find themselves in a fix, because Quadris is essentially the only chemical available to control target spot.

“This is a potential problem,” says Mila. “You can use only 32 ounces of it per acre per season. Some farmers may have exhausted the 32-ounce limit in treating blue mold.”

Again, there is a non-chemical tactic that can help. If you are in this predicament, try taking some of the pressure off chemical control by removing your lower leaves early.

“Once you prime your lower leaves, target spot is less of a problem,” says Mila.

It also helps to ensure adequate nitrogen to the plant.

Mila says if blue mold doesn’t recur, the May/June outbreak in the Wilson area may not result in any significant loss.

“There are a few fields with economic damage,” she says. “But most farms look like they will come out of this okay.”

So far, most of the blue mold in North Carolina appeared in flue-cured tobacco, says Mila. “An outbreak like this is unusual for eastern North Carolina. The last one was 10 years ago.”

Blue mold is spread on wind currents and usually does well only in cool, wet conditions. For these reasons, its spread is to a certain degree predictable. The North American Plant Disease Forecast Center issues blue mold forecasts from March through August. They can be seen on the World Wide Web at www.ces.ncsu.edu/depts/pp/bluemold/ or by calling 1-800-662-7301.

But nothing can replace close observation of your tobacco.

“You can miss early infections by ‘scouting from the truck,’ as you will not see the lesions,” says Wilson tobacco agent Harrell.

“With blue mold, the best control measure is always regular scouting,” says Mila.

e-mail: [email protected]

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