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Fungus Control With a New Mode of Action

June 29, 2016 |

By Julia Ellis

When weather conditions are ideal for fungus growth, tobacco farmers now have one more tool to protect their plants.
In early 2015, the Environmental Protection Agency registered Presidio fungicide for tobacco use. The advent of Presidio means farmers can use a suite of fungicides that attack pathogens like black shank and blue mold in different ways.

Suite Success
Valent U.S.A. Corp., the makers of Presidio, claim their product is an excellent fungicide with good residual activity. “Meaning, it hangs around long after you apply it,” says Kenneth Seebold, product development manager for Valent. Seebold explains that Presidio’s active ingredient, fluopicolide, has a “unique mode of action” in the way it attacks the pathogen: Presidio breaks down the structural proteins of pathogens’ cells so they can’t multiply.
Fungicides are grouped by the way they attack the pathogen, and Presidio is the only one that works this way. Using a variety of fungicides that have different modes of action can prevent or slow down the development of resistance to the products. In order to impede resistance, it’s important to use fungicides only when necessary, and Presidio in particular should be used only once per season, says Seebold.

Application Tips
For black shank prevention, Presidio should be applied to the soil beneath the lower leaves at first cultivation or layby. No sequential applications of Presidio should be made to the soil or foliage. For blue mold treatment, Presidio should be tank-mixed with another fungicide with a different mode of action and can be sprayed on the leaves more than once, but multiple sprays should not exceed 12 ounces total for foliar and soil applications,
emphasizes Seebold.
Other fungicides with different modes of action include Ridomil Gold or Orondis Gold. Using the
fungicides in conjunction allows multiple applications without exceeding the labeled application limit of any
one chemical.
Charles Johnson, Extension plant pathologist at the Virginia Tech Southern Piedmont Agricultural Research and Extension Center, agrees that a suite of chemicals is more effective than a single one. But he warns that the strategy also can be pricey and confusing. “Using either Presidio or Orondis Gold with Ridomil is more effective than treating with Ridomil Gold on its own,” he says. Johnson acknowledges that growers have to decide which combination of fungicides works best for their situation.
Northeastern tobacco farmers are more frequently plagued by blue mold than are southern growers, according to James LaMondia, chief scientist and plant pathologist at the Connecticut Agricultural Experiment Station. LaMondia says Connecticut growers will benefit from Presidio use. “Presidio is a big advantage for us,” he says. “We’ve really had only one fungicide we’ve been using—dimethomorph.”
And dimethomorph, the active ingredient in Forum, may be losing some of its power due to resistance development. Not every fungicide is compatible with the high-end tobacco used to wrap and fill cigars, says LaMondia. Growers of wrapper tobacco must have perfect leaves without lesions or fungicide residue. A single lesion can drop the price of a leaf to a third or a quarter of the value of a perfect leaf. LaMondia reported that quality testing with foliar applications and drenching indicated that Presidio “seemed to be OK.”
Presidio should not be used in transplant water, as growers in Alamance County, North Carolina, discovered in 2015. “Around here last year, Presidio was applied in the transplant water like Ridomil Gold has been done for many years, and there were some growers who had problems with that method,” reported Dwayne Dabbs, North Carolina Cooperative Extension agent, referring to the Alamance County region. “I saw a few fields that showed signs of the leaves hooding, and after about a week or two the plants grew out of it, but it was a great concern to growers. I think it was a concern for the makers of the product, and since then the label has been changed to where you cannot use it in the transplant water.”
Alamance growers using Presidio at last cultivation did not have any problems, according to Dabbs. But some growers, like Hugh Davis of Alamance County, don’t want to take any more chances. Davis doesn’t plan on trying a soil application of Presidio in 2016, saying he’ll use Ridomil as he did in previous years, as well as relying on crop rotation.

Nonchemical Fungus Prevention
Crop rotation is vital to avoiding continuous black shank problems, and giving a field a year between tobacco crops “can provide a minimum benefit and make a big difference,” says Johnson. “The longer a field lays out of tobacco, the more effective it’s going to be in controlling black shank.”
Organic tobacco farmer Jane Iseley of Iseley Farms in Burlington, North Carolina, hasn’t seen black shank in 10 years and attributes its absence to a three-year rotation, with the fields used as pastures for the two years between tobacco crops. Planting outlays with buckwheat and sunflowers helps keep beneficial insects around the tobacco and may also help with drainage. She also tries new varieties after reading the reports.
Experts recommend destroying stalks and roots immediately after harvest to rid fields of fungus. Re-disking and harrowing the soil two weeks later helps kill any remaining roots and dries the soil.
Planting black shank-resistant tobacco varieties is another way to reduce crop loss. Production guides list varieties resistant to various races of black shank. The tobacco varieties grown may determine how many fungicide treatments are used.
But, as Davis notes, “timing is a whole lot of everything” when it comes to rainfall and black shank, with extremely wet weather early on fostering fungal growth followed by dry spells that weaken the plants. “Most of the time if the plant is suffering, that’s when you’ll see it real bad,” says Dabbs. While farmers like Davis and Iseley haven’t seen blue mold more than once since the 1980s, Connecticut farmers have experienced blue mold almost every year in the last 20 years, says LaMondia. He recommends keeping fields well drained, controlling irrigation and plant spacing to decrease the likelihood of blue mold challenges.

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