April 4, 2007 | By: Terri McLean
LEXINGTON, KY.

When using fungicides to manage the “big three” tobacco diseases in Kentucky, proper timing of applications will give growers the most bang for their buck.

That, according to Kenny Seebold, plant pathologist with the UK Cooperative Extension Service, is the “take-home” message from the University of Kentucky’s 2006 field trials evaluating fungicide products for management of the state’s top tobacco diseases – black shank, target spot and blue mold.

“Timing is everything,” Seebold said. “Fungicides must be in place before plant pathogens arrive or, at the latest, when the first symptoms of disease appear.” 

Consider black shank, one of the most destructive of all tobacco diseases and the most important to Kentucky growers in terms of cumulative losses. Black shank is caused by a fungus that lives in the soil, and once soil becomes infested the disease must be managed each year on a continuing basis. “It doesn’t go away,” Seebold said.

Black shank trials conducted in Breckinridge, Bullitt and Clark counties demonstrated that an application of fungicide with the chemical mefenoxam (Ridomil Gold EC, 1 to 2 pints per acre) prior to planting is needed in cases where the disease has been severe, Seebold said. Supplemental applications at cultivation and layby also might be warranted. 

In addition, the black shank trials showed that planting tobacco varieties with the highest level of resistance to the disease is important to a management program. The KT 204 variety, which Seebold called the “gold standard” for black shank control, once again proved to offer the highest resistance.

“With low host resistance to black shank … losses were around 70 percent even when Ridomil Gold was applied,” he said. “If you don’t have enough resistance, there’s no amount of chemical you can pour on the ground – and that you can afford – that will get you where you want to be.”

Trials conducted on target spot, a fungal disease that is fast becoming a problem for tobacco growers, also illustrate the importance of the timing of fungicide applications, Seebold said. As with black shank, target spot is caused by a soil-borne fungus that can persist once a field is infested and requires careful management to reduce its survival in soil.

“If target spot is a problem, I think they need to consider being on a spray schedule aimed at controlling target spot, which means getting an application on before the row middle is closed – when the crop gets big enough so that the leaves overlap in the center of the row,” he said. 

More specifically, Seebold said the target spot trials conducted in Jackson and Owsley counties indicate that an early season application of the azoxystrobin (Quadris; 8 fluid ounces per acre) beginning when the plants are between 24 and 36 inches tall and a mid- to late-season application between topping and harvest is adequate for suppression of target spot. A 12-fluid-ounce application may be necessary where disease pressure is higher.

Quadris was labeled in 2006 for use on tobacco. Of the products registered in Kentucky for disease management, it is the only one that includes target spot on its label. Early applications prevent buildup of the target spot pathogen, suppressing disease later in the season, Seebold said. 

“The beauty of Quadris is that it allows us to target more than one disease with a fungicide spray program,” Seebold said. “Quadris offers protection against two other diseases, including blue mold and frogeye.”

Blue mold trials conducted in Jessamine and Jackson counties confirmed that preventive applications of fungicide are most effective in management of the disease, Seebold said. 

“Begin fungicide applications for blue mold control when the disease is forecasted to threaten your area or has been found nearby,” said Seebold.

Of the three major tobacco diseases, blue mold often gets the most attention because it immediately and drastically affects tobacco plants. It is also capable of spreading from one farm to the next, Seebold said. However, unlike the other two major tobacco diseases, it does not survive the winter and, thus, is not a continuous threat.

“When it’s here, nothing could be more important,” he said. “Some years, we’ll have outbreaks of blue mold and some years we won’t. But if you look at black shank, it’s bad every year. Some years it’s worse than others and so the cumulative losses to growers are way worse.” 

The recommendations Seebold and his extension colleagues make as a result of the field trials are often considered old news by many tobacco growers. But reiterating the importance of such things as timely fungicide applications gives those growers the confidence that they’re doing the right thing, Seebold said.

“We may be repeating what they already know, but it’s always valuable information. A lot of times what we’re doing with these trials are almost demonstrations. … There’s value in that,” he said.

Other recommendations for fungicide application include using a volume that gives the best coverage of plants and calibrating the sprayer for accurate delivery.

“Timely and accurate application of fungicides is essential for best performance,” Seebold said.

For more information about the fungicide field trials conducted in 2006, visithttp://www.uky.edu/Ag/kpn/kyblue/kyblue.htm. Or contact your county extension office.

Contact: 

Kenny Seebold, 859-257-7445, ext. 80721