July 18, 2008

In the last few weeks, instances of black shank on burley tobacco have been on the rise. Those numbers could increase as the summer wears on, said Kenny Seebold, extension plant pathologist with the University of Kentucky College of Agriculture.

Black shank is the biggest annual threat to tobacco yields in the state. It is caused by a pathogen that produces spores, which are attracted to the roots and stalk of the plant. The pathogen invades the roots and stem of tobacco, producing dark brown to black lesions. Infected plants will become yellow and wilt before dying. Since there is no way to completely eliminate the black shank pathogen once it is in a field, producers must carefully manage the field each year to prevent future outbreaks.

The black shank pathogen is most active during warm, moist weather. Wet weather this spring provided ideal conditions for its spread, and the wet conditions may continue.

"We should have another month of good rain. Then it should dry off around the middle of August, and we should return to normal rainfall amounts," said Keys Arnold, staff meteorologist with the UK College of Agriculture.

            Dry weather exacerbates black shank damage. While it doesn't look like there will be a drought this year, its symptoms could become more noticeable if the weather does dry out some in August.

Since the disease is an annual problem for tobacco producers, many varieties were developed to provide moderate to above-average protection from the disease. While many producers use these resistant varieties, black shank still occurs in some plants.

"Even though it is resistant, it is not immune," Seebold said, adding that the disease could be a result of a lack of crop rotation and poor sanitation methods.

Producers who are unsure whether they have black shank should take a soil sample to their county's extension office. Agents there will send it to the UK Plant Disease Diagnostic Laboratory for evaluation. Black shank can often be confused with Fusarium wilt, which doesn't produce the yield loss that can result from black shank.

Producers who find black shank on their plants should consider the plant variety, maturity level and extent of damage before deciding to apply a fungicide.  The fungicide, Ridomil Gold, works best when applied to varieties that are at least moderately resistant. It should be applied directly to the soil, which may be hard to do for fully developed tobacco.

            "The later you wait, the less successful you are going to be," Seebold said.

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