A minor blue mold outbreak in Shelby, Henry and Oldham counties is a late-season reminder to Kentucky tobacco farmers not to let their guards down just yet.
Kenny Seebold, plant pathologist in the University of Kentucky College of Agriculture, said it is unclear how much of the disease is out there. The confirmed sightings have occurred in a fairly tight-knit area around the adjoining borders of the three counties.
Sporulating lesions were present in the Oldham County sample, as well as in an earlier sample taken near Chestnut Grove in Shelby County. The Shelby County plants showed lesions that appeared to be seven to 10 days old.
He thinks that last week's cooler temperatures along with a few showers were prime conditions for blue mold development and spread.
"The disease could have been dormant over the last month and became active when a cold front passed through Kentucky about 10 days ago," he said. "We're heading into a warming period — clear, sunny weather and warm temperatures in the daytime — which is unfavorable for blue mold."
Such weather will significantly slow down not only the speed at which the disease develops but how far it spreads. Blue mold is caused by Peronospora tabacina, an airborne fungus that prefers cool, humid conditions.
Seebold assesses the risk level as low to moderate.
"It's still cool enough at night that in places where it's active, it's likely to stay active. It's just that it won't pick up and move very far, as long as it's clear and sunny during the daytime," he said.
Also working in farmers' favor is the timing of the outbreak. The latest figures from the U.S. Department of Agriculture indicate that 11 percent of Kentucky's tobacco has been harvested and 64 percent topped. Topped tobacco and plants treated for suckers are less susceptible to the fungus, which leaves a grayish-blue mildew on the undersides of leaves.
"The pathogen needs an actively growing host to infect and produce spores," Seebold said. "Once farmers top the tobacco and spray to control suckers, it pretty much shuts the plant down physiologically. You get some leaf maturity, but you don't get the active growth. So those plants tend to be a lot less susceptible."
Due to weather patterns, untopped tobacco southeast of the infected counties could be most affected by the outbreak. Seebold said farmers should base their decisions to apply fungicides on the age of the crop and whether a particular field lies in the path of spores.
"If they've topped their crop and applied their sucker control material, there's really not much they need to do," he said. "But if you have susceptible crop in the field, you need to be looking for it.... If you're in the right kind of environment, you can still get some pretty good spread inside a field, and it may be worth considering applying a fungicide if the crop is far enough away from being topped and cut."
The appearance of a lot of target spot, another fungal disease, is also a reason for applying a fungicide. Target spot causes major leaf damage if left uncontrolled. Seebold called target spot an opportunistic disease, which will often settle into the damaged lower leaves of plants that have been compromised with blue mold.
He said observers have seen a big jump in target spot activity over the past two weeks, particularly in low-lying fields or those that have been planted in tobacco for more than two years.
He recommends Quadris at a rate of eight or more ounces per acre. With good coverage, this application should give a couple of weeks' protection against blue mold and target spot.
For more information on controlling fungal diseases in tobacco, contact your local Cooperative Extension office or consult the Kentucky Tobacco Disease Information Web site.