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Blue mold finally moved out of Pennsylvania, and fortunately for Kentucky growers, it went north. On July 21, a case of the disease was reported in an eight-acre field of tobacco in Massachusetts.
Apparently, this was a very mild outbreak that affected a handful of plants in the planting. Current information from the North American Plant Disease Forecast Center at NC State indicates that the threat to Kentucky and surrounding states from blue mold remains extremely low. It’s still a good idea to keep watch, though, and be prepared to act if blue mold threatens in the coming days.
It does seem that both target spot and frogeye leaf spot are on the increase, however, based on what I’m seeing and hearing.
As was mentioned in a previous article (KPN No. 1276), Quadris fungicide, applied at 8 fl oz/A, is the recommended material for target spot and gives good control of disease if applied at least once beginning at layby. A second application could be needed at topping to help with a late-season flush of disease. Timing Quadris applications for frogeye control should be the same as for target spot unless disease pressure is heavy before layby.
In these cases, treat with Quadris as soon as possible. A follow-up treatment may be necessary later in the season if disease is active, particularly around topping time. If multiple applications of Quadris are needed, the label requires alternation with a fungicide, which has a different mode of action from Quadris. For frogeye and target spot, our only options would be Manzate Pro-Stick, Dithane DF, or Penncozeb (mancozeb fungicides). So a grower applying Quadris at layby could come back with mancozeb 2-3 weeks later, and then treat with Quadris either before or after layby to stay compliant with the label.
Given the recent spell of very hot and humid weather, it’s important to remember that Quadris can cause injury to tobacco, and the potential for damage seems to increase at higher temperatures and higher application rates. To reduce the potential for phytotoxicity, growers should try to treat earlier or later in the day to avoid very high temperatures and full sunlight.
As a reminder, never mix Quadris with foliar fertilizers, sucker control materials, surfactants, liquid Dipel, or pesticides formulated as emulsifiable concentrates (EC) since serious leaf burn can occur. Other materials, such as Warrior and Orthene, are used routinely with Quadris, but have caused injury in a small number of cases; growers should use caution with these materials as tank-mix partners and follow the guidelines mentioned earlier for spraying in hot weather.
For recommendations on the control of tobacco diseases, consult past issues of the Kentucky Pest News, or the Kentucky-Tennessee Tobacco Production Guide (ID-160), available at http://www.ca.uky.edu/agc/pubs/id/id160/id160.pdf