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Disease update provided for week of July 22

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By Rick Greenwell

Current Status
After a short break of hot and mostly sunny weather, rain has returned to many parts of Kentucky. Conditions have been very favorable for target spot and frogeye leaf spot, and we’re seeing quite a bit of these on burley around the state. Quadris fungicide, applied at eight fl oz/A, is the recommended material for target spot and frogeye, and gives good control of disease if applied at least once beginning at layby. For growers making their first application of Quadris on tobacco after this stage, and where target spot or frogeye is present, a rate of 10 to 12 fl oz/A is recommended. In these cases and especially if tobacco is nearing topping-time, drop extensions should be used to insure that fungicide reaches the lower, more disease-prone part of the canopy.

Some growers may be considering a follow-up treatment to an earlier application of Quadris to suppress active disease prior to topping. If multiple applications of Quadris are needed, the label requires alternation with a fungicide which has a different mode of action than Quadris. For frogeye and target spot, our only option would be Manzate Pro-Stick or Penncozeb 75DF. Previously, Dithane DF (another mancozeb product) could have been used as well; however, this product appears to have been pulled from all but the turf and ornamental markets. Mancozeb products cannot be used within 30 days of harvest, while Quadris has a 0-day pre-harvest interval. Treating tobacco with Quadris right before harvest is not warranted nor recommended; however, the 0-day pre-harvest interval means that the fungicide can be used after topping if needed.

Blue Mold Update

Currently, blue mold is known to be active in the areas around Lancaster, Pa. and Abingdon, Va. We have not found blue mold in Kentucky as of July 22 and very warm temperatures have helped keep the risk low. At this point in the season, growers should stay alert and monitor closely for disease. Cooler temperatures and changes in weather patterns could move the blue mold pathogen into Kentucky, and growers should be prepared to act if necessary.

Tobacco that has been topped and treated for suckers tends to be much less susceptible to blue mold than tobacco that has not been topped. Crops in the latter stage should be watched carefully in the coming days, and a fungicide application should be made if blue mold is found.      

Quadris is an effective blue mold fungicide, and growers who have made a recent application for target spot or frogeye can expect up to two weeks of protection against the disease. Growers who haven’t treated with Quadris should consider an application to protect against both target spot and frogeye, as well as blue mold. As mentioned earlier, back-to-back sprays of Quadris are prohibited by the label and a fungicide with a different mode of action must be used. Other fungicides that are labeled for suppression of blue mold in Kentucky include Actigard, Forum, Revus, Manzate Pro-Stick, and Penncozeb.  Actigard (0.5 oz/A) is a highly systemic product that induces tobacco plants to become resistant to blue mold; this material needs an activation period of 4-5 days for full induction of resistance and will not do well if disease is active already. Forum SC (2-8 fl oz/A) has worked well in our research trials against blue mold; however, Forum must be tank-mixed with a product that has a different mode of action as well. Manzate ProStick or Penncozeb are recommended for this purpose. The newest product on the market for control of blue mold is Revus (8 fl oz/A). Revus is very effective against blue mold, but has the same mode of action as Forum.  These products cannot be tank-mixed, but can be used in alternation.  Actigard, Forum, and Revus will not control frogeye leaf spot or target spot; Quadris is the only product labeled for these two diseases on tobacco.

We will continue to monitor for blue mold and will send alerts if the disease is found.  Please contact me if you find or suspect blue mold in your area.  For recommendations on the control of tobacco diseases, please consult past issues of the Kentucky Pest News, or the Kentucky-Tennessee Tobacco Production Guide (ID-160), available at www.ca.uky.edu/agc/pubs/id/id150/id160.pdf.