Managing target spot in tobacco

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By Rick Greenwell

Over the past years, Kentucky’s burley producers have had to contend with some fairly serious outbreaks of target spot on seedlings in float beds.  These outbreaks often delay transplanting and may even provide a source of inoculums for epidemics for target spot in the field.  Recommendations for managing target spot include taking steps to maintain good ventilation, adequate fertility, and application of mancozeb fungicides (such as Dithane, Manzate, or Penncozeb) on a 5-7 day schedule.  Mancozebis not specifically labeled for target spot, but is reasonably suppressive when applied in a preventive manner.  Mancozeb falls short, however, when the disease becomes established prior to application, or when the environment remains favorable for disease for long periods of time.  Clearly, there has been a need for a stronger chemical tool for managing target spot.
Quadris fungicide, used widely on field-grown tobacco, received a Special Local Need, or 24 ( c)  label for control of target spot in float beds in 2009 and 2010 to help fight notably severe epidemics of these diseases.  However, these labels expired each year with no guarantee that they would be reinstated the following year.  This situation has changed.  We now have a very useful fungicide tool for use on tobacco seedlings, and it is in place early enough to be used in a preventive program for controlling target spot.
Effective now through December 31, 2012, growers will be able to make ONE application of Quadris at a rate of 6 fl. Oz/A, which is the equivalent of 0.14 fl. oz. (4 cc) of product per thousand square feet of float bed (roughly 400 trays), applied in a recommended volume of 5 trays), applied in a recommended volume of 5 gallons – enough to get good leaf coverage.  Thus a grower mixer 20 gallons of spray solution would add 16 ml of Quadris, or roughly 3 teaspoons of product.  For best control of disease, we recommend making the Quadris application before symptoms are seen (or when they first appear at the very latest) and at a time when the plants would be the most susceptible to target spot.  Generally, target spot becomes active in Kentucky in late April or early May.  To get ahead of the disease a good strategy would be to apply Quadris after the first or second clipping.  After making the Quadris treatment, producers should switch back to mancozeb until plants are set in the field.  This is critical to stay in compliance with the label and to minimize the risk of resistance developing.
In cases where target spot is severe on float plants, a follow-up application of Quadrismay be needed at about 5-6 weeks after setting to suppress the disease in the field.  Quadris is mostly likely compatible with products like mancozeb or Orthene, but it would be advisable not to mix Quadris and agricultural streptomycin to avoid potential injury.  In terms of resistance to rain and weathering, we can expect Quadris to be fairly rainfast if it can dry for a couple of hours after treatment.
It’s important that growers who use Quadris in who use Quadris in the greenhouse have a copy of the 24 ( c)label in their possession.  It is equally important to remember that the greenhouse application counts toward the seasonal limit (4-5 applications) allowed on tobacco.