Act Now to Prevent Target Spot
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Since the 1980s, target spot has been a recurring disease problem in flue-cured tobacco. In 1989, many Pitt County fields were severely affected by this disease and many county farmers realized reduced profits that year. Target spot caused the greatest losses of any tobacco disease in 1995.
Target spot is caused by a fungus called Rhizoctonia solani and this fungus is present in every Pitt County field. This disease is generally first observed in June when the weather is warm and humid.
Symptoms of Target Spot
With target spot, a series of necrotic rings form around the primary lesion, giving the characteristic “target” symptom. When the spot grows large enough, the necrotic tissue in the center will split or fall out, creating a large hole in the leaf. Fortunately, not all primary lesions grow in size, however, the ones that do have potential to cause a serious amount of tissue loss. Plants low in nitrogen often display larger necrotic lesions and broader concentric rings compared to plants with sufficient levels.
Source for the Disease
Spores from target spot are released from the soil surface onto mature leaves closest to the ground. This causes the first symptoms to appear on the surface of foliage closest to the soil level. As the disease progresses, it moves up the tobacco plant.
If growing conditions are favorable for disease development, many cycles of infectious spores can occur in one season. This disease pathogen reproduces on the underside of tobacco leaves.
Target spot can spread quickly and has the ability to severely reduce yield when it is not adequately managed. Since the fungicides which are labeled for use do not stop the spread of the disease, growers who suffer from this disease every year may want to consider a preventative approach to target spot management. Azoxystrobin (e.g. Quadris) is labeled for use in the field and greenhouse. Mancozeb (e.g. Manzate Pro Stik) has a 24c label for use in tobacco in North Carolina, and may be moderately effective for reducing target spot.
Dr. Matthew Vann, NC State Tobacco Extension Specialist, has outlined a preventative approach to reducing target spot. These steps are as follows:
1) Use a post application of Quadris over-the-top 4 weeks after TP but before layby. Apply 8 fluid ounces in a volume of 25 gallons of water. Apply using one nozzle directly over the row.
2) Use Manzate Pro Stik as a post application at the rate of 1.5 pounds per acre 2 weeks later. At this stage of growth, it is recommended to use a 3-nozzle arrangement for good coverage in 25 gallons of volume per acre.
3) Use a second application of Quadris at a rate of 9 ounces per acre if there are signs of target spot development or if this disease is an annual problem. Use drop nozzles with a volume of 40-50 gallons per acre for good coverage. Note: There is a 21-day pre-harvest interval with Quadris.
Adequate coverage of the foliage is essential with any attempt made to reduce target spot. Since target spot is a disease that “comes out of the ground,” any treatment made to control this disease needs to reach the bottom of the plant.
As tobacco increases in size, it is important to provide an adequate spray volume. Poor performance of fungicides is believed to be linked to inadequate application methods.
Taking Corrective Action
Nitrogen-deficient plants are more susceptible to target spot; therefore, it is critical to maintain the proper nutrient level to help suppress the pathogen’s devastating effects. If a field experiences high rainfall amounts, it is important to adjust the total nitrogen amount to reduce losses due to target spot.
Practices that increase air circulation and reduce leaf wetness are helpful to avoiding disease formation. Harvesting or removing the lower leaves of the plant after the disease begins to develop is also a recommended management practice to reduce target spot.