After a slow start, tobacco transplant production is now underway in Kentucky and disease issues are beginning to crop up. Temperatures have begun to rise slowly, creating conditions favorable for Rhizoctonia damping-off. A few cases have been reported so far, and seeing this disease at this point in the production cycle is an indicator of possible problems with target spot in the coming weeks. Taking steps early in the transplant season can help reduce the likelihood of losses to these diseases before plants go to the field. The following are suggested practices for managing Rhizoctonia damping-off and target spot effectively. Detailed information about these diseases can be found in the fact sheet PPFS-AG-T-02, Managing Target Spot and Rhizoctonia Damping-off in the Float System, available online at www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/ppfsagt2.pdf.
Infested soil or Styrofoam trays are the most common sources of R. solani in transplant production. Sclerotia and dormant hyphae associated with organic debris are the principal resting structures of R. solani. These can be found easily on the surfaces of infested trays and in cracks and crevices in older Styrofoam trays. Infested trays thus become a source of inoculum in subsequent years if not sanitized properly or replaced. Using new trays for each crop of transplants will all but eliminate the risk of carrying over inoculum from previous transplant cycles, but this option can be expensive and may create issues with disposal of old trays. Used trays should be sanitized properly (refer to ID-160, 2013-2014 Kentucky-Tennessee Tobacco Production Guide, for more information) to reduce carryover of inoculum.
Minimizing leaf and stem wetness should be a priority, and providing adequate ventilation to plants will help keep plants as dry as possible. Properly-timed fungicide applications can be effective in controlling damping-off. We get reasonable suppression with Manzate Pro-Stick, a protectant fungicide that contains the active ingredient mancozeb. The application rate of Manzate is 0.5 lb/100 gallons of finished spray solution (or 1 level teaspoon per gallon) once plants have reached the size of a dime. Use 3 to 5 gallons of the fungicide solution per 1000 square feet, applied as a fine spray (to ensure good coverage) on younger plants; and increase spray volume to 6 to 12 gallons on older plants. Begin applications before symptoms (Figures 1 and 2) develop, or immediately after the first symptoms are observed at the latest, and continue on a 5 to 7 day schedule until plants are ready to go to the field.
As with damping-off, sanitation and good growing practices are the best defense against target spot. Because nitrogen deficiency can increase the susceptibility of tobacco seedlings to the target spot pathogen, maintain nitrogen within the recommended range of 75 to 125 ppm. This practice won’t eliminate the risk to target spot, but it will reduce the likelihood of a severe outbreak. Mancozeb fungicides, as described for damping-off, should be applied when conditions favor disease. These products work reasonably well when used in conjunction with sound management practices. A better option for target spot in the float bed is Quadris fungicide, which received a “Special Local Need” or ‘24 (c)’ label for control of target spot in float beds through December 31, 2018. Under the provisions of this label, growers can make ONE application of Quadris at a rate of 6 fl oz/A, which is the equivalent of 0.14 fl oz (4 cc, or roughly 1 tsp) of product per thousand square feet of float bed (roughly 400 trays) applied in a recommended volume of 5 gallons. This should be adequate to get good leaf coverage and rundown of fungicide solution onto stems. For best control of disease, we recommend making the Quadris application before symptoms (Figure 3) 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. To get ahead of the disease, a good strategy would be to apply Quadris after the 1st or 2nd clipping. After making the Quadris treatment, producers should switch back to mancozeb until plants are set in the field. It is also important to note that the float-bed application counts towards the seasonal limit for Quadris in the field. Following the manufacturer’s use guidelines for Quadris is critical to stay in compliance with the label, and to minimize the risk of the target spot pathogen developing resistance to this important fungicide.
by Kenny Seebold, Extension Plant Pathologist