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Weather conditions seem to be changing to a warmer pattern. These conditions can lead to a number of environmental disorders such as damping off in tobacco float beds.
The float-system environment is near-ideal for Rhizoctonia solani, the fungus causing damping-off (or soreshin) in tobacco seedlings. Damping-off usually occurs early in the development of the seedling and first appears as a water-soaked lesion at the base of the plant.
Later, the lesion will take on a sunken, brown appearance and will eventually girdle the plant which then falls over and will eventually die. Occasionally, the entire stem of affected plants may show discoloration and decay may spread into leaves. Leaves in contact with the surface of Styrofoam trays or peat-based media can become infected and develop water-soaked lesions that enlarge over time, often spreading to the stems on young seedlings. Seedlings with mild infections that are later transplanted may contribute to outbreaks of soreshin in the field, and may be more susceptible to black shank and Fusarium wilt.
High humidity and temperatures above 70 degrees F are optimal for growth of R. solani. Commonly found in agricultural soils, the fungus can survive on organic matter and will colonize growth media used in tobacco transplant production. Infections can spread from plant to plant, and plant debris can serve as a bridge between infected and healthy seedlings.
Infested soil or Styrofoam trays are the most common sources of infection in transplant production. Infective structures 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.
Complete control of soreshin with fungicides is not possible; however, some suppression can be achieved with the mancozeb-based fungicides Dithane DF or Manzate Pro-Stick. These products can be applied at a rate of 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 3-5 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 6-12 gallons on older plants.
Begin applications before symptoms develop, or immediately after the first symptoms are observed at the latest, and continue on a 5-7 day schedule until plants are ready to go to the field.
Target spot is caused by another R. solani. Target spot begins in localized areas and commonly occurs after the plant canopy has fully formed. Small, water-soaked lesions appear on leaves and will expand rapidly under conditions of warm temperatures (> 75 degrees F) and high humidity.
Lesions normally have a transparent-light green appearance and may be surrounded by a chlorotic (yellow) halo. Dead leaves will turn brown and adhere to the float tray. Web-like strands of mycelia of fungal growth may be present on leaves, stems, and growth media when humidity is high. Seedlings with target spot that are transplanted can contribute to epidemics in the field later in the season.
As with soreshin, sanitation and good growing practices are the best defense against target spot. Nitrogen-deficient plants show increased susceptibility to target spot. Severe outbreaks have occurred where nitrogen has dropped below 50 ppm. This can be particularly common in outdoor float beds that have received significant rainfall that dilute fertilizer levels.
Maintaining nitrogen within the recommended range of 75-125 ppm will help suppress, but not eliminate, this disease. Reasonable control of target spot can be obtained with Dithane DF or Manzate Pro-Stick as described for damping-off.