TOBACCO

DAMPING-OFF AND TARGET SPOT CAUSED BY RHIZOCTONIA SOLANI: MANAGEMENT IN THE FLOAT SYSTEM

by Kenny Seebold

Damping-off

The float system creates near-ideal conditions for Rhizoctonia solani AG (anastomosis group)-4, the causal agent of damping-off (or soreshin) to grow and infect tobacco seedlings. Damping-off usually occurs early in the development of the tobacco 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. Girdled seedlings will fall over and eventually die. Occasionally, the entire stem of affected plants may show discoloration, and decay may spread into leaves. Leaves alone may become infected and will first develop water-soaked lesions that enlarge over time, often spreading to the stems on young seedlings. Seedlings with mild infections of R. solani that are later transplanted may contribute to large-scale outbreaks of soreshin in the field, and may also be more susceptible to black shank and Fusarium wilt.

High humidity and temperatures above 70 °F are optimal for growth of R. solani. A common inhabitant of agricultural soils, R. solani can survive on organic matter and will colonize growth media used in tobacco transplant production. Primary infections occur when actively growing hyphae, or fungal threads, come in contact with roots or stems. Hyphae then form infection cushions that produce enzymes that will degrade plant tissues. Infections can spread from plant to plant, and organic matter (plant debris) can serve as a bridge between infected and healthy seedlings. Survival structures called sclerotia are formed after the food source has been exhausted.

Infested soil or Styrofoam trays are the most commons inoculum sources of R. solani. As with Pythium spp., tobacco roots embedded in Styrofoam float trays will serve a source of inoculum of R. solani (in the form of sclerotia or dormant hyphae) if trays are re-used.

Good sanitation is the best way to manage soreshin in the float system. The first step is to limit the amount of fungal inoculum in the transplant system. New trays will all but eliminate the risk of carrying over inoculum from previous transplant cycles, but this option is expensive and creates issues with disposal of used trays. Used trays should be steam-heated to 165-170 °F for 30 minutes (after the heating chamber reaches operating temperature). Dipping used trays in bleach or other disinfectants will not eliminate R. solani from old trays because the chemicals cannot penetrate and reach pathogen-infested roots that have grown into the tray. Proper ventilation, which minimizes leaf and stem wetness, and maintenance of fertility are important considerations as well. Complete control of soreshin with fungicides is not possible; however, some suppression can be achieved with Dithane DF. Dithane DF 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. Begin applications before symptoms appear and continue on a 5-day schedule until transplanting. Refer to “Management of Tobacco Float Systems”, pp. 12-22 in ID-160, the 2008 KY Tobacco Production Guide, for more information.

Target Spot

Target spot is caused by the sexual stage of R. solani AG-3, Thanatephorus cucumeris. Target spot begins in localized areas, or foci, 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 °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 (mycelia) of fungal growth may be present on leaves and stems when humidity is high. The target spot pathogen can also cause dampin-off of younger transplants. Seedlings with target spot that are transplanted can contribute to epidemics in the field later in the season.

Inoculum carried over on R. solani-infested trays is the most common way for the pathogen to enter the float system, although inoculum may move in on air from sources outside the transplant facility. Basidospores, generated by the sexually reproducing phase of this fungus(Thanatephorus cucumeris), are released under favorable conditions and contribute to spread of the disease within the float house.

As with soreshin, sanitation and good growing practices are the best defense against target spot. Research suggests that plants that are nitrogen-deficient show increased susceptibility to target spot; maintaining nitrogen within recommended levels will help suppress this disease. Reasonable control of target spot can be obtained with Dithane DF, as described for damping-off.