North Carolina Plant Disease and Insect Clinic
Plant Disease Fact Sheets

Collar Rot in Tobacco Greenhouses

Damaged tobacco seedlings from collar rot. The whit mycelium is very characteristic for the presence of this disease.

Stems of tobacco seedlings affected by collar rot.

M. Mila, W. A. Gutierrez, and H. D. Shew


Symptoms of the disease are usually observed after the canopy has formed among seedlings, or approximately 7 to 10 days after the first clipping.  First symptoms of collar rot are observed as small foci 4" to 16" in diameter in different areas in the greenhouse (Figure 1). Early infection is characterized by the presence of a soft, water-soaking lesion at the base of the stem.  Later on a cottony white mycelium is present in the surface of the infected tissues (Figure 2). In very advanced infections the cottony mycelium will form sclerotia, and infected plants melt down in the foci, leaving a blank of dead plants on the tray (Figure 3).

Causal Organism/Disease Cycle

Collar rot is caused by the fungus Sclerotinia sclerotiorum. The fungus produces sclerotia that are black, 0.1" - 0.5" long, and resembles black beans (Figure 4). These sclerotia can overwinter and then next spring will produce apothecia that release ascospores and infect next season's seedlings (Figure 4). Ascospores are wind disseminated from a few feet to 1.5 miles, but most spores travel less than 100 yards from their source of origin. The infection can either start from the upper leaves, growing down through the petiole, infecting stems, and other plants; or start from the lower senescent leaves, colonizing them first and then infecting the stems, and killing the plants.

Factors that Affect the Development of Collar Rot

Seedling age. Older seedlings (> 45-days) are the most susceptible to collar rot.   Clipping. Clipping alone has little effect on the incidence of collar rot, but when clipped debris is left on seedlings, incidence of collar rot is significantly increased.  Apparently this is favorable for germination of S. sclerotiorum and infection by the fungus of tobacco seedlings.

Leaf injuries. During the transplant production period, injuries other than clipping may occur. Injuries may predispose tobacco seedlings to infection by S. sclerotiorum. The most commonly observed injuries in commercial greenhouses are those due to poorly adjusted heating systems, which may blow dry, hot air over seedlings and desiccate leaf margins. Propane gas leaks in the presence of high humidity may result in the condensation of propane onto leaves in water drops and cause small necrotic lesions. Incompletely burned gases, damage due to cold temperatures, or injuries caused by oil drippings from mower machines may also occur. Inadequate pest control also may cause foliar damage. High incidence of collar rot is observed with injuries that results in necrotic areas on leaves that serve as infection courts for ascospores of S. sclerotiorum

Disease Management

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Creation Date: 2001

Revision Date: April 2010

Key Words: tobacco, disease, greenhouse, Sclerotinia sclerotiorum

Glossary Terms: foci, sclerotia, apothecia, ascospores