Several diseases routinely cause seedling damage in float-type greenhouses.  One of the most potentially devastating of these diseases is collar rot, caused by Sclerotinia sclerotiorum.  Incidence of this disease has increased as the percentage of transplants produced under greenhouse conditions has increased. 

Symptoms of Collar Rot

Symptoms of the disease are usually observed in the second half of the transplant production period after the canopy has formed among seedlings, or approximately 7 to 10 days after first the clipping.  First symptoms of collar rot are observed as small foci (small group of plants with symptoms) 4'' to 16'' in diameter in different areas in the  greenhouse. Early stages infection are characterized by the presence of a soft dark green (water-soaking) lesion at the base of the stem and the presence of a cottony white mycelium in the surface of the infected tissues with advanced infections.

In very advanced infections the cottony mycelium will form a sclerotia, and infected plants melt down in the foci, leaving a blank of dead plants on the tray.  The sclerotia, which is the resting structure, is black, 0.1'' –0.5'' long, and resembles mouse droppings or a black bean. These sclerotia will germinate as a thread-like mycelium (not observed in tobacco greenhouses) or as an apothecium after overwintering in the soil.

In the spring, apothecia release ascospores which will infect seedlings. 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.  This steep dispersal gradient increases the possibility of finding and eliminating sources of primary inoculum of the pathogen.  The presence of ascospores can be easily monitored by trapping them with the use of a semi-selective medium.

The infection of collar rot 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.  Seedlings 45 and 52-days are the most susceptible to collar rot.
Leaf extract and inoculum concentration. High numbers of spores and the presence of a food supply on the leaf surface of seedlings favor the development of collar rot.  However, the presence of leaf sap is so conducive to disease development that it mitigates the effect of inoculum levels.
   Clipping effects. Tobacco seedlings are typically clipped several times during the production period to increase plant uniformity at transplanting.  Clipping alone has little effect on the incidence of collar rot, but when clipped debris (macerated tissue and sap) 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. Large clumps of debris are most favorable.
Effect of 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. 

CONTROL RECOMMENDATION

-  Seed trays no more than 55 to 65 days in advance of transplanting. 
-  Reduce the moisture inside the greenhouse by increasing air circulation and ventilation. 
-  Reduce early stage injuries of seedlings by correct setting of heat systems and correct application of chemicals used in greenhouses. 
-  Use good clipping practices that reduce the production of leaf debris. Increasing the frequency of clippings and reducing the amount of leaves clipped at a time.  Reduce the deposition of debris and pieces of leaves on seedlings (use mower machines with high vacuum power). 
-  Discard leaf debris at least 100 yds from the greenhouse or bury them. 
-  Dispose of infected tobacco seedlings apart from leaf debris, burying them in the soil. 
 -  Avoid having family gardens or weeds close to the greenhouse because it is possible that plants other than tobacco also are involved in sclerotia and ascospore production. The fungus has a host range of more than 300 plant species.

Additional pictures
Symptoms in greenhouses

• Initial infection of seedling on trays (Figure 1, 2)
• Foci and spread of collar rot (Figure 3, 4)
• Old infection on trays (Figure 5, 6)

• Initial infection of seedling (Figure 7, 8)
• Stem lesions (Figure 9, 10)
• Water soaking, cottony mycelium, and sclerotia (Figure 11, 12)

• Debris on leaf surfaces (Figure 13, 14)
• Injury on seedlings (Figure 15, 16)

• Disposal of infected plants and debris (Figure 17, 18)

• Sclerotia (Figure 19)
• Sclerotial germination (Figure 20)
• Apothecia, asci, and ascospores (Figure 21)

Blue medium (semi-selective medium)
A method to induce apothecia formation

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Reformatted July 2001 by Plant Disease and Insect Clinic