Collar rot, caused by Sclerotinia sclerotiorum, is a severe disease of tobacco seedlings grown in greenhouses. A semiselective medium was adapted and used to detect the presence and quantity of ascospores in commercial greenhouses. Petri dishes of the semiselective medium were placed inside and outside of greenhouses in four counties during the transplant production period in 1995 and 1996. Ascospores were present throughout the production period each year (February to April) and were confirmed to be the primary inoculum for the disease. Significant differences were observed in the number of ascospores trapped within and between counties. Peak numbers of ascospores were trapped between 10 and 12 a.m., and higher numbers of ascospores were trapped outside than inside houses. In general, distribution of ascospores inside houses was uniform unless a high concentration of apothecia was present very close to one section of the greenhouse. The semiselective medium and trapping technique used in this study may allow development of a forecasting system for collar rot of tobacco based on the presence and level of pathogen inoculum.