Ascospore discharge was monitored from black knots excised from sour cherry trees in May 1992 and 1993, and March, April, and May 1994 and 1995. Rain, temperature, and wetness duration were monitored and a regression model was developed to describe the relationship between environmental conditions and ascospore discharge. Regardless of the date of excision, the peak period for ascospore release occurred from late May (shuck split) through the end of June (shoots 20 cm long), considerably later than previously reported from knots on plum. Fewer ascospores were released from knots excised in early March compared to later dates in 1994. In 1995, the spring was much warmer than in 1994 and ascospore release from March-excised knots was comparable to that from knots excised later in the spring. The period of release and total number of ascospores released was similar from non-excised knots and from knots excised in April or May 1994. Release of ascospores was dependent on rainfall and temperature, but not on the duration of wetness.