Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a recurrent disease on potato in the Columbia Basin of Washington. The role of ascospores in association with disease onset and stem rot incidence in commercial fields and the role of flower blossoms on plant infection were investigated in 10 fields over 2 years. Ascospores of S. sclerotiorum were detected on a semiselective medium over several weeks, with a peak in number of ascospores near initial full bloom. A high proportion of blossoms at initial full bloom were contaminated with S. sclerotiorum prior to blossom fall in most fields. Stem lesions occurred after row closure and blossom drop, and were associated with mycelial mats growing from contaminated blossoms that had dropped on plant stems in the plant canopy or blossoms that dropped on the ground and stems contacting the ground. Incidence of Sclerotinia stem rot was reduced significantly when blossoms were removed from plants before blossom drop. Flower blossoms were shown to be a paramount bridge between airborne ascospores of S. sclerotiorum and stem infection in the potato canopy.