Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. The disease develops in cool, wet conditions and is more prevalent when plants form a complete, compact canopy that holds high relative humidity. The fungus can colonize dead organic plant matter and survive many years. 'Profusion White' zinnia is very susceptible. In the potato system, flowers are very susceptible to infection and are the main source of stem infection.
Note Although it has never been reported, the OSU Plant Clinic has found Phytophthora on samples of zelkova with root rot and stem cankers. Collar rots also seem to occur when trees are planted too deep.
Cause Coniothyrium bartholomaei, a fungus, has been found on yucca in Oregon. Cladosporium herbarum (formerly Mycosphaerella tassiana) and Kellermania yuccifolia have been reported from Washington.
Notice the dark spots on these leaves.
Photo by Jay W. Pscheidt, 2000.
Note the dark fruiting bodies (pycnidia) in the center of the tan zone of this leaf spot.
