Cause Several fungi have been found to cause leaf spots on Ceanothus in the Pacific Northwest. Blumeriella ceanothi (formerly Cylindrosporium ceanothi) and Septoria ceanothi have been reported and/or found from all states. Cercospora sp. has been found by the OSU Plant Clinic. Several cultivars get Cercospora leaf spot in western Oregon but 'Autumnal Blue' had severe defoliation reducing its ornamental appeal and vigor. In general, warm wet weather favors disease development.
Symptoms Blumeriella produces brown, circular lesions with a distinct but irregular margin. Spots may coalesce when numerous. Numerous spots can cause defoliation of the older leaves.
Cultural control
- Space and/or prune plants for increased air flow and better drying.
Chemical control
- Heritage is registered for use against powdery mildew and may be effective for several leaf spots. Use at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
Reference Bell, N.C. 2009. Evaluation of growth, flowering, and cold hardiness of Ceanothus in western Oregon. HortTechnology 19:411-417.