Cause Butterfly bushes (Buddleja sp.) are generally disease free, however, a few pathogens have been found in the PNW. The leaf spot disease caused by Cladosporium herbarum has been reported from Washington and found by the OSU Plant Clinic. Additional problems found by the OSU Plant Clinic include Botrytis sp., downy mildew (likely a Peronospora sp.), root rots caused by Pythium, Phytophthora and/or Phoma (also involved with a leaf spot and dieback), and Cucumber mosaic virus.
Buddleia davidii is considered a noxious weed and is illegal to sell.
Symptoms Downy mildew develops on leaves as yellow, brown and occasionally purple-red angular patches. Sporulation may occur on the underside of the lesions but can be sparse and difficult to see due to dense leaf hairs.
Cultural control Will be dependent on the type of pathogen involved. Leaf spots are managed through sanitation and lack of leaf wetness. The other problems, such as downy mildew, when found will require removal and destruction of the plants.
Reference Shattock, R. C., Smith, A. M. and Cooke, D. E. L. 2006. Downy mildew on Buddleja. Field Mycology. 7:22-27.