Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: 12/21/2003
Publication Date: 4/1/2004
Citation: Plant Disease 88:425
Technical Abstract: In 2003, landscape and potted nursery plants of laurustinus (Viburnum tinus) in Monterey County, California, were found to be infected with a powdery mildew. White ectophytic mycelial and conidial growth was present primarily on adaxial leaf sides, with only sparse growth on abaxial surfaces. Severely infected leaves were buckled and slightly twisted. Affected leaf tissue exhibited slight purple to brown discoloration. Appressoria were opposite and lobed. Conidia were produced singly, cylindrical in shape, and measured 31 to 42 x 14 to 19 µm. No fibrosin bodies were observed in the conidia, and conidia germinated at the ends. Ascomata were not observed. The fungus was identified as Erysiphe (sect. Microsphaera) viburni Duby, (=Microsphaera sparsa Howe) (1, 2). Pathogenicity was demonstrated by gently pressing infected leaves having abundant sporulation onto recently expanded leaves of potted laurustinus and then incubating the plants in a greenhouse (23 to 25 C). After 9 to 10 days, powdery mildew colonies developed on the test plants. Such colonies were morphologically similar to the original fungus. Uninoculated control plants did not develop powdery mildew. Using the same technique, inoculating fully mature leaves of the same plants did not result in disease. To our knowledge this is the first report of Erysiphe viburni infecting Viburnum tinus in California. At some landscaped areas the powdery mildew was extremely severe, causing plants to take on a whitish appearance and resulting in all new foliage being misshapen.