Evaluation of Fungicides for the Control of Phytophthora ramorum Infecting Rhododendron, Camellia, Pieris, and Viburnum

Fungicides were evaluated for pre- and post-infection control of ramorum blight, caused by Phytophthora ramorum, on Rhododendron cvs. Cunningham's White and Irish Lace, Camellia japonica, Pieris japonica, and Viburnum tinus. Cyazofamid, dimethomorph, mefenoxam, pyraclostrobin, and fenamidone applied as foliar sprays consistently provided preventative control as indicated by reduced lesion size compared to water controls. These fungicides provided preventative activity for at least 28 days in the tested species except in Rhododendron where fungicides were active for at least 14 days following application. With preventative fungicide applications, the pathogen was recovered from most fungicide-treated leaves by isolation onto selective media. Dimethomorph consistently reduced the percent recovery from diseased Rhododendron leaves. With post-infection treatments, the fungicides did not significantly reduce lesion growth and percent recovery of the pathogen. The pathogen was recovered from lesions consistently at least 6 weeks after fungicide application in Rhododendron regardless of treatment on intact and fallen diseased leaves. However, the cultures resulting from isolations of diseased tissue treated with cyazofamid and dimethomorph were significantly slower growing than those cultures from other fungicide treatments. P. ramorum management issues relating to fungicide use in commercial nurseries are discussed Accepted for publication 12 October 2007. Published 8 February 2008.