Evaluation of fungicides for the control of Phytophthora ramorum infecting Rhododendron, Camellia, Viburnum, and Pieris

Phytophthora ramorum has been detected infecting ornamental hosts in European nurseries and gardens beginning in 1993, and detected in North American nurseries beginning in 2000. Nursery operators need a comprehensive program to insure that nursery stock remain disease free. Fungicides could be part of an integrated pest management approach to meet that goal. It is also important to evaluate whether fungicides are masking symptoms and the detection of the pathogen. For two years we evaluated fungicides for the prevention and eradication of P. ramorum on four important ornamental genera of nursery hosts: two cultivars of Rhododendron (R. 'Cunningham’s White' and R. 'Irish Lace') in 2003, and Camellia japonica 'Elena Nobile', Pieris japonica 'Whitewater', and Viburnum tinus 'Compacta' in 2004. First, we screened prospective fungicides by evaluating the preventative control of a wide range of commercially-available and experimental Oomycete fungicides. Second, we selected the fungicides that provided the best efficacy in the fungicide screen and repeated these applications to determine their residual action to prevent infection. Efficacy was also judged by how well the pathogen was recovered from lesions resulting from infections. Finally, the eradicative potential of these fungicides were judged by the success of recovery of the pathogen from lesions that were treated with these fungicides.