Identity and potential pathogenicity of Phytophthora species found on symptomatic Rhododendron plants in a Finnish nursery

In this study, microbial isolations were made from symptomatic Rhododendron plants from a large Finnish nursery, known to be harbouring Phytophthora based on PCR screenings. The nearby waterways were also sampled. A diversity of common Nordic plants was screened for Phytophthora susceptibility. Isolates recovered from Rhododendron plants included P. ramorum, P. cactorum, P. plurivora, P. pini and Pestalotiopsis sp. Baits floated in water samples from nearby waterways did not become infected with Phytophthora. In infection trials, all Phytophthora detected here were pathogenic to Rhododendron but nonpathogenic to Pinus sylvestris and Quercus robur. Phytophthora plurivora infected Betula pendula, Alnus glutinosa, Picea abies, Viburnum lentago, Vaccinium myrtillus, V. uliginosum, V. angustifolium and Fragaria x ananassa, the latter four species being new host records for this pathogen. Phytophthora ramorum caused small lesions on B. pendula, A. glutinosa and V. uliginosum, and more serious symptoms in Rhododendron, Viburnum lentago, V. lantana, Vaccinium myrtillus and V. angustifolium. Phytophthora pini was pathogenic to most plants tested, including Rhododendron, V. lentago and P. abies. In spite of an annual eradication programme in the nursery, P. ramorum was detected in annual samples taken during 2004-2010. Microsatellite analysis revealed that all isolates of P. ramorum belonged to the EU1 lineage.