Fourteen German isolates, eight isolates from other European countries and ten American isolates were examined. The study mainly focused on comparing vegetative growth at different temperatures, comparing size and shape of sporangia and chlamydospores, and investigating mating reactions by crossing experiments. The German isolates chosen for this study originated from different Rhododendron and Viburnum species. The other European isolates originated mainly from Rhododendron spp. The American isolates were from hosts other than Viburnum.
The twenty-two European isolates were very homogenous. They showed only small variations in colony morphology, growth rate, size and shape of sporangia and chlamydospores. However, there was a high degree of variability amongst the ten American isolates. Some were identical to the European isolates but others showed quite different colony patterns and had a very slow vegetative growth rate. Furthermore, some of these American isolates produced sporangia that were different in shape to those of the European isolates.
All European isolates belonged to the A1 mating type and could be successfully crossed in vitro with P. cryptogea. A small number of oogonia were also produced by most of these isolates after pairing with P. cinnamomi. In contrast, those American isolates which could be crossed sucessfully belonged to the A2 mating type. The most successful partners for the American isolates were P. cryptogea and P. cambivora. However, two American isolates did not produce oogonia with any of the partners.
The in vitro pairing experiments of the European isolate BBA 9/95 (the type strain of P. ramorum) with the ten American isolates were not successful. Oogonia were not produced on either carrot piece agar or on Rhododendron agar after an incubation period of eight weeks. However in first studies with green Rhododendron twigs inoculated with the European isolate BBA 9/95 and the American isolate PR6, oogonia were produced on the outgrowing hyphae. The studies have not been finished.
