In experiments under controlled conditions, P. ramorum isolates from different European countries and ten American isolates were tested for their pathogenicity to Rhododendron. For these experiments, freshly cut green twigs of the Rhododendron hybrid 'Cunninghams White' were inoculated at the cut ends with a mycelium plug. The twigs were incubated at 25°/20°C and a daylength of 16 hours.
All the European isolates caused a twig dieback and disease development was identical for all isolates. Nine days after inoculation, the length of the discolouration on the twigs ranged from 5 to 6 cm for nearly all isolates. The pathogen was successfully re-isolated from all the inoculated twigs.
In contrast, the virulence of the American isolates was highly variable: some caused a twig dieback identical in severity to that caused by European isolates; some caused only a moderate dieback; and others caused no discolouration on the Rhododendron twigs, or else the discolouration was less than 1 cm in length. Isolates causing symptoms were all successfully re-isolated from the inoculated twigs; the single isolate that did not produce symptoms could not be re-isolated from the inoculated twig.
The host origin of the isolates did not seem to influence their agressiveness towards Rhododendron. However, agressiveness did appear to correspond to their vegetative growth on carrot piece agar. American isolates with a very slow growth rate on carrot piece agar did not cause a severe twig dieback on Rhododendron in these experiments; it has not yet been determined whether this is due to potential degeneration of these isolates, or whether this reflects true genetic variation within the American P. ramorum population.
