Since 1993, a hitherto unidentified Phytophthora species has been found associated with twig blight disease in Rhododendron and, sporadically, Viburnum. The fungus was described in 2001 as P. ramorum. Between 1993 and 2000, a total of 18 samples of infected Rhododendron plants had been collected in The Netherlands from 15 locations of public green and nurseries. A single finding had been made in 1998 on Viburnum x bodnantense 'Dawn'. Unexpectedly, this fungus proved to be conspecific with a new Phytophthora species identified in 2000 as the cause of large scale oak mortality in California. The awareness of the potential destructiveness of P. ramorum for our ecosystem, landscape, recreation, and, secondarily, trade caused great concern.
In order to make a first assessment of the phytosanitary situation, a general and a targeted survey were carried out in 2001. For the general survey, 214 locations with Rhododendron or Viburnum were checked. Infestations were found in 11 out of 78 nurseries and garden centers, and in 7 out of 136 locations in public and private green. In the targeted survey, 15 locations where P. ramorum was previously found were visited. At 5 locations inspections were not possible due to the absence of host plants. One out of 2 nurseries infected in the previous years still was infected, and 3 out of 8 locations in public or private green. In both surveys, potential host plants including Quercus, Fagus, and Castanea approximately 100 m around infected Rhododendron or Viburnum plants were checked for symptoms of P. ramorum. In none of the cases bleeding cankers or other symptoms suggesting Sudden Oak Death were encountered. Our experience so far suggests that in The Netherlands the disease is restricted to Rhododendron (particularly R. ponticum and R. x catawbiense) and Viburnum (particularly V. x bodnantense 'Dawn'). In nurseries, P. ramorum was restricted to infested lots of these two genera; no spread was found in adjacent lots of (potential) host plants. In public and private green, only Rhododendron was infected. No spread from Rhododendron to other plants was observed. Control measures taken in nurseries were aimed at securing that plant material for trade is free from P. ramorum. Infected plants and neighbouring plants within 1 meter were destroyed; remaining plants had been treated chemically beforehand as a preventive measure. Re-inspection took place during the quarantine period of three months. Lots found to be infected at re-inspection were destroyed. On the plots where plants had to be removed because of infestation, a prohibition (3 years) on growing host plants was imposed. In 2001, in public and private green, infested plant parts were pruned and destroyed, fallen leaves were removed and wound treatment was applied.
All member states of the EU are concerned about the disease. In July 2002, the European Commission decided to take precautionary measures for P. ramorum. The member states have to carry out national surveys in addition to the implementation of the official measures. The precautionary measures of the EU are partly based on the assumption that P. ramorum strains from the USA are different from those in Europe. This assumption is based on genetic differences and differences in epidemic development between California and Europe. The assumed differences are subject of international research.
In The Netherlands, in 2002 and 2003, the Plant Protection Service is performing inspections on all 700 nurseries cultivating Rhododendron and Viburnum. Potential host plants including Quercus receive special attention during inspection for symptoms of P. ramorum. Measures in nurseries are according to the new EU legislation. Furthermore, 2000 locations where Rhododendron is present in public and private green will be inspected. Inspections are aimed at determining the spread of the disease and at checking for the presence of additional host species in our ecosystem. Following findings in public or private green, measures are imposed aiming at preventing fungal spread and reducing the inoculum pressure; the infested plants are to be removed.
The results of this survey will be used by the government of The Netherlands to determine the approach for 2003 and thereafter regarding the eradication or management of P. ramorum. The Plant Protection Service also initiates research in areas where basic information for this policy is lacking, such as pathogenicity testing, population genetics of the pathogen, and development of detection methods.
