First Report of Phytophthora ramorum in Ornamental Plants in Norway
Authors: Herrero M. L. , Toppe B. , Klemsdal S. S. , Stensvand A. 
Published: May 2007

In November 2002, Phytophthora ramorum was isolated from Rhododendron catawbiense with wilted branches in a nursery in Bergen. The isolate was identified by characteristic deciduous, semipapillate sporangia, abundance of large chlamydospores, and slow growth (2). The identification was confirmed by ITS rDNA sequencing. After the first detection, the Norwegian Food Safety Authority (NFSA) started a survey of different ornamental plants during 2003. Of 21 samples from 10 locations, two rhododendron samples were positive. The rhododendron plants containing positive samples in 2002 and 2003 had been imported that same year as the disease was detected on them. In 2003, NFSA made regulations similar to those in the EU for P. ramorum, including the destruction of all infected plants and all plants susceptible to P. ramorum within a 2-m distance of the infected ones. The production of rhododendron in Norwegian nurseries is limited, and most rhododendrons marketed in the country are imported from March to May from other European countries. The main sale of rhododendron occurs in May and June, often before symptoms of P. ramorum are easy to observe. In 2004, 133 samples from 53 locations were analyzed. P. ramorum was found in 29 new locations. It was detected in 57 samples of rhododendron, in one sample of Pieris japonica, and one of Kalmia sp. Symptoms on pieris were similar to those on rhododendron with blighted twigs and leaf spots. In Kalmia sp., P. ramorum was isolated from small foliar spots. In 2005, special efforts were directed to detect P. ramorum before the spring sale. Between January and May, 142 samples were analyzed (including plants from 45 import shipments) and 19 yielded positive (including six samples from five import shipments). In 2005, 370 samples from 74 nurseries and garden centers were analyzed and 97 samples from 43 locations were positive (all were rhododendron). Ten of the 43 locations had been positive in 2004. Some of the samples that yielded positive in the summer and autumn came from import shipments or nurseries controlled earlier and found free from P. ramorum. As suggested previously, the disease is probably moving in trade as symptom-free plants (1) and also likely in batches with few infected plants with mild infections that are difficult to detect when random control is carried out in large shipments. Most nurseries receive new plants every year. It is thus difficult to determine if it is a reintroduction or an eradication failure when a nursery yields positive to P. ramorum in two consecutive years. In 2005, P. ramorum was detected on well-established Viburnum fragrans and rhododendron plants in a private garden in Bergen. The viburnum plants of this garden were heavily infected, with wilting of whole branches from the root collar to the top. The pathogen was also found on established rhododendron shrubs in four public greens in Bergen and two in Stavanger. The two cities are located at the southwestern coast of Norway and have more than 2,000 mm of annual precipitation, cool summers, and mild winters. The pathogenicity of 26 isolates from rhododendron, one from pieris, and one from Kalmia sp. was tested by placing mycelial plugs (18 isolates) or drops of zoospore suspension (7 isolates) on the unwounded abaxial surface of rhododendron leaves of cv. Cunninghams White. After 7 days, all isolates produced lesions larger then 2 cm at each inoculation site. P. ramorum was reisolated from the leaves.

References: (1) C. M. Brassier et al. Mycol Res. 108:1107, 2004. (2) S. Werres et al. Mycol. Res. 105:1155, 2001.