Updated pest risk assessment of Phytophthora ramorum in Norway
Phytophthora ramorum is present in the PRA area but has a restricted distribution, mainly being detected in the southern and southwestern parts of Norway. The only P. ramorum lineage considered to be present in Norway is EU1 with mating type A1. The other lineage in Europe, EU2, has so far mainly been documented from the UK. The most widely distributed multilocus genotype of P. ramorum in Norway is EU1MLG1, which became dominant in Europe (including Norway) after 2008. In North America, the NA1, NA2, and EU1 lineages are known from both nurseries and forests. NA1 and NA2 are of the opposite mating type (A2) than European lineages. Recently, various other lineages of P. ramorum have been described from Asia. The main risks for future problems with P.ramorum in Norway are related to entry and establishment of non-European isolates (of all lineages), as well as emergence of new genotypes in European P. ramorum populations.
here are several options for diagnosing P. ramorum to species and lineage (mainly EU1, EU2, NA1, and NA2). From a management perspective it is more important to distinguish these entities than mating type and isolate groups (genotypes). The latter are mainly relevant for research purposes or in cases of unexpected disease developments, such as new hosts, increased spread or more severe symptoms on known hosts. However, for more detailed regulation, monitoring, and management of P. ramorum it could also be useful to test for genotypes, i.e. to distinguish EU1MLG1 from other genotypes.
Rhododendron remains the most important host plant for P. ramorum in Norway, both in terms of imported plants and detections (mainly in nurseries, garden centres, and public parks). Species in other ornamental plant genera, such as Viburnum, Pieris, and Kalmia, are also listed as major hosts in Europe, and P. ramorum has been detected at least once on species in all these genera in Norway. In the US, Rhododendron, Viburnum, Pieris, Syringa, and Camellia are considered to be the main ornamental hosts. In Norway, there has been one documented detection of P. ramorum on Syringa. Vaccinium and several tree species are potential hosts in the wider environment in Norway, but these hosts are most likely to be infected on sites where rhododendrons are affected by P. ramorum.
We consider the probability of entry of P. ramorum to Norway to be very likely, with a low level of uncertainty. Plants for planting, in particular rhododendron and other ornamental hosts, remain the most important entry pathway for P. ramorum. Due to the high import volumes to Norway from Europe, nurseries in the EU are still the main sources of infected plants.
If efforts to prevent import of infected plants and to eradicate P. ramorum infestations are discontinued, we consider it very likely that the pathogen will eventually establish in or spread to new areas in Norway. There is a high potential for establishment and spread of P. ramorum along the southwestern and southern coast of Norway, where climatic conditions are favourable for the pathogen and rhododendron and other hosts are common.
We consider the overall probability of spread of P. ramorum in Norway after establishment to be moderately likely, with a medium level of uncertainty. Despite repeated detections of the pathogen in some locations, further spread seems to be local and limited. New sites with P. ramorum outbreaks have been rare in Norway in the last decade. Whether this is due to import regulations, eradication efforts (removal of infected plants) or other factors is difficult to determine. Despite the limited spread of P. ramorum in Norway so far, the potential for persisting infections and spread in areas with a conducive climate (high precipitation) cannot be ignored.
Phytophthora ramorum still meets the criteria for being regulated as a potential quarantine pest, at least all other lineages than EU1 – thought to be the only lineage present in Norway. For the EU1 lineage, a possible categorization for European isolates is ‘regulated nonquarantine pest’, whereas non-European EU1 isolates fulfil the criteria of being a quarantine pest. Within the EU1 lineage there are different isolate groups, and new genotypes may arise. If the genotype of EU1 isolates detected in imported plant materials differs from isolates that are already present in Norway, European EU1 isolates also fit the category ‘quarantine pest’.
The potential effect of introducing new lineages, mating types or isolate groups is considered to be similar for new areas in Norway and areas where P. ramorum is already present. If the pathogen becomes widely spread and/or more genotypically diverse, the potential for damage is considered to be high, due to disease development in infected trees and the possibility of shifts in host plants. In addition to preventing new introductions, it is important to limit domestic spread of the pathogen from known infestations and, if possible, to eradicate P. ramorum from those sites. The longer P. ramorum is present at a site, and the more widespread the pathogen becomes, the higher is the risk that the pathogen will adapt to (new) local hosts and environmental factors.
Monitoring host plants for symptoms and testing for the presence of P. ramorum, especially on imported plants, remain the best risk-reducing options. Other effective risk-reducing options are prompt removal and destruction of infected ornamental hosts, in particular rhododendron, and to not replant with susceptible plant species. For infected trees, the best management measures depend on the situation, but infected larch trees should always be removed and destroyed. For non-transmissive tree species, such as beech, the risk of inadvertently spreading the pathogen during felling activities should be weighed against the risk associated with leaving an infected tree on site. Finally, it can be useful to run public awareness campaigns about the importance of cleaning soil from footgear and other items after visiting areas where P. ramorum is present (both in Norway and abroad), as well as other risk-reducing options for private gardens.
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