Phytophthora ramorum and P. kernoviae are fungus-like organisms closely related to those causing potato blight, holly blight and Phytophthora root rot & bleeding canker.
In the mid-1990s, P. ramorum began to cause widespread death of tanoak (Lithocarpus densiflorus) and some native oak (Quercus) species in parts of the USA (notably coastal California and Oregon). Here, the disease was given the common name of sudden oak death. Plant health authorities in the rest of the world were alerted to the problem and began to check for P. ramorum. The first UK finding was in 2002 and P. ramorum has now been found at hundreds of sites in England and Wales as well as in Scotland, Northern Ireland and the Channel Islands.
In the UK the majority of findings of P. ramorum have been on nurseries, affecting container-grown ornamental plants such as Rhododendron, Viburnum and Camellia. There have, however, also been many outbreaks in gardens, amenity areas and woodland, usually associated with infected Rhododendron ponticum. The disease is often referred to as ramorum dieback or ramorum disease in Europe and the UK, rather than sudden oak death, as our native oak trees are more resistant to the pathogen than their American counterparts.
Until 2009, cases on trees had been relatively few in the UK, with beech (Fagus sylvatica) appearing most susceptible. However, in August 2009, the disease was found on Japanese larch (Larix kaempferi), killing significant numbers of trees. There have now been significant outbreaks on Japanese larch in many areas, but predominantly in south west England, south Wales, south west Scotland and Northern Ireland. In 2011, Phytophthora ramorum was confirmed on European larch (Larix decidua) in an area with infected Japanese larch trees nearby in Cornwall. The discovery of the disease on larch has had significant implications as the fungus can reproduce abundantly on this host. This means the disease will be more difficult to contain and will pose more risk to trees, shrubs and heathlands. Thousands of hectares of Japanese larch trees have already been felled as a result of the disease.
P. kernoviae was first detected in Cornwall in 2003, during surveys for P. ramorum. At the time it was a species new to science, but it has now been detected at dozens of sites in England and Wales (mainly in south-west England), and has also been found in New Zealand. It causes similar symptoms to P. ramorum, but appears to be more aggressive to rhododendrons.
Notifiable plant pathogens
In 2008 there were also confirmed cases of these pathogens on the important heathland wild plant bilberry (Vaccinium myrtillus). P. ramorum and P. kernoviae are regarded as posing a serious threat to the environment and commerce. A major epidemic could have far-reaching consequences for woodland and heathland habitats, as well as gardens, amenity plantings and the horticultural industry.
These two fungus-like (Oomycete) organisms are notifiable plant pathogens, and suspected outbreaks must be reported to the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal.
Common hosts for the two species are as follows:
Phytophthora ramorum: Rhododendron, Viburnum, Camellia, Pieris, Kalmia, Leucothoe, Quercus ilex (holm oak), Fagus (beech), Larix kaempferi (Japanese larch)
Phytophthora kernoviae: Rhododendron, Magnolia, Quercus ilex, Fagus
Forest Research profile on P. ramorum
Forest Research profile on P. kernoviae
Scientific paper on P. ramorum on larch
Video on P. ramorum and P. kernoviae