Phytophthora ramorum and P. kernoviae are fungus-like organisms closely related to those causing potato blight, holly blight, Phytophthora root rot and Phytophthora bleeding canker.

Phytophthora ramorum

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, a large proportion of the 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, frequently 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 large 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 Cornwall, in an area with infected Japanese larch trees nearby.

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.

Since 2015 there have also been increasing numbers of cases on sweet chestnut (Castanea sativa) in southern England.

Phytophthora kernoviae

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 Scotland, the Republic of Ireland, Chile and 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

Phytophthora ramorum and P. kernoviae are aerial pathogens and do not cause root decay. Symptoms vary according to the host and the Phytophthora species, but there are a number of general features that you should look out for:

On shrubs such as Rhododendron, Camellia, Pieris and Kalmia:

• Brown, spreading lesions develop on leaves, often starting at the petiole, leaf tip or margin
• No fungal growth is visible
• The lesion often progresses more rapidly down the main vein (midrib) of the leaf, giving a V-shaped appearance
• Lesions or cankers may form on twigs or stems, progressing back from or spreading into sideshoots and leaves. The internal tissues below the bark at these points are a brown colour. Wilting and dieback often result
• Viburnum differs with infections occuring at the base of the stems that quickly cause wilting and collapse of the plant. Sometimes leaf blight symptoms are present as well

On trees such as beech:

• Patches of dead bark known as bleeding cankers develop, weeping a brown or black liquid. These are usually found on the lower stem, but occasionally up to a few metres above ground level
• Removal of the outer bark reveals a mottled brown decay of the inner bark
• It should be noted that other disease problems (eg: other Phytophthora species, honey fungus, bacteria), as well as pests or environmental factors (eg: drought) can also lead to weeping patches of dead bark. Unless rhododendrons or other understory plants affected by P. ramorum or P. kernoviae are in the immediate vicinity it is likely that one of these alternative causes is responsible

On trees such as ash and holm oak:

• Symptoms are confined to the leaves, and often consist of browning and death of the margins

On larch:

• Needles discoloured becoming purple or black and fall off prematurely
• Bud flushes abort, shoots wilt and dieback. The entire crown of trees can dieback
• Bleeding cankers can occur on trunks, stems and side shoots

If you suspect that Phytophthora ramorum or P. kernoviae could be present in your garden you should not attempt to control the disease yourself. You should report your suspicions immediately to the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal. 

If an outbreak of either pathogen is confirmed, a Statutory Notice will be issued detailing the eradication and containment action required.

Links

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

Spread of these Phytophthora species is favoured by wet or humid conditions. Spores produced on leaf lesions on hosts such as Rhododendron and Larix kaempferi are splashed around to create new infections on leaves or bark of neighbouring susceptible trees and shrubs.

Each species is also capable of producing a long-lived resting structure which can contaminate soil, and can be spread around on boots, vehicle tyres and the feet of animals.

Water is also a transport mechanism, and P. ramorum has been found in rivers and streams near to some outbreak sites. The most important means of long-distance spread, however, is the transportation of infected plants.