Defra

What is Phytophthora kernoviae?

Phytophthora kernoviae is a recently-discovered, exotic, fungus-like pathogen which causes damage to trees and shrubs.  It was first found in the south-west of Great Britain in 2003 during official surveillance activities for Phytophthora ramorum causing similar damage to beech trees and rhododendrons.  P. kernoviae is taxonomically a distant cousin of P. ramorum and appears to behave in a similar fashion in GB, sporulating on rhododendrons and causing bleeding stem cankers on susceptible trees.  However, there is some evidence that it may be more aggressive on rhododendron under GB conditions than P. ramorum.

It is mainly found in woodlands, parks and managed gardens in the south-west of England and south Wales but has recently also been found on heathland in Cornwall.  It has also been found at one managed garden and one nursery in the north-west of England, as well as two nurseries in the south-west.  In January 2008 it was found for the first time in Scotland at an established garden site. 

The only other country where P. kernoviae has been reported is New Zealand, where official reports were made in March 2006.   It has recently been announced that the pathogen has probably been present in New Zealand since at least the 1950s.   There is currently insufficient information to judge whether the pathogen was introduced to New Zealand or whether it is endemic or native there.  Prior to this it was speculated that P. kernoviae may have been introduced to GB from Asia or Patagonia.

Is Phytophthora kernoviae a mutation of Phytophthora ramorum?

P.  kernoviae was initially discovered in Cornwall, a county known to have higher than average levels of natural radioactivity (due to the presence of radon natural gas in uranium-bearing granite rocks.) This has led to the suggestion that these naturally occurring levels of radioactivity have caused P.  ramorum to mutate, giving rise to P.  kernoviae.  However, if P.kernoviae were to be a mutant of P. ramorum then we would expect the two species to be similar in morphology and DNA sequence.  This is not the case and there are, indeed, marked differences.  It appears that P. kernoviae is more closely related to other Phytophthora species (e.g.  P. boehmeriae) in a group of Phytopthora species which are relatively distantly related to the main Phytopthora groups that contain P. ramorum.  Genetic analysis of P. kernoviae isolates has provided evidence that it is a recently introduced pathogen since the amount of genetic variation in the pathogen population is small.

What does ‘kernoviae’ mean?

The name is derived from Kernow, the Cornish name for Cornwall, where the organism was first observed. It has previously been referred to as Phytophthora taxon C and Phytophthora kernovii.

Why is Phytophthora kenoviae described as a fungus-like pathogen?

P.  kernoviae belongs to a group of organisms known as the oomycetes. Until recently oomycetes were believed to be fungi as they have many characteristics, such as spores and hyphae, which are also shared by fungal species.  However, DNA sequence analysis in the 1990s indicated that the oomycetes are actually more closely related to algal groups, particularly the diatoms and brown algae.  Consequently oomycetes were placed in a separate taxonomic kingdom, the kingdom Chromista as opposed to kingdom Fungi.

How was it discovered?

In October 2003, whilst surveying for P.  ramorum, Forestry Commission scientists isolated a Phytophthora from a large bleeding canker on a mature beech in the South-West of England.  Meanwhile, scientists at the Central Science Laboratory had identified the same organism on established Rhododendrons in the South-West.   Further investigations led to the conclusion that this was a previously undiscovered pathogen.  All samples submitted are now routinely analysed for Phytophthora ramorum and Phytophthora kernoviae.

What are its hosts?

A ‘host’ in biological terms is something which a pathogen can infect and colonise.  Infected plants typically display symptoms of disease.

In GB, shrub and tree species in 15 host genera are affected, representing 9 different families.  The main shrub host affected is rhododendron.  About 60 trees have exhibited bleeding cankers in GB, and these are mainly beech trees.  Trees with foliar infections have been predominantly magnolias and Drimys.    The first official report of P. kernoviae in New Zealand was on the orchard fruit tree known as cherimoya or custard apple.  It has recently been discovered that an unidentified Phytophthora isolated from beneath stands of the conifer tree Pinus radiata in the 1950s in NZ is P. kernoviae.  These trees exhibited no disease symptoms.

The known hosts of P. kernoviae to date include Fagus sylvatica (beech), Rhododendron spp., Quercus robur (English oak), Quercus ilex (holm Oak), Gevuina avellana (Chilean hazelnut), Liriodendron tulipfera (tulip tree), Magnolia stellata, Michelia doltsopa and Pieris formosa. Many of these are also hosts of P.  ramorum.

In December 2007 P. kernoviae was confirmed on bilberry (Vaccinium myrtillus) at a woodland site in Cornwall and in February 2008 in open heathland in Cornwall.

Information on plants reported as natural hosts of P. kernoviae can be found at: www.defra.gov.uk/planth/kernovii/kernhost.pdf

What are its symptoms?

P. kernoviae causes three main types of disease:

• ‘Kernoviae bleeding canker’ refers to cankers (discoloured lesions) on trunks of trees, which emit a dark ooze.  As they increase in size they can lead to tree death. 

The other two types of disease affect both shrubs and trees.

• ‘Kernoviae leaf blight’ refers to infection of the foliage, leading to discoloured lesions on the leaves. 

• ‘Kernoviae dieback’ refers to shoot and bud infections which result in wilting, discolouration and dieback of affected parts.

A detailed breakdown of symptoms specific to species is available at: www.defra.gov.uk/planth/pestnote/newram.pdf

How is it spread?

P. kernoviae is considered to be adapted to a temperate climate as it has an optimum temperature for growth of 18°C, with an upper limit of 26°C, and a requirement for moisture.  It produces spores known as sporangia (containing motile infective zoospores) on the leaves and shoots of shrub and tree hosts; these are known as sporulating hosts.  These sporangia are mostly spread locally over relatively short distances during rain.  P. kernoviae can be found in soil and leaf litter and can be moved on the footwear  of humans and possibly on the feet of other animals, and potentially by vehicles.  It is also found in watercourses but it is not known whether this can lead to new infections of shrubs or trees.  Long-distance spread is primarily considered to be by movement of infected plant material, e.g. in trade.

Tree hosts only produce infective sporangia if the foliage becomes infected (tree hosts with susceptible foliage include magnolia, holm oak, Drimys, Michelia, Chilean hazelnut, tulip tree, holly and cherry laurel).  Some trees only develop bleeding cankers (beech and English oak): these cankers do not produce sporangia and so are not a source of infection for these hosts or for other hosts; they become infected as a result of being in the proximity of sporulating foliar hosts.  In GB, all the trees that have developed bleeding cankers have been adjacent to, or in most instances actually in contact with, infected rhododendron, invariably Rhododendron ponticum.

Sporulating hosts vary in the amount of infective sporangia that they produce with rhododendron being the greatest sporulator (compared to magnolia, Michelia and holm oak).  Rhododendron is the most abundant sporulating host in GB woodlands, especially now that invasive R. ponticum has become so widespread. 

Monitoring work in woodlands in Cornwall has shown that by completely removing infected rhododendron from the woods, no new trees have developed bleeding stem cankers within the two years since rhododendron removal.  However, some rhododendron re-growth and seedlings have continued to become infected. The pathogen can still be detected in raintraps and in soil, although the level of inoculum has declined significantly and may be below that required to initiate stem infections on trees.  Comparative data on persistence of P. ramorum and P. kernoviae in soil  indicates that levels of contamination of P. kernoviae may decline more quickly but it is not known in what form the pathogen may be surviving.  It is speculated that P. kernoviae could be surviving in the form of a robust spore known as an oospore.  Oospores are produced through sexual reproduction and as P. kernoviae is self-fertile it can achieve this without needing a mate.  No evidence has been found for the presence of oospores of P. kernoviae in the GB environment to date but they have been occasionally observed in infected plant material in laboratory experiments.

Examination of bleeding cankers on beech trees has shown that P. kernoviae can be found extending up to 12mm into the wood and can survive there for at least 24 months.  This appears to be a dead end for the pathogen but it may be possible for this to lead to further spread via movement of infected timber.  Currently no wood has been harvested from known infected trees in GB.

Why are Defra and the Forestry Commission so concerned about Phytophthora kernoviae?

Currently the pathogen is subject to an eradication/containment programme and so its full potential has not been realised.  However, the extent of the damage to trees and shrubs, and the apparent speed at which symptoms develop, indicate that this disease poses a potentially serious threat to woodland and heathland environments.  P. kernoviae may continue to be damaging in woodlands infested with rhododendron and in historic gardens with susceptible plants, especially in the west of GB or in other parts with favourable microclimates.

P. kernoviae was  found infecting Vaccinium myrtillus (bilberry) in woodland in Cornwall in December 2007 and in open heathland in February 2008.  These are the first records on vaccinium in the UK.  The potential for the pathogen to spread further in heathland environments is not known; however, prior to these findings laboratory experiments have shown that this and other heathland species were susceptible to P. kernoviae.   The UK has 20% of the world’s lowland heath and approximately 75% of the total resource of upland heath.  The Government has a Public Service Agreement target for 95% of the area of Sites of Special Scientific Interest (SSSI) in England to be in ‘good condition’ by 2010.  Much of the heathland resource is notified as SSSI and these habitats form a significant proportion of the total SSSI area.  Whilst the potential for the spread of P. kernoviae on open heathland is not known, if significant areas become infected or need to be destroyed as part of control measures there is a risk that this could impact on the achievement of the target.   Work is currently underway to help determine risks to heathland plants and inform management approaches in heathland environments.

If the pathogen were not controlled it is not known whether or when the whole of GB could become affected.  However, in the absence of measures, potential spread into and within the environment is likely to increase; the scale of environmental damage is uncertain but the maximum development of the epidemic in GB habitats is likely to take decades.

What damage is being caused by Phytophthora kernoviae in GB?

In England and Wales, between October 2003 and January 2008 there have been 52 outbreaks in locations other than nurseries and all but one are subject to on-going eradication or containment action.  The most significant damage has been in the south-west of England (Cornwall and one site in Devon) and at five sites in south Wales, with only one finding on a single mature R. ponticum in a managed garden in north-west England.  These western areas of the country appear most favourable for the disease.

Although the number of trees that have become infected is low, it is increasing, and a few trees with bleeding cankers have died.  Ornamental plants and trees in managed gardens involved in tourism have been badly affected and some rare or historically-important specimens continue to be at risk. Visitors to some historic gardens have complained about the appearance of the plants. 

The nursery industry has not been significantly affected by the disease or by the phytosanitary measures taken to date because there have been only three nursery outbreak sites: one in the north-west of England which has been eradicated; and two in the south-west of England.  One of these nurseries adjoins the woodland in the managed garden where P. kernoviae was first found on beech and rhododendron in October 2003.  

In January 2008 it was found for the first time in Scotland at an established garden site.

What is the Phytophthora kernoviae Management Zone?

P. kernoviae was found to be sufficiently widespread in one area of Cornwall, south east of Redruth, that statutory measures were put in place at the end of 2004 (The Plant Health (Phytophthora kenovii* Management Zone) England Order 2004)   to prohibit the removal of host plants, or parts of host plants (such as cut foliage for decoration) from a designated area without an inspection and written authorisation.  The Order also provides powers to close public footpaths to enable eradication measures to be undertaken.

Unlike P. ramorum there are no specific EU measures, but like any Member State with a new disease problem, we have to report to the European Commission and the other Member States the measures which we are taking to prevent its spread and protect other Member States.  The European Commission may, if it deems it necessary for the purpose of protecting the rest of the Community, seek to introduce regulatory provisions by proposing measures to the Standing Committee on Plant Health.  At the present time there is no indication that any such proposals are planned.

*Subsequent to publication of the Order the spelling was changed, replacing ‘kernovii’ with ‘kernoviae’.

What action are you taking in the zone?

Clearance of woodlands with infected Rhododendron ponticum in and around the zone started in August 2005 and is continuing, concentrating on high-risk sites based on a risk matrix . To date (April 2008) clearance of R. ponticum  has been undertaken in conjunction with 21 landowners at 25 sites, with 89 hectares predominantly of woodland being either cleared or in the process of being cleared. 

Why do you destroy infected plants?

The threat P.  kernoviae poses to trees and the environment, and the function of infected plants as a pathway for infection, necessitates a policy of destruction as the only guaranteed means of achieving eradication in a diseased specimen.

Some commentators suggest that the trees which have become infected were particularly weak specimens?

Promoters of this point of view have cited heavy metals present in the soil at outbreak locations and have speculated that planted trees, possibly enduring less compatible soil conditions, might be less robust than native specimens in oak woodland.  However, we know from laboratory tests that, regardless of site and growth conditions, some trees are intrinsically more susceptible to P. kernoviae and P.ramorum. In the case of P. kernoviae, beech is more susceptible to infection, whilst European beech, Turkey oak and red oak are particularly vulnerable to P. ramorum.  In laboratory tests, the pathogen can infect unwounded bark of several tree species.  Another key factor is the proximity of trees to infected rhododendrons. Of all the trees which have become diseased, most are only a few metres away from infected rhododendrons and many are actually touching them.

Have you undertaken any research into Phytophthora kernoviae?

A wide-ranging research and development programme has been implemented by Defra and the Forestry Commission, with contributions from the Scottish Government and the Horticulture Development Council.  Areas covered include: analysing the means by which the pathogen spreads; development of diagnostic methods to detect and identify the pathogen; determination of the risk posed to native flora; and disease management strategies.  The results of this research are used to underpin policy and to determine the most effective eradication measures.  For example, epidemiological investigations have helped us to understand the pathways by which P.  kernoviae spreads.   Research into diagnostic methods has already generated better detection techniques, including the development of an on-site diagnostic methods that are being used by PHSI and Forestry Commission field staff.  Studies into the control of the disease have earmarked disinfectants appropriate for use in decontamination procedures as well as nursery practices that can reduce risk.

Further details of Defra funded P. kernoviae and other projects can be found at: www.defra.gov.uk/science/default.htm

A summary of all the UK-funded research on both P. ramorum and P. kernoviae can be found at: www.defra.gov.uk/planth/science/resdec04.pdf

What should I do if I think I have observed an instance of Phytophthora kernoviae?

A number of other plant diseases and disorders can cause similar symptoms. Please consult the online information at: www.defra.gov.uk/planth/pkernovii3.htm.

If you continue to be concerned please contact:

• For suspect symptoms on plants

Defra Plant Health HQ on 01904 455174

or your local Defra Plant and Seeds Inspector in your local area, details available at: www.defra.gov.uk/planth/offices.pdf

• For suspect symptoms on trees

Forestry Commission Plant Health Service on 0131 314 6414