The latest on Phytophthora ramorum and kernoviae
The UK government announced recently that £25 million will be spent over the next five years on activities in England and Wales in relation to the detection and control of the notifiable plant pathogens Phytophthora ramorum and Phytophthora kernoviae. This increased activity is being taken ‘with a view to reducing disease levels to epidemiologically insignificant levels’. What was the thinking behind this decision, and what threat do these organisms pose?
The problem
Since the mid-1990’s Phytophthora ramorum has caused the death of many thousands of tanoaks (Lithocarpus densiflorus) and oaks (Quercus species) in parts of the USA. The disease was given the name in America of ‘sudden oak death’. The pathogen was soon found in Europe, with the first UK case confirmed in 2002. Numerous outbreaks have occurred on nurseries and in parks, gardens, woodlands and heathland. The majority of European outbreaks have been on shrubs such as Rhododendron and Viburnum species, and in Europe the disease is referred to more commonly as ‘ramorum dieback’. Native UK oaks have more resistance to the disease than their American cousins, but there have been cases in the UK and Europe on a range of tree species, with beech (Fagus sylvatica) appearing most susceptible.
Phytophthora kernoviae was discovered in 2003 during surveys for P. ramorum, and causes very similar symptoms. It has now been found dozens of times in England (mainly in the south-west) and Wales, with cases also confirmed in Scotland and Ireland.
The symptoms
On shrubs such as Rhododendron, Camellia and Pieris brown lesions, without obvious fungal growth, develop on the leaves. They may originate at the leaf tip, petiole or margin and often spread most rapidly down the midrib to give a v-shaped effect. Lesions may spread into twigs and branches to cause dieback. In Viburnum a common symptom consists of lesions girdling the stem base to cause wilting and death of the plant.
Some affected trees (e.g. beech) develop patches of dead bark that weep a brown or blackish liquid. These ‘bleeding cankers’ may increase in size over time and can lead ultimately to the death of the tree. It should be noted, however, that similar bark symptoms can be caused by other pests, diseases or environmental problems. In species such as holm oak (Quercus ilex) and ash (Fraxinus excelsior) infection is confined to the leaves.
The pathogens
Phytophthora ramorum and P. kernoviae are microscopic, fungus-like organisms closely related to the pathogen causing potato blight (P. infestans). Spores produced on leaf lesions on shrubs such as Rhododendron are splashed around to create new infections. Lesions that develop on the bark of trees are invariably caused by spores arising from infected under-storey plants, particularly Rhododendron ponticum.
A long-lived resting spore is also produced that can contaminate the soil on affected sites. The pathogens can be moved around with contaminated soil or water, but most long-distance spread of the problem occurs as a result of the transport of infected plants.
EU control methods
The current EU minimum level of control requires surveys of ‘cultivated plants’, and measures to eradicate P. ramorum where it is found. Surveys of ‘uncultivated’ or’ unmanaged’ plants are also required – here action must be taken to at least contain P. ramorum where it is found.
Where P. ramorum is found on cultivated plants all susceptible plants within a 2m area must be destroyed, and all plants within 10m monitored for three months. Certain susceptible plants such as Rhododendron, Viburnum and Camellia require a Plant Passport to be moved within and between EU states.
There are currently no EU-wide control measures for outbreaks of P. kernoviae, as this problem is at present confined to the UK. All EU measures for these pathogens are currently under review.
Additional UK measures
The UK plant health authorities have been applying the EU control measures outlined above, but with additional inspections, surveys and controls, aiming for the eradication of both P. ramorum and P. kernoviae. However, P. ramorum has now been found at well over 700 sites, in England, Wales, Scotland, Northern Ireland and the Channel Islands.
A recent scientific analysis, combined with an extensive consultation process involving interested parties (including the horticultural industry, research and consultancy organisations and charitable bodies such as the Royal Horticultural Society and the National Trust), led to the decision to increase funding in England and Wales to tackle the pathogens (a separate consultation process is being undertaken by the Scottish government). The threats posed to the horticultural industry, historic parks and gardens, tourism, and woodland and heathland ecosystems are all thought to be of great significance.
The extra funding will allow for increased nursery inspections, surveillance, diagnostic testing and public awareness programmes. There will also be considerable expenditure incurred in the clearance of Rhododendron ponticum from affected sites, although in some parts of the UK funding is already available for the removal of this alien invasive species.
Tests available from Pocket Diagnostic™
The Phytophthora Pocket Diagnostic™ test can be used to detect a range of Phytophthora species, including P. ramorum and P. kernoviae. As this test is not species-specific it will also react to other Phytophthora species.
The presence of P. ramorum or P. kernoviae can be confirmed using the PDplus™ service. This uses the DNA from the positive test line on the Pocket Diagnostic test to confirm the exact identity of the pathogen.
As P. ramorum and P. kernoviae are notifiable pathogens in Europe, any positive results must be reported to the relevant plant health authority.