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ANNEX B: PHYTOPHTHORA KERNOVIAE: SCIENCE SUMMARY
Background
1. Phytophthora kernoviae is a recently-discovered, exotic, fungus-like plant 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. It is mainly found in woodlands, parks and managed gardens in the south-west of England and south Wales. 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.
2. The only other country where P. kernoviae has been reported from 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.
3. Whenever P. kernoviae is found in the GB it is currently subject to emergency phytosanitary measures, aimed at containment and eradication. More specifically, under the Plant Health ( Phytophthora kernovii Management Zone) England Order 2004, a defined area of the south-west of England was established in 2004 within which specific measures are taken aimed at containing and eradicating the pathogen within the zone. However, since that time new findings have been made some distance from the zone.
What kind of diseases are caused by Phytophthora kernoviae?
4. 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 dying back of affected parts.
Which shrubs and trees are affected by Phytophthora kernoviae?
5. 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. More detail is given below. 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.
What do we know about the biology of Phytophthora kernoviae?
6. 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 feet of humans and possibly 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.
7. Tree hosts only produce infective sporangia if the foliage becomes infected (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.
8. 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 invasive R. ponticum has become so widespread.
9. 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. However, no evidence has been found for the presence of oospores of P. kernoviae in the GB environment.
10. 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.
What damage is being caused by Phytophthora kernoviae in GB?
11. P. kernoviae was found for the first time in Scotland in January 2008 at an established garden site. In England and Wales, between October 2003 and January 2008 there were 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 areas of the country appear most favourable for the disease. Although the number of trees that have become infected is low ( ca.60), 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. Nurseries have 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.
What potential does Phytophthora kernoviae have?
12. Currently the pathogen is subject to an eradication/containment programme and so its full potential has not been realised. P. kernoviae may 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 has the potential to affect GB heathland environments but is yet to be found there; species of V accinium, a common heathland plant, could be at risk as laboratory experiments have shown these to be susceptible. Vaccinium myrtillus was found infected with P. kernoviae in woodland in the south-west of the UK in December 2007. This was the first record on Vaccinium in the UK. If the pathogen is not controlled it is not known whether or when the whole of GB would 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 management options are available to limit Phytophthora kernoviae?
13. To limit further spread of the pathogen into the environment it would be necessary to remove rhododendron and other foliar hosts that are significant inoculum sources from woodlands and managed gardens where the disease occurs, as well as those in the vicinity. Surveillance and monitoring of these sites would have to be continued for at least two years after clearance of the foliar hosts to monitor the management regime; regrowth of rhododendron should be targeted. Extension of the existing Phytophthora kernovii Management Zone could be considered, or the development of new management zones. Surveillance and testing of nursery stock in GB and entering GB, especially from New Zealand, would be necessary to prevent spread into the nursery trade and the potential for movement from nurseries to the natural environment or to managed gardens. Continued surveillance would also be required to determine the pathogen's GB distribution in the environment. Consideration of controls on imports of timber, especially from New Zealand would also be needed. Fungicides are not currently recommended as a control strategy for P. kernoviae. Although a range of fungicide active ingredients have activity against P. kernoviae, they have not been shown to be very effective as eradicant treatments. There are also concerns over whether use of fungicides on nurseries may mask symptoms on plants resulting in further spread of the pathogen in trade.
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