« Previous | Contents | Next »
Listen
ANNEX A: PHYTOPHTHORA RAMORUM: SCIENCE SUMMARY
Background
1. Phytophthora ramorum is an exotic fungus-like plant pathogen which causes damage to trees, shrubs and other plants. Since the early 1990's, it has caused widespread death of millions of trees in forest environments in coastal California and Oregon in the USA. Because the most commonly affected trees that have been killed are tanoaks (not true oaks) as well as several true oak species, this extensive phenomenon is commonly known as 'Sudden Oak Death'. The pathogen was first found in the nursery trade in the USA/Canada in 2003.
2. Between October 2003 and December 2007, 27 trees became affected with bleeding cankers in GB and at least another 14 trees with these symptoms over the same time period in the Netherlands. In Britain, one of the affected trees (the first) was in the south-east, with most of the remainder in the south-west of England. Recently (October 2007) a beech tree has been found with a bleeding canker in a historic garden in West Yorkshire, and around the same time a red oak was diagnosed with the disease in northern Ireland. All the diseased trees have been close to large numbers of infected rhododendron.It is thought that P. ramorum was introduced separately to North America and Europe from an unknown origin (or origins), speculated to be Asia, possibly Yunnan, Taiwan or the eastern Himalayas.
3. P. ramorum is subject to emergency phytosanitary measures in the EU. Here, including GB, measures are aimed at eradication on nurseries and eradication or containment in managed gardens and woodlands.
What kind of diseases are caused by Phytophthora ramorum?
4. P. ramorum causes three main types of disease. 'Ramorum 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. This is referred to as 'Sudden Oak Death' in the USA. 'Ramorum leaf blight' refers to infection of the foliage of trees, shrubs and some herbaceous plants that leads to discoloured lesions on the leaves. 'Ramorum dieback' refers to leaf and shoot/stem infections which result in wilting and dying back of affected parts.
Which plants are affected by Phytophthora ramorum?
5. P. ramorum has a very wide natural host range. Currently numerous species in 70 host genera are affected, representing 33 different families. The types of hosts that are affected varies between countries. In GB, the majority of nursery hosts are species of rhododendron, viburnum and camellia. 26 trees have been confirmed exhibiting bleeding cankers in GB, these are mainly European beech, but individual trees of horse chestnut, sweet chestnut, several oak species, sycamore, southern beech and Schima have also become affected. In the Netherlands, the trees with bleeding cankers are European beech and northern red oak.
What do we know about the biology of Phytophthora ramorum?
6. P. ramorum has an optimum temperature for growth of 20°C and a requirement for moisture; it is therefore well suited to a cool-temperate climate. It produces sporangia (containing infective motile zoospores) on the leaves and shoots of a wide range of plants; these are known as sporulating hosts. These sporangia are mostly spread locally over short distances during rain. P. ramorum 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 to contaminate and persist in watercourses at infected sites but it is not known whether this can lead to new infections of plants. Long-distance spread is primarily by movement of infected plant material.
7. Tree hosts only produce infective sporangia from infected foliage. Some tree species only develop bleeding cankers; these do not produce sporangia and so are not a source of infection for themselves or for other host species. These trees become infected as a result of being in the proximity of sporulating hosts. In GB, all of the trees that have developed bleeding cankers have been adjacent to, or very close to, infected rhododendron, particularly Rhododendron ponticum.
8. Sporulating hosts vary in the amount of infective sporangia that they produce. Although rhododendron produces less sporangia than some of the sporulating hosts in woodlands in California and Oregon (predominantly California bay laurel and tanoak), it is the most abundant sporulating host in GB woodlands, especially now R. ponticum has become widespread. It has the potential to produce inoculum all year round within Great Britain.
9. Experiments in historic gardens have shown that complete removal of infected rhododendron and other foliar hosts can prevent new plant infections. In one garden, no new plant infections have been recorded four years after all the infected plants were removed. It can still be detected in soil and watercourses; however, the level of residual inoculum appears to be epidemiologically insignificant, at least with respect to trees. Regrowth of rhododendron shoots has become infected. In soil, the pathogen is most likely surviving in the form of a robust spore known as a chlamydospore. This type of spore maintains the pathogen in the GB environment for at least two years.
10. Chlamydospores are large, thick-walled spores which have a major role in survival. They are produced asexually in infected leaf and (possibly) shoot tissue; they are also reported to occur in bark phloem and xylem tissue of tanoaks in the USA. The tissue in which these are formed can vary with the host; they can also be formed on mycelium growing out of leaf lesions but are not as readily detached as sporangia. Chlamydospores formed within rhododendron leaves are smaller with thicker walls than those formed in the laboratory. Chlamydospores typically germinate to produce hyphae and sporangia.
11. Examination of bleeding cankers has shown that P. ramorum can be found extending up to 25mm into the wood of some tree species and can survive there for at least 27 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.
12. P. ramorum has the potential to reproduce sexually but this depends upon the presence of two mating types, A1 and A2. European/ GB isolates are mainly A1 and North American isolates A2. A few European A2 isolates have been found in Belgium and a few European A1 isolates in North America. However, to date, sexually produced spores (oospores) have not been observed naturally and there is evidence to suggest that the mating system is not fully functional. Oospores have two roles: one is to increase the genetic diversity of the population, which may then lead to changes in behaviour; the second is a survival role, since these are thick-walled and potentially longer-lived spores than sporangia. European and North American populations differ: some European isolates tested are generally more aggressive towards plants than the North American isolates. Asexual recombination also has the potential to occur between the two populations; this may also change the behaviour of the pathogen. There is evidence that the US population is more genetically diverse than the European population. For these reasons we are trying to prevent entry of non-European isolates to GB/ EU.
What damage is being caused by Phytophthora ramorum in GB?
13. In Scotland, between April 2002 and January 2008, there were 42 outbreaks at 31 sites; these were mainly on nurseries or garden centres except one outdoor finding (garden/landscape) in 2002, two landscape sites in 2007 and 4 outbreaks in established gardens to January 2008. No findings occurred in 2006 and so it was thought that the pathogen was eradicated. However, in 2007 to January 2008, two new nursery findings were made in addition to the four outdoor finds.
14. In England and Wales, between April 2002 and February 2008 there were 217 outbreaks at 198 sites in locations other than nurseries, of which 65 have been eradicated. These woodland or garden/park sites are fairly widely distributed but the highest incidence and severity of disease has been in the south and west of England and in south Wales; these areas of the country appear more favourable for the disease since they are mild and wet. Although the number of trees that have developed bleeding cankers is low (around 26), it is increasing; a few trees have also been infected outside of the SW. Ornamental plants in historic gardens involved in tourism have been badly affected by the pathogen and some rare or historically-important specimens or collections are now considered to be at risk. Visitors to some historic gardens have complained about the appearance of the plants; gardens which rely on spring-flowering rhododendrons and camellias to attract visitors have been most affected. Nurseries have mainly been affected by the phytosanitary measures that have been implemented to try to prevent spread to the environment. Between April 2002 and January 2008 in England and Wales there were 576 nursery outbreaks at 488 sites of which 464 have been eradicated.
What potential does Phytophthora ramorum have?
15. In GB/ EU the pathogen is subject to an eradication/containment programme and so its full potential in terms of impact on the environment has not been realised. An epidemic on the scale of California has not yet occurred. The geography, climate, hosts and mixture of hosts in California and Oregon is different to GB and favours disease development. In California, California bay laurel, tanoak and Pacific rhododendron are the main sporulating hosts in affected woodlands. In GB, evergreen rhododendron (especially R. ponticum) is the main sporulating host that will drive woodland epidemics, with beech and some oak species being particularly threatened. In California, sporulation is seasonal with little or no sporulation in hot dry summers. Under GB conditions, the pathogen can produce spores all year round on rhododendron. P. ramorum may be particularly damaging in GB 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. ramorum has the potential to affect GB heathland environments but is yet to be found there; species of Vaccinium, a common heathland plant, could be at risk as laboratory experiments have shown these to be susceptible and V. myrtillus was recently found infected (with P. kernoviae) in woodland in Cornwall. The longer-term risk may increase if additional non-European isolates are introduced or if climatic conditions become more favourable for the pathogen. If the pathogen was not controlled it is not yet clear when or whether the whole of GB would become affected. However, in the absence of existing measures, potential spread into and within the environment is predicted 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 ramorum?
16. 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 historic gardens where the disease occurs, as well as those in the vicinity. Surveillance and monitoring of these sites would need 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. Surveillance and testing of nursery stock in GB and entering GB, especially from North America and the rest of Europe would be necessary to limit further spread in the nursery trade and the continued potential for movement from nurseries to the natural environment or to historic gardens. Further surveillance would also be required to determine the pathogen's distribution outside nurseries. Consideration of further controls on imports of timber, especially from the USA, might also be needed. Fungicides are not currently recommended as a control strategy for P. ramorum. Although a range of fungicidal active ingredients have activity against P. ramorum, 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, which might result in further spread of the pathogen in trade.
« Previous | Contents | Next »