Phytophthora kernoviae: addition to the EPPO Alert List
During surveys done on Phytophthora ramorum (EPPO Alert List) in United Kingdom, a new and unknown Phytophthora species was isolated in Cornwall from dying rhododendrons and Fagus sylvatica (beech). The pathogen was then described and named Phytophthora kernoviae sp. nov. (Brasier et al., 2005). This pathogen causes symptoms similar to those of P. ramorum. On mature F. sylvatica, P. kernoviae causes bleeding stem lesions. On rhododendron (particularly R. ponticum), it causes foliar and stem necrosis. Since the first discoveries, other forestry or ornamental hosts have also been identified. Considering the risk that this pathogen may present to both the forestry and nursery industries, the EPPO Secretariat decided to add it to the Alert List.
Why: In late autumn 2003, during surveys on Phytophthora ramorum, an unknown Phytophthora species was isolated in Cornwall (GB) from rhododendrons showing leaf and stem necrosis in a woodland area near a commercial nursery. The same pathogen was isolated at another site from a large bleeding canker on a mature Fagus sylvatica and from nearby rhododendrons with foliar necrosis and shoot dieback symptoms. The pathogen was initially referred to as Phytophthora taxon C and then described as a new species called Phytophthora kernoviae (from ‘Kernow’, the Cornish noun for Cornwall - Brasier et al., 2005). It is morphologically distinct from other known Phytophthora species. In phylogenetic studies (comparison of ITS rDNA sequences), its closest relative was P. boehmeriae. It is hypothetized that it could be an exotic species of recent introduction.
EPPO region: United Kingdom (England and Wales).
So far, P. kernoviae has not been reported from other countries. P. kernoviae has been found at a few sites (24 sites as of September 2005) in England and Wales, with most findings on rhododendron in small areas of woodland in Cornwall. Diseased Fagus trees were found in woods dominated by rhododendrons. There have been limited findings on rhododendron bushes in South Wales and on a commercial nursery in the North West of England (Cheshire). Eradication measures are being taken in UK (P. kernoviae is there a ‘notifiable pest’). In the nursery found contaminated, all infected plants have been destroyed and the outbreak is considered eradicated. In infected woodlands, rhododendrons are eliminated to contain the disease.
On which plants: Mainly Fagus sylvatica (Fagaceae) and Rhododendron spp. (notably R. ponticum - Ericaceae), but also found on other plant species: Drimys winteri (Winteraceae), Gevuina avellana (Proteaceae), Liriodendron tulipifera (Magnoliaceae), Magnolia spp. (Magnoliaceae), Michelia doltsopa (Magnoliaceae), Pieris formosa (Ericaceae), Quercus ilex (Fagaceae), Quercus robur (Fagaceae). Full host range is not known and needs to be further investigated. For example, studies are currently being done on the susceptibility of heathland species in UK.
Damage: As for P. ramorum, two different types of symptoms are observed: bleeding cankers and leaf lesions.
On F. sylvatica, Q. robur and L. tulipifera, bark necrosis and bleeding lesions are observed. Lesions often develop into sunken and bleeding cankers (gummy brown to black ooze). Cankers size can range from a few centimetres to large lesions (> 3 m).
On Rhododendron, shoot dieback, foliar necrosis and wilting are observed. In severe infection, bush may be killed. Similar foliar necrotic lesions are observed on other ornamental host species. Leaf and shoot dieback are observed on Q. ilex.
Symptoms can be viewed on Internet: http://www.defra.gov.uk/planth/pkernovii3.htm
Dissemination: Caduceus sporangia (containing zoospores) can most probably be spread locally by water splash or in airborne mist droplets. Under suitable conditions, asexual reproduction takes place and new sporangia are being produced. Oospores (sexual reproduction) have been produced by P. kernoviae in the laboratory but have not been observed in naturally infected plants. Chlamydospores (ensuring survival under adverse conditions) have never been observed nor in the laboratory or in the field. Further studies are still needed on the biology and epidemiology of P. kernoviae. Long distance spread can be ensured by movement of infected plants of rhododendron, beech and other hosts (it is thought that isolated occurrences of P. kernoviae in south Wales and Cheshire may reflect its further spread via plant trade). P. kernoviae is apparently not a root pathogen, but it can be isolated from soil. Movements of soil (or litter and plant debris) could probably spread the disease.
Pathway: Plants for planting, cut branches, soil, wood? (apparently no sporulation has been observed on mature bark lesions).
Possible risks: In the EPPO region, F. sylvatica is an important forest tree, also planted for amenity purposes. Rhododendrons are commonly grown as ornamentals in parks and gardens, although R. ponticum is considered as an invasive plant in woodlands (EPPO list of invasive alien plants). Q. ilex is more important for the Mediterranean area. The other ornamental species which are hosts of the pathogen are also valuable trees or shrubs. P. kernoviae appears more virulent on some hosts than P. ramorum. It has killed some established R. ponticum and apparently caused lethal cankers on F. sylvatica. So far, control measures are essentially based on the destruction of infected plants, and in particular of rhododendrons in infected woodlands. More studies are needed on possible control measures. Finally, from a more fundamental point of view, the potential risk of lateral transfer of genes from other Phytophthora needs to be further investigated. Like P. ramorum, P. kernoviae represents a risk to both the forestry and nursery industries.
EPPO RS 2005/164
Panel review date - Entry date 2005-10
Brasier CM, Beales PA, Kirk SA, Denman S, Rose J (2005) Phytophthora kernoviae sp. nov., an invasive pathogen causing bleeding stem lesions on forest trees and foliar necrosis of ornamentals in Britain. Mycological Research, 109(8), 853-859.
Phytophthora kernoviae. A new threat to our trees and woodlands. http://www.defra.gov.uk/planth/pestnote/kern.pdf
Pest Risk Analysis for P. kernoviae (2005-02). http://www.defra.gov.uk/planth/pra/forest.pdf
Host plants of P. kernoviae. http://www.defra.gov.uk/planth/kernovii/kernhost.pdf
P. kernoviae - Latest findings (2005-09). http://www.defra.gov.uk/planth/pkernovii2.htm
Forestry Commission website. Phytophthora kernoviae. http://www.forestry.gov.uk/forestry/infd-66jlgb
BBA website. BBA factsheet (in German) by Dr S. Werres. http://www.bba.de/inst/g/pkernoviae/p_kernoviae.pdf