Phytophthora kernoviae

An invasive pathogen causing bleeding cankers on beech tree trunks, necrosis on leaves of rhododendrons, pieris and magnolias, and extensive dieback of bilbery.

Disease overview

The novel species Phytophthora kernoviae was first isolated from beech and rhododendron by scientists from Forest Research and the Food and Environment Research Agency (Fera) in 2003.

Our research into this highly aggressive pathogen shows it attacks leaves, buds and shoots of understorey rhododendron in woodlands, as well as ornamental garden plant species.  Woodland trees such as beech are also at risk of infection if growing within a few metres of affected rhododendron because of the number of spores released from infected foliage of this shrub.

This resource provides information from our research about the distribution and spread of this infection. Our guide to symptoms will help you to identify infected rhododendrons so you can take precautionary action to destroy these plants.

Disease details

  • Fungus-like pathogen that affects the aerial parts of the trees and shrubs
  • Causes leaf necrosis and/or stem dieback in shrubs and bushes and some tree species
  • Principal host is rhododendron, mainly R. ponticum
  • Bilberry (V. myrtillus) is highly susceptible and suffers extensive dieback
  • Attacks the inner bark of European beech (Fagus sylvatica) and very occasionally English oak (Quercus robur) leading to bleeding cankers
  • Our research suggests only trees in close proximity to affected shrub vegetation such as rhododendron are at risk from infection, and that trees are not contagious and can even recover from infection  
  • Several garden ornamentals such as Magnolia, Pieris and Drimys species can suffer from leaf and shoot blight

Incidence and ditribution

  • Found mainly in south-west England sites, where it is particularly aggressive and virulent
  • Occasionally found in other locations including Wales and Scotland , typically in individual plants or outbreaks in rhododendron
  • Infecting thousands of rhododendrons and heathland bilberry plants in Cornwall
  • Spread to a variety of woodland tree species (especially beech) and ornamental plants, usually found in close proximity to infected rhododendron

Latest distribution map .

Full list of plants and trees reported with P. kernoviae infection


  • Leaf necrosis and/or shoot dieback in susceptible  plants and shrubs
  • Bleeding cankers in trees caused by necrosis of inner bark tissue
  • Leaf spot in many Magnolia species

These can also be general symptoms caused by some other plant pathogens, including the more widespread Phytophthora ramorum in rhododendron. Please consult our symptoms and diagnosis page  for help and advice on identifying P. kernoviae infections in plants and trees. A fact sheet is also available from FERA.

Our Tree Health Diagnostic and Advisory Service offers laboratory-based infection diagnosis. Phytophthora screening provides an accurate diagnosis to support your management decisions and obligations.

Disease management

Access the Forestry Commission’s P. kernoviae pages for advice on action being taken to prevent and control disease outbreaks.

As a precautionary measure to prevent or limit the spread of disease, you should consider removing and destroying all infected host plants, following P. ramorum management procedures. Please consult the Forestry Commission’s P. ramorum pages for more details.

Our research

Our scientists first isolated, identified and characterised Phytophthora kernoviae as a new species in 2003. Our research, funded by Defra and the Forestry Commission’s Phytophthora Programme has investigated how the organism spreads, the process of infection and disease management strategies.

Our work covers:

  • Effective diagnosis and detection of Phytophthora pathogens
  • Distribution and the impact of recently introduced Phytophthoras, such as Phytophthora ramorum, P. kernoviae and P. lateralis, to trees in Britain
  • Susceptibility of forest and woodland tree species to P. ramorum and P. kernoviae
  • Extent of variation in some of the recently arrived Phytophthora species to identify the possible origins, entry pathways and potential for genetic change
  • Understanding the infection process and disease development in relation to key variables (e.g. host availability, inoculum production and climate)
  • How Phytophthora pathogens spread in natural and seminatural environments
  • Assessment of chemical treatments of chemical treatments
  • Steps toward integrated management of P. kernoviae and P. ramorum on magnolias in British heritage gardens

We are among the leading research institutes looking into Phytophthora austrocedri on juniper, Phytophthora on alder, Phytophthora lateralis and Phytophthora ramorum, the cause of sudden larch death in the UK.

Related resources 


For further information contact:

Dr Joan Webber