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RHS project team
Dr Béatrice Henricot
Start date
29/01/2011 00:00:00
End date
29/01/2013 14:25:24

Phytophthora ramorum, Phytophthora kernoviae, susceptible hosts, cultural controls, fungicides, replanting

The problem

Phytophthora ramorum and Phytophthora kernoviae are exotic plant pathogens that have only recently been described in the last decade.

In the UK the majority of findings of P. ramorum has previously been on nurseries, affecting container grown ornamental plants such as Rhododendron, Viburnum and Camellia.

Ornamental plants in gardens, amenity areas and woodlands in the UK can also be seriously affected, as can timber trees in woodlands, where hosts such as Rhododendron ponticum occur and drive epidemics. Heathland species, especially Vaccinium species (bilberry) have also been affected. More recently, there has been a significant new development, with P. ramorum now being found causing extensive mortality on Japanese larch.

Since 2003, P. kernoviae has been found on Rhododendron, Magnolia, Pieris and a number of trees (particularly beech), principally in the south-west and in south Wales, causing symptoms similar to P. ramorum. Vaccinium has also been found infected in a small number of heathland and woodland sites.

Statutory controls have been imposed because of the threat identified by Defra and the EC to trees and heathland shrubs. P. ramorum has caused extensive native tree loss in California.


Techniques being evaluated for management of P. ramorum and P. kernoviae include the use of trunk injection technology to treat infected tree specimens, and the use of pruning together with fungicide applications (to foliage or around stump bases) to protect susceptible plant re-growth. Treatments will be tested for their effects on P. ramorum and P. kernoviae surviving in soil and leaf litter, while mulches will be used to determine whether disease spread from the soil can be reduced.

Increased knowledge of P. ramorum and P. kernoviae will help to better target management strategies. For example, laboratory studies are being done to investigate the infection process for the two pathogens in a number of important plant genera (Rhododendron, Magnolia, Viburnum, Camellia and Pieris) commonly found in managed garden situations. Monitoring of susceptible plants at outbreak sites is providing information on the susceptibility of key host plants at different times of year, to determine optimum timings for control measures.

A desk study has commenced to produce an inventory of plant varieties, species and genera which are important in terms of their use in gardens and parks, for which there is no previous evidence of susceptibility to P. ramorum and P. kernoviae. Subsequent infection studies in a laboratory will be done to confirm whether identified species show tolerance to the two pathogens, and therefore have potential as species for replanting in situations of high disease pressure.

Benefits to gardeners

The project will provide new information on managing and reducing the spread of tree and shrub diseases caused by Phytophthora ramorum and P. kernoviae in historic gardens and parks.

This is especially important in gardens with valuable susceptible specimens, where tree and shrub removal, which is currently the only option, is undesirable. In addition, an inventory of plant varieties and species that show resistance to P. ramorum and P. kernoviae, will enable gardeners to replant even under conditions of high disease pressure.

Summary of results

Preliminary results
1) Control methods for disease management

Laboratory work on detached leaves has identified fungicides with protectant activity against P. ramorum and P. kernoviae. Subsequently, some fungicide product applications to Rhododendron and Pieris receiving natural infection by both pathogens gave a reduction in the area affected, particularly for Fubol Gold when five foliar sprays were given between June and September.

In the mulch trial, recording of the treatments (copper coated non-woven fabric, woven ground- cover, wool waste mats and bark) is to continue following periods with little rain and low potential for soil-splash. Soil treatments including heat-treated wood shavings, beneficial fungi, mustard meal and a chemical sterilant have also been used prior to re-planting and to date the greatest number of plots with soil remaining uninfested has followed use of the chemical.

2) Studies on the epidemiology of P. ramorum and P. kernoviae

In inoculation tests, Camellia and Magnolia were only infected by P. ramorum and P. kernoviae at high spore concentrations, compared with the infection of Rhododendron and Pieris. Viburnum only became infected by P. ramorum and this was confirmed in natural infection field tests. These field studies also showed that Rhododendron, Viburnum and Vaccinium are susceptible to infection by P. ramorum at all times of the year. In 2011, most plants showed infection by P. ramorum in February, March, May and September, whereas infection of Pieris only occurred between January and May. Camellia and Magnolia were confirmed to be the most resistant species to both pathogens. Rhododendron and Pieris were the most susceptible to P. kernoviae in both the laboratory and natural infection tests with, in 2010/2011, Pieris showing infection over most of the year while rhododendron infection was not seen between August and April. These studies will continue for a third year as weather conditions are likely to influence infections.

Further information

Fera profile on P. ramorum
Fera profile on P. kernoviae
Defra Project PH0604

Advisory information

Phytophthora root rot
Phytophthora ramorum and P. kernoviae
Video on P. ramorum and P. kernoviae
Advice on Phytophthora bleeding canker
Holly leaf blight
Advice on potato and tomato blight


Henricot B and Prior C (2004). Phytophthora ramorum, the cause of sudden oak death or ramorum leaf blight and dieback. Mycologist 18 (4): 151-156.

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