Advice
RHS Help & Advice
Phytophthora
The phytophthoras are a group of soil- and water-borne fungus-like pathogens, which are a common cause of death in a variety of trees and shrubs. Phytophthora species can attack plants at or below ground level, causing crown and root rots. Phytophthora is spread by water, or in contaminated soil.
How Phytophthora affects the garden
Introduced to the garden in various ways - on infected plants, in irrigation water, in soil or windblown dust - the fungus may remain dormant in the soil for many years. Most often during wet weather or on poorly drained soils, infection may occur through the roots of susceptible plants, which are killed. The foliage shows symptoms only when the root infection is well advanced, with the production of small, sparse and yellow leaves and eventually some die-back. Infected roots are black or dark brown, and fine roots are absent. Secondary infection of dead tissues by other fungi is common.
Symptoms in detail
Above-ground symptoms of plants infected with Phytophthora include dull foliage (in conifers), wilting, yellow or sparse foliage and branch die-back, followed in many cases by whole plant death. Below-ground examination of the roots, collar and stem base reveals a poor root system. Fine root infection can cause the roots to snap, resulting in black discolouration of the root tips and a reduced root system. Larger roots and collar infections exhibit dark brown or black lesions on peeling back the bark. This discolouration can extend up the plant and be seen at the stem base as an inverted ‘V’. The dark areas normally have a clearly defined division between healthy and infected tissue. This is seen beneath the bark, but on severe infections of woody plants, oozing and black markings can be seen on the surface near the stem base. When infections are at or near the stem collar, Phytophthora can cause girdling, resulting in plant death. Although primarily a root pathogen spores can be carried higher up plants and can cause bleeding cankers, twig die-back (as seen with the new species Phytophthora ramorum and P. kernoviae) and fruit rot. All these infection show the same black discolouration and lesions with defined edges.
What to do about Phytophthora
No chemical treatments are available for amateur gardeners to control this disease. Phytophthora, being water or soilborne is encouraged by poorly drained and waterlogged soils, so improvement of drainage can greatly reduce occurrence.
Where the disease is new or localised in a garden, affected plants should be destroyed and the soil from the root-run should be replaced with fresh topsoil, or treated by a contractor with a chemical soil sterilant. There are no soil sterilant products available for amateur use. Replanting should be done with less susceptible species, which will also benefit from measures to improve soil drainage.
Susceptible plants
The species most commonly affected by Phytophthora in the British Isles are:
Acer
Aesculus hippocastanum
Calluna
Camellia
Castanea sativa
Chamaecyparis lawsoniana
Erica
Eucalyptus
Fagus sylvatica
Hamamelis
Hebe pinguifolia 'Pagei'
Malus
Nothofagus
Pieris
Prunus
Rhododendron
Rubus idaeus
Syringa
Taxus baccata
Tilia
Potato and tomato blight
Perhaps the best known disease caused by Phytophthora is potato blight and tomato blight. This is one of the few species of Phytophthora which is wind dispersed and can attack the leaves.
Current work on Phytophthora at Wisley
Find out more about investigation into Phytophthora being carried out at RHS Garden Wisley