After honey fungus, Phytophthora root rot is the most common cause of root and stem base decay of a wide range of trees and shrubs. There are a number of different Phytophthora species, all causing very similar symptoms. Herbaceous perennials, bedding plants, pot plants and even bulbs can be affected, in addition to woody plants.
What is Phytophthora root rot?
Phytophthora species are microscopic, fungus-like organisms. The species causing root and stem base decay of plants are found within the soil, where they may survive for many years in the absence of a host plant. Phytophthora root rot is primarily a disease of heavy or waterlogged soils, and the symptoms can be very difficult to separate from those arising due to waterlogging itself.
A few Phytophthora species act primarily as foliar pathogens, spread by air-borne spores. Foliar diseases caused by Phytophthora are not discussed in this profile. Potato and tomato blight (Phytophthora infestans) is the disease of this type most likely to be encountered by gardeners.
Above-ground symptoms often do not develop until the root decay is well advanced. They are not specific to Phytophthora root rot, and merely indicate that the plant is having trouble taking up water and nutrients through a poorly-functioning root system. Other factors causing root problems, such as waterlogging, drought or other root diseases (e.g. honey fungus) will cause similar foliar symptoms.
Symptoms include wilting, yellow or sparse foliage and branch dieback. In many cases the symptoms get progressively worse until the plant dies. A common symptom in conifers is a gradual fading in the colour of the foliage, from a vibrant to a dull green, through to greyish and finally brown.
Below-ground examination of the roots, collar and stem base of an affected plant will reveal a poor root system. Many of the fine, feeder roots will have rotted away. Some or all of the larger roots will also show evidence of decay – they will be brown or black internally, softer than normal and may break easily. Because Phytophthora species are microscopic organisms there will be no evidence of the fungus itself in association with the decay, unlike honey fungus where a prominent white fungal growth may be found below the bark. Very similar root symptoms to those of Phytophthora infection can be caused by prolonged waterlogging, and the situation is further complicated by the fact that Phytophthora root rot is frequently associated with waterlogged soils. Laboratory examination is often required to determine whether root decay in these situations is due to waterlogging or Phytophthora root rot.
In severe infections Phytophthora invades the collar or stem base of the plant, causing a brown or black discoloration below the bark (often seen at the stem base as an inverted ‘V’). This area of infection is sometimes visible externally as bark discoloration and/or weeping, although once again such symptoms can be caused by other factors such as drought, waterlogging or pest attack.
Improving soil drainage can greatly reduce the risk of plants succumbing to the disease.
Where the disease is new or localised in the garden, affected plants should be destroyed and the soil from the root-run replaced with fresh topsoil. Replanting should be done with less susceptible species.
The following species are commonly affected by Phytophthora in the UK, and should be avoided where the disease has been a problem:
Chamaecyparis lawsoniana (especially ‘Elwoodii’)
Taxus (very susceptible)
In practice, the host range will vary between different Phytophthora species.
See the download for a more complete list of susceptible and resistant plants.
Phytophthora root rot – resistant and susceptible plants (Adobe Acrobat pdf)
No chemicals treatments are available to gardeners for the control of Phytophthora root rot.
Phytophthora spreads by tiny spores that swim in the water films between soil particles. These spores can travel only very short distances (a few millimetres) under their own power, but chemicals exuding into the water films from the roots of a susceptible plant may attract the spores toward them.
Phytophthora also produces long-lived resting spores that are released into the soil from the decaying roots. These can contaminate the soil in the vicinity of an affected plant for a considerable time, often several years.
Long-distance spread of Phytophthora may occur if the spores or soil containing them are carried in drainage or run-off water (e.g. down a slope). Movement of infested soil during cultivations or transplanting can also spread the problem. Bought-in plants carrying low levels of infection may introduce the disease into a garden.
Common name Phytophthora root rot
Scientific name Phytophthora species
Plants affected Various woody plants, esp. Taxus
Main symptoms Wilting, yellow or sparse foliage and branch dieback
Caused by A microscopic, fungus-like organisms
Timing All year