Phytophthora Root Rot
Phytophthora is a genus of soil-borne fungi which cause root rot of herbaceous and woody plants. Some of them cause Damping Off disease and P. infestans has been around for over 150 years causing potato and tomato blight. An strain of P. ramorum has been found in England, it is different to the American form which causes Sudden Oak Death. No trees are known to have died on this side of the Atlantic so the alternate name Ramorum Dieback, may be more appropriate - also the oaks are a different species. Rhododendrons, camellias and viburnums are the affected plants in Europe, the disease has been known in the Low Countries for a decade. The oak species here appear to be more resistant, but imports of wood products from California and Oregon have been banned as a precaution.(PLANT HEALTH The Plant Health (Forestry) (Phytophthora ramorum) (Great Britain) (No. 2) Order 2002)
Horse Chestnut trees are now showing signs of this disease and have been turning brown prematurely.
On affected trees cracking or bleeding cankers occur at the base of the trunk or higher up, where complete girdling leads to rapid death. The symptoms on viburnums are wilted stems leading to death, signs are seen at the base. Rhododendrons develop a twig and leaf blight - twigs become brown to black, usually starting at the tip and moving towards the base; the leaves show dark brown blotches.
A secondary infection by a bacterium has been found in the oozing sap and could be entering the tree to cause its death. Removing the oozate may help to prevent this damage.Phytophthora cinnamomi has been identified in the Antipodes where it attacks woody plants including Azalea, Chamaecyparis, dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, Pieris, Rhododendron, Taxus and white pine. It requires higher temperatures (15 to 28 C) for growth than P. ramorum (2 to 28 C). Mild symptoms are reduced size of foliage, dead feeder roots and dark streaks up the stem wood. In severe cases the entire plant becomes stunted with no new shoots being produced; there is a reddish-brown discolouration of wood at the soil line and in the roots.
The spores favour soil moisture at just below saturation level. Fruiting bodies take 4 to 6 hours to develop and they can start releasing asexual spores 10 to 60 minutes later. If these zoospores are washed into the soil they infect the feeder roots just behind the root cap (growing tip). Soil pH does not affect development.
The spores overwinter in the soil and can be moved around in run-off water from field sites or from potted plants carrying them. The zoospores are carried down in irrigation water and can be taken away in wind-blown rain. The movement of infected soil to another site on plant roots, as landfill, on vehicle tyres or on footwear, are all means of spreading this disease.There are no treatments to clear up infected plants so they should be removed and destroyed. If practical the soil can be fumigated to a depth of 15cm before replanting. There are some fungicides which can be used on healthy plants as a preventitive.
As yet there is no evidence that P. ramorum is established here so prevention is the main course of action; it is important to purchase only healthy plants. Planting too deeply with the soil line more than 2.5cm over the upper roots can create conditions favourable to infection.Back to article on Plant Diseases