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.
Phytophthora ramorum causes a bleeding canker. A strain of P. ramorum which causes Sudden Oak Death disease in North America, has been found in the United Kingdom, although it is different to the American form. The two main Oak species here appear to be more resistant, although trees standing very close to infected shrubs can pick up the disease - the alternate name Ramorum Dieback, may be more appropriate. The two affected American species, Tanoak and American Oak, are not true oaks.
Horse Chestnut trees are now showing signs of this disease and have been turning brown prematurely. Also Beech and Ash. It has also been found in a couple of conifers, Japanese Larch (Larix kaempferi) and Western Hemlock (Tsuga heterophylla) in Cornwall, Devon and Somerset.
Rhododendrons, camellias and viburnums are the mainly affected plants in Europe. P. ramorum was first found in Great Britain in 2002, and since then a number of outbreaks have been detected in England, Scotland and Wales. In January 2009 infected Rhododendron ponticum bushes in the New Forest have caused an exclusion zone to be declared to prevent spread.
There have been several outbreaks in Northern Ireland affecting Japanese Larch trees. In August 2010 it was reported that two Northern Ireland Forestry Service sites at Ballyboley and Woodburn near Broughshane in County Antrim have had signs of the disease and a nearby wood owned by Lord Rathcavan has had the disease confirmed, so the eight acres of woods are to be felled and destroyed. In total about 100,000 trees are involved in these outbreaks.
On affected trees cracking of the bark occurs at the base of the trunk or higher up, where complete girdling leads to rapid death. There is a weeping of sap from the lesions. The symptoms on viburnums are wilted stems leading to death, signs are seen on branches near to 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.
The spores favour soil moisture at just below saturation level at temperatures between 2 and 28°C. 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 preventative.
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.
Infected shrubs will be producing the spores which are blown around to infect trees, so to reduce the risk, there are proposals to increase the spending on programs to remove Rhododendron ponticum which has become naturalised in some areas.
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)
Another species, 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. The recently discovered Wollemi Pine (Wollemia nobilis) is also susceptible.
It requires higher temperatures of 15 to 28°C for growth. 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.
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