Fork It - Sudden Oak Death
SOD. For years, gardeners would have used this word to describe the earth beneath their feet. Or else as an expletive when they missed said earth and speared their feet with the fork! However, there is now a very worrying meaning that haunts gardeners, as it is an acronym for Sudden Oak Death, which is causing devastation and not only on oaks.
The culprit is phytophthora ramorum, which can be carried by many different host plants, with rhododendron and viburnum the chief culprits for spreading the spores. But whereas in these shrubs the pathogen causes the less serious “ramorum dieback” – with symptoms of stem dieback and dark spots on foliage – on trees it causes bleeding cankers on the trunk and foliage dieback, often with lethal results on younger trees.
In the US, it has devastated swathes of their native oaks, but the strain affecting British plants is thought to have come from Europe or Asia, via an imported shrub at a Sussex nursery in 2002. In 2003, it seemed to have been contained, but in 2009 it jumped species and was found on Japanese and European larch. These are both important commercial species, but as they are grown in the intensive monocultures of forestry plantations the disease can spread like wildfire. Last year, carried by wind or rain, the spores crossed the Bristol Channel to South Wales and SOD has caused huge amounts of damage in Forestry Commission woodlands near Port Talbot and Bridgend.
The organism is from the same family as potato blight and there is no treatment available, just preventative measures such as early detection and proper disposal of infected plant material. We have a line of 20 oaks bordering our field, which are probably about 40 years’ old and I have not detected any signs of the disease, but I am on my guard!
The same cannot be said for the dozen horse chestnuts that line the road, at the edge of our orchard. Sadly, some of them do have bleeding canker, although in this case it is caused by the bacterium pseudomonas syringae pv aesculi. Suggested controls are the removal of the cankered branches and treatment of the wounds with wound paint, plus a spray of the foliage with Bordeaux mixture or a copper fungicide in late summer. The problem is that the orchard has certified organic status, so I shall have to check with the Soil Association if there are any permissible sprays available. Along with many horse chestnuts in the South of Britain, ours are also infected with cameraria ohridella. This leaf-mining moth, first spotted in Wimbledon in 2002, severely damages leaves, causing them to shrivel, turn brown and fall early. It is not lethal and the trees develop a full canopy of leaves the following year, but it is an aesthetic problem, as the leaves can start to brown as early as July. As the moth pupae overwinter in the leaf litter, removal and burning of the fallen leaves in the autumn will reduce the infestation in spring, giving the leaves a chance to get going, although it is impossible to stop mature moths flying onto the tree later in the season. A biological control will be possible only when they have established the country where the moth originated, as this will hold the clue to its natural predators.
For those who remember the devastation wrought by Dutch elm disease, these are worrying times. Are we facing a future where the British landscape will be changed forever, bereft of the mighty oak and dotted with sickly-looking horse chestnuts?
