Being the festive season with mistletoe decorations and mistletoe kisses, this strange plant seemed a suitable subject for a blog, so here are some facts, superstitions and folklore...
As a plant without obvious roots or sources of food, it was once thought entirely magical and credited with extraordinary powers - giving rise to its use in medicine and a rich history of folklore. In the Middle Ages it was thought to break the death-like trances of epileptics, dispel tumours, divine treasure, keep witches at bay, and protect the crop of the trees on which it grew. Its use in the treatment of epilpsy continued into the 17th century and, as recently as 1993, a sixty-six year old man told of how, as a boy, he was given mistletoe by a gypsy woman to treat his epileptic fits, and that he had had no seizures since. I have seen mistletoe most commonly on Limes and Apples, but also on Hawthorn, Poplar, Willow, Rowan and False Acacia. It will undoubtedly grow on many other species but Pliny refers to its rarity on Oak, making 'Oak Mistletoe' highly valued for its 'powers' - Oliver Rackham questions if it is ever found on ancient Oak.
Enough myth and folklore - now for something completely different - a few mistletoe facts:
Mistletoe is in the Order Santales which includes the valuable and scented Sandalwood tree. The Order contains many epiphytes, parasites and hemi-parasites - including the Sandalwood tree itself which, when young at least, taps into the roots of a variety of host trees. Foresters growing Sandalwood in plantations will often use Acacia to start them off. Mistletoe is termed a hemi-parasite because its green leaves manufacture sugars by photosynthesis and it draws only water and mineral nutrients from the host.
There are over seventy species of Mistletoe worldwide, the European Mistletoe being the only one native to Britain. It prefers a mild, humid climate and good numbers of suitable host trees and is particularly common in a wide circle of land around the Severn estuary, where the valleys are moist and there is a tradition of fruit growing.
Whilst host trees can often tolerate some uninvited mistletoe, it can also be harmful, certainly killing branches and even whole trees if the 'infection' is heavy enough. Trees 'pull' water up through millions of tiny vessels, the pulling force being exerted by the osmotic pressure within the leaves and transpiration (evaporation) of water through leaf pores. The force is considerable and the millions of threads of water are as taught as piano wire. They can snap - something that is called cavitation but that a plumber might call an air lock, or a surgeon an embolus - and cells relying on the water from those vessels will die. Mistletoe can increase the tendency to cavitation and sometimes the water supply can give out and the host is killed by desiccation.
Fireblight is a potentially fatal disease of trees and shrubs of the sub-family Pomoideae (those of the Rosaceae family with apple-like fruits). Common hosts in Britain are Hawthorn, Apple, Pear, Whitebeam, Cotoneaster and Pyracantha. The disease is caused by the bacterium Erwinia amylavora which originates from the USA and was first recorded in Britain in Kent in 1956. The disease can cause significant losses in orchards and nurseries where large numbers of trees can be killed in local outbreaks and cropping of more resistant species seriously reduced.
The bacteria quickly multiply and spread to kill the inner bark of shoots and spurs, spreading into branches and even the stem, causing cankers and death of bark. If branches or stems are girdled by the infection, the branch or tree will die. The blackening of flowers and leaves can have the appearance of being scorched by fire, giving the disease its name.
The disease cannot be cured but the spread of infection can be stopped by pruning out infected branches. These should be cut well below the last sign of staining and in dry conditions. Pruning tools should be swabbed with methylated spirits between every cut. Infected material should be promptly burned.
The Oak Processionary Moth (Thaumetopoea processionea) is here and, 10 years after it was first spotted, it looks like it might be here to stay.... and it could be heading your way if it's not there already.
The moth is native to southern Europe where natural predators and climatic factors have tended to keep numbers in check. Unfortunately trade in plant material has led to a rapid expansion of its range over the last 20 years (as with a number of pests and diseases) and it is now established in the Netherlands and north Germany and has a foothold in the UK. The Forestry Commission now advise that it is considered impossible to eradicate the pest and that resources will concentrate on containing its spread. Some years ago I heard one London tree manager expressing deep frustration that he could not get the necessary budget and coordination to attempt eradication when the pest was first discovered in south west London in 2006. It was perhaps an opportunity missed and, some say, demonstrated a lack of policy and coordination in the management of urban trees at the time.
Despite control efforts, the spread seems to be inexorable and is now a major problem at some important sites such as Richmond Park, Kew Gardens and Syon House where enormous effort and expense is expended in removing nests. From the initial outbreaks found in several boroughs in West and South-West London and the Elmbridge and Spelthorne districts of Surrey in 2006, it was then found in the Pangbourne area of West Berkshire in 2010; Bromley, Croydon and Lewisham Boroughs in South London in 2012; Hackney, Waltham Forest, Tower Hamlets and Newham Boroughs in East London in 2014 and the Guildford District in Surrey, in 2015. A small number of nests were found in Watford this year and at the time of writing the critters are suspected in southern parts of Buckinghamshire and Hertfordshire.
This is not just bad news for Oak trees which are defoliated by the caterpillars, but also to us and to our animals. The caterpillars have thousands of hairs containing an irritant, thaumetopoein, which can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why the nests should not be touched. The greatest risk period is May to July, but they can be present on old nests, and could be blown or touched at any time of year.
The moths are named after their most distinctive identification feature, namely the manner in which the caterpillars form nose to tail processions when moving around the tree, or across the ground to move from one Oak tree to another. This video footage show one such column...they could almost be goose-stepping!
Whilst the implications to human and animal health posed by this caterpillar are currently the principal cause for concern, it could be increasingly damaging to the Oak trees. The larvae munch through the leaves and can strip a tree bare. This leaves them vulnerable to attack by other pests and diseases, and less able to withstand adverse environmental events such as drought and flood. It is possible that repeated heavy attacks could kill trees. Although the nests have only been reported on Oak, if they run short of oak leaves to eat, the larvae have been observed on hornbeam, hazel, beech, sweet chestnut and birch trees. Both of Britain’s native species of oak, pedunculate and sessile oak, and several other oak species grown here are susceptible to attack. In a very broadly descending order of susceptibility, they are:
Turkey oak (Quercus cerris), English or pendunculate oak (Q. robur), chestnut-leaved oak (Q. castaneifolia), white oak (Q. alba), Turner's oak (Q. x turneri), Holm oak (Q. ilex), Algerian oak (Q. canariensis), Hungarian or Italian oak (Q. frainetto), sessile oak (Q. petraea) and cork oak (Q. suber).
If you see this pest on your Oak trees you should report it to the Forestry Commission using the tree alert facility on their website . To be most effective control measures should be carefully timed and carried out by professionals with appropriate training and equipment. Do not try to remove caterpillars or nests yourself, because of the health risks.
The Forestry Commission advises:
Control efforts have currently seen the establishment of 'control zones'. It is intended to contain infestation within the 'Core Zone' which includes boroughs of Ealing, Richmond, Hammersmith & Fulham, Hounslow, Kensington, Chelsea and Kingston, as well as parts of Brent, Merton, Wandsworth, Westminster and Elmsbridge. The 'Control Zone' is a bufferzone of surrounding districts and where some presence has been found. The 'Protected Zone' is currently (largely) unaffected areas. Government policy is to contain spread from the West London 'Core' outbreak, and to eradicate it elsewhere. In general terms, Forestry Commission England will take charge of all aspects of treatment of privately owned infested trees in the Control and Protected Zones in 2016, and there will be no costs for the owner. In the 'Core Zone' control by owners is 'strongly advised' but it would appear that no help is offered. This can only be due to the scale of the problem and cost of control. But without determined efforts in the Core Zone, there will surely be a perpetual need for vigilance and control in the Control and Protected Zones. The current efforts must certainly have restricted its progress, but if containment isn't completely effective, there must surely be an exponential expansion of its presence across the country. Lets hope this has taught us to act swiftly and thoroughly when the next pest arrives because, despite Brexit, it surely will.
For information about regulations and requirements applying to the importation of oak plants, or to the movement, handling and disposal of oak material in affected areas, contact the Forestry Commission's Plant Health Service.
T: 0300 067 5155