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A miscellany of tree related posts - from important current issues to anything that's even tenuously connected to trees

MISTLETOE

12/12/2016

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European Mistletoe (Viscum album). Image By H. Zell (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
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.
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Mistletoe on Willow. Image By David Monniaux (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) , via Wikimedia Commons
Though now widespread, the origins of our traditional use of mistletoe in Christmas decorations, and of kissing beneath a sprig are uncertain. Many a time I have loitered beneath the sprig at parties but, uncannily, I always seem to do so at the precise moment  that all the ladies suddenly decide to loiter elsewhere. Still, there is always the mulled wine...

Mistletoe's association with Christmas is coincidental - its use in mid-winter customs pre-dates Christianity. Indeed, at least until the 1960's its inclusion in church decorations was frowned upon on in many parishes - so has the plant a darker past?

The excellent Mistletoe Pages website explains the mixtures of mythology and folklore surrouning the plant, some examples of which are described below.
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19th Century Punch cartoon
The kissing tradition is thought related to the use of mistletoe as a fertility symbol. Richard Mabey notes that the green mistletoe on a tree that has shed its leaves seemed an example of spontaneous generation and that  the 'milk white berries held between splayed leaves...seemed signed as a human fertility potion and aphrodisiac. Women who wished to conceive would tie a sprig around their waists or wrists'.  Other local customs to improve soil fertility include a number of variations of burning bundles of Hawthorn twigs and mistletoe on the field, or a man would run over the field carrying this burning globe. These ceremonies would usually end with cider drinking.
These once local traditions are thought to have been revived by more recent revival of interest in Druidism - though the Druidic and Celtic fertility rites involving golden sickles and white-robed virgins (see below) would become sanitised to a Christmas kiss. Nevertheless, the kissing custom soon spread to many English speaking countries - though most of our Christmas mistletoe is imported from the orchards of Normandy.
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In Norse, Greek and Roman mythology, and European culture, mistletoe is more associated with peace than fertility. In legend, the Nors god Baldr, was killed by a weapon made from mistletoe. Baldr was was one of the most popular gods, known as Baldr the Beautiful, but he was plagued by dreams foretelling his death and so, in an effort to reassure and protect him, his mother made everything, plant, animal or rock, living on or growing in the earth swear never to harm him. But jealous god Loki realised that mistletoe had been left out of the oath and contrived a weapon made from it – variously described as an arrow, dart or spear. Rather than do his own dirty work he persuaded Hod, Baldr’s blind brother to strike with this weapon, ensuring that Hod took the immediate blame. Baldr died from this single wound, and all the gods mourned for him. Baldr's mother, Frigg, wept tears that turned into the pearlescent berries of mistletoe and rather than punish the plant, she decreed that it should be a symbol of peace and friendship evermore.

In France it was often given as a Porte Bonheur - a gift for luck, particularly for the New Year, rather than at Christmas.
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Baldr the god from a 17th century Icelandic manuscript
According to Pliny the druidic priesthood valued, worshipped even, mistletoe where it grew on their sacred trees, particularly their oaks.  They would climb the tree to harvest it, cutting it with a golden sickle, then let it fall naturally to be caught in a hide or cloak before it touched the ground.  If it did reach the ground it would lose its special powers.  The special harvest would then be used in ritual or in medicine - two white bulls would be sacrificed and the mistletoe used to make an elixir to cure infertility and the effects of poison. It was the eighteenth century fad for Druidism that revived old customs into national fashion. Modern Druid groups still take an interest in mistletoe, especially on Oak. In 2004 a new Druid initiative called the Mistletoe Foundation was established to review and rekindle interest in the mistletoe ritual described by Pliny.  The group is open to all, druid or non-druid, and they have events each year in the Tenbury Wells area. In the Asterix comics, venerable Druid Getafix is often depicted among oak trees, robed in white, and bearing a golden sickle
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Getafix whose potion used Mistletoe as a key ingredient
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.

The stem is yellowish and smooth with tongue-shaped leaves, 1 to 3 inches long, thick and leathery, arranged in pairs. It is much branched and grows to form an often spherical bushy mass about 2 feet across. The flowers are small and inconspicuous and arranged in threes, in close short spikes or clusters in the forks of the branches, and are of two varieties, the male and female occurring on different plants.  They open in May. The fruit is a round, smooth, white berry, ripening in December
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By BerndH (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) via Wikimedia Commons
The white berries contain a single seed enclosed in a pulp which is very sticky (giving the plant its botanical name Viscum). This helps them stick to branches when they land on them, usually by the agency of birds. They are said to be particularly enjoyed by the Mistle Thrush - earning the bird its name. The sticky pulp has historically been used in the manufacture of birdlime, a sticky substance spread on branches to trap unfortunate birds. I guess that's known as coming to a sticky end.


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By Stefan.lefnaer (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

A root, actually called an Haustorium, grows from the seed, penetrates the bark and taps into the host tree's pipework to appropriate water and nutrients for itself. These can also grow through the branch and produce 'buds' breaking back out to form another plant as at 'g' on the diagram below.
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Germinating seedling
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Diagramatic cross section of mistletoe growing into a branch. Typical swelling - and reduced growth to the right - are clearly shown. (Julius Sachs 832-1897).
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Image:Gerhard Elsner (Own work) CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
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.
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FIREBLIGHT

4/9/2016

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Fireblight of Pear. Image By Ninjatacoshell (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)
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.
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Infected flowers on Apple. Image P W Steiner
Infection is commonly through flowers which rapidly blacken and die - the bacteria being carried by pollinating insects - although the blossoms of Pears can escape damage with the first symptoms appearing on new shoots. This is because Pear blossom tends to appear a week or two earlier than on apple trees when the conditions are not yet warm enough for the bacteria to flourish. But the bacteria can also be carried by wind, birds and pruning tools and can infect a plant through any injury or natural opening (pruning wounds, lenticels, stomata, hail damage etc.)
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.
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Infection spreading from shoot into parent branch. Image avtreefarm.com
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Cracking and cankering of infected branch. In autumn the bacteria become dormant and overwinter in bark at the canker margins. Image Ontario MAFRA
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To confirm the disease, peel back the bark from the affected area to reveal a red-brown staining of the inner bark or cambium. Image Royal Horticultural Society
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.
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Serious infection of Gala Apple Orchard (Photo by Mark Longstroth, Michigan State University)
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OAK PROCESSIONARY MOTH

26/8/2016

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Close up of the larvae and their irritating (to put it mildly) hairs. Image By Kleuske (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
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!
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Larvae in a nest on the trunk of an Oak. Image By Falko Seyffarth ("FWHS") [GFDL (http://www.gnu.org/copyleft/fdl.html)
The caterpillars also build silken nests on the trees' stems and branches. The caterpillars rest up in these nests during the day between feeding periods, and later in the summer they retreat into the nests to pupate into adult moths.

The nests can occur in a range of shapes, including hemispherical, tear-drop shaped, bag-like, and like a blanket stretched around part of a trunk or branch. Sizes range from as small as the width of a 50p coin to stretching several feet up the oak tree trunk in some cases. They can occur anywhere from ground level to high in the oak tree, and can fall out of oak trees and be found on the ground.
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With apologies to the squeemish..the rash. Image By Kwouters001 (Own work) [CC0], via Wikimedia Commons
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The culprit. Image By G.Hagedorn (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Larval emergence begins about mid to late April in an average spring, but can be as early as March. As they grow, the caterpillars descend lower in the trees to feed and build nests, and this is when they are most likely to be seen by the public. It is also when they develop the irritating hairs which pose the health problems.
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Male and female moths. In July the caterpillars congregate in the nest to pupate into adult moths, which can take up to four weeks.Image By Orchi (Self-photographed) [GFDL (http://www.gnu.org/copyleft/fdl.html)
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Spent nests fallen from the tree after adult moth emergence will invariably contain pupae cases.. Image Christian Fischer [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
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:


  • DO NOT:
  • touch or go near nests or caterpillars
  • let children touch or go near nests or caterpillars
  • let animals touch or go near nests or caterpillars
  • try to remove nests or caterpillars yourself.

  • DO:
  • teach children not to touch or go near nests or caterpillars;
  • train or restrain pets from touching or going near nests or caterpillars
  • see a pharmacist for relief from skin or eye irritations after possible contact
  • call NHS111 or see your GP if you think you or someone you care for has had a serious allergic reaction
  • speak to a vet if you think your pet or livestock has been affected;
  • call in a suitably qualified pest control expert to remove infestations in your trees
  • warn any neighbours who have oak trees that they might also have an infestation
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.
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Two columns of caterpillars swarm over an Oak - note the shoots stripped of leaves . Image By Jörg-Peter Wagner (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)
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
Email: plant.health@forestry.gsi.gov.uk

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