18 posts in the category

Pests & diseases

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PSAUS defoliated by lesser willow sawfly (Nematus pavidus) larvae

It’s only the middle of July but the PSAUS, a willow (Salix) has already lost most of its leaves…

Nope, autumn hasn’t come early. It has been munched to destruction by these hungry fellas: lesser willow sawfly (Nematus pavidus) larvae.

I recognised them straightaway as sawfly larvae, rather than caterpillars, after the Set A grey alders played host a similar species a few years ago.

This photograph shows a birch sawfly aka hazel sawfly (Croesus septentrionalis) larva on grey alder No. 3 in October 2009...

…and on the same day I took this photograph of an alder sawfly (Eriocampa ovata) larva on grey alder No. 2.

Little wonder the PSAUS has been almost entirely defoliated – there are dozens and dozens of larvae! I’ve decided to leave them to do what they do best, and wait and see whether or not the PSAUS can weather the storm.

In this photo the second-right larva is caught in a classic pose while the furthest-right larva has an injury halfway along its body.

Posted in Pests and diseases + The treeblog trees

Chicken of the woods?

About five weeks ago I was driving along the bottom of the Ewden Valley when I caught sight of a big bracket fungus growing eight or nine feet up the trunk of a wild cherry or gean (Prunus avium). I returned later in the week with my camera, but as the fungus was a few feet above my head and it was quite dark under the canopy of trees my photos didn’t turn out very good. I intended to come back with a step-ladder and take some better pictures but for whatever reason I didn’t get back again until yesterday. Over the last month the fungus has degraded somewhat. It’s sadly no longer the fresh specimen it was at the end of May. It has a not-unpleasant, cheesy sort of smell to it. The big brackets were more or less a foot wide.

A month ago the fungus was quite yellow (at least it was below, which was all I could see of it), and my first thought was that it was chicken of the woods (Laetiporus sulphureus). I still think this may be the case, but I’m not so sure. When am I ever sure? Maybe it’s something like Inonotus cuticularis? Listen - dear reader, if you are able to shed any light on this matter I would greatly appreciate it! (Just bear in mind that the fungus is past its best and is starting to look a bit ropey.)

This is the best of the photos I took when I first discovered the fungus five weeks ago.

This is the host tree: one very tall cherry. Its lower branches are dead, but it’s uppermost branches are still leafy. There is a big, old wound on the stem just below the fungus, and the fungus itself grows from an old wound. A shiny new tag identifies it as ‘0852’. 0852 isn’t a brill tree.

Is this weird? It’s a little weird, right?

Posted in Pests and diseases

Farewell, horse chestnut 32, thou sufferer of bleeding canker

Horse chestnut No. 32 on the first day of May.

Right at the very beginning of this year, by Broomhead Reservoir in Ewden Valley, I discovered a horse chestnut (Aesculus hippocastanum) with several Daedaleopsis confragosa (blushing bracket / thin walled maze polypore) brackets poking from its trunk. The tree obviously wasn’t in good health; it had recently shed a large branch and much of the tree was dead. I realised that it was suffering from bleeding canker, the bacterial disease which has stormed Britain and now infects roughly half of all our horse chestnuts.

I went to check up on the tree again on the 1st of May and noticed that a number had been painted on it. A few other bleeding canker-infected horse chestnuts nearby had also been labelled. I guessed that these trees were going to be felled soon. Someone else had noticed that these chestnuts were infected!

Maze-like gills on the underside of one of the D. confragosa brackets.

A different type of fungus had appeared a little further around the trunk. I think these are Pleurotus ostreatus (oyster mushrooms).

Here’s the wound where the largish branch had dropped off. I’ve noticed that while bleeding canker doesn’t have a noticeable impact on leaf growth – from a distance diseased trees look pretty normal – infected chestnuts are highly prone to losing whole limbs. The bleeding canker infection must seriously weaken the structure of the tree.

The most noticeable symptoms of bleeding canker are massive bark cracks on the trunk or major branches; these can lead to whole sections of bark breaking away from the tree. I suppose this is how the disease can eventually kill its host. Bleeding cankers like this one are (surprise, surprise) another symptom of… bleeding canker.

Anyway - despite its troubles, No. 32 was still putting on a brave face! It was just beginning to flower at the start of May.

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I drove past at the weekend and noticed that the numbered trees had all been felled, so I returned with my camera on Tuesday (May 24th). There isn’t much left of No. 32. A section of the trunk has been left in situ, along with the stump, but there is no trace of the rest of the tree. Everything (branches, twigs, leaves, the lot) has been taken off-site – perhaps to be disposed of in accordance with whatever regulations apply regarding trees infected by bleeding canker.

The stump. It looks like there was some decay in the centre. Was the whole right-hand side of the tree dead? It’s hard to say from the stump.

The stump and the stem, with some of the D. congragosa brackets still attached.

The other end of the stem (top end). Decay is apparent in the bottom-right quarter.

Some of the D. confragosa brackets had broken off and were lying on the floor, where they have begun to go mouldy. I pulled the best remaining specimen off the trunk to take home as a keepsake.

Posted in Pests and diseases

Oyster mushrooms (Pleurotus ostreatus) on a dead horse chestnut

Oyster mushrooms (Pleurotus ostreatus) on a dead horse chestnut tree (Aesculus hippocastanum).

I’m sorry if you’re thinking “What? Another post about fungi on a horse chestnut?”, but that’s just how the dice are falling! This particular tree stands not too far from the horse chestnut in the last post, set back a little from the road that runs the length of More Hall Reservoir in the Ewden Valley. It is completely dead, and probably died a few years ago. I don’t think it was killed by bleeding canker, which probably didn’t reach these parts until after the tree had died anyway. Nowadays it’s a big chunk of standing deadwood happily rotting away under a host of hungry fungi, the most noticeable one being the oyster mushroom.

As you can see, the stem is home to a fair few clusters of them. I made a little post last year when I found some of the same on a knackered lime tree (Tilia sp.) on a street in Sheffield.

The oyster mushroom is common on dead trunks and branches of broadleaved species, also occurring rarely on conifers. It can be found throughout the year although it fruits mainly in summer and autumn. Young mushrooms are excellently edible.

This fungus was growing from the base of the tree. I think it’s Xylaria polymorpha, commonly known as dead man’s fingers.

This was also at the base of the tree, tucked between two buttresses. It looks to me like another fungus, but I’ve no idea what species it is. I’ll have to keep a look out for any developments!

Posted in Pests and diseases

Velvet shank fungi (Flammulina velutipes) on a horse chestnut destroyed by bleeding canker

This post is all about the fungi growing on a horse chestnut tree (Aesculus hippocastanum) with bleeding canker in the Ewden valley. In a previous post all about different fungi growing on a horse chestnut tree with bleeding canker in the Ewden valley, I wrote “…a road running along the northern shore of More Hall Reservoir is lined predominantly by horse chestnut and cherry trees. Virtually every single horse chestnut on this mile-long stretch is in a … bad way, and I am quite sure that bleeding canker is the cause. I think I’ll have much to say on this in future posts!” Well, here’s a start.

First let me introduce you to the tree. It’s in catastrophically bad condition; it has a serious bleeding canker infection and, as apparent from this massive wound, half the tree has broken off sometime last year.

The remainder of the tree is covered in extensive bark cracks symptomatic of bleeding canker. I can’t imagine it being long before the large branch heading out to the right in this photograph breaks off. The orange fungi you can just about make out in this photo – growing on the trunk, the branch, and in the union between the branch and the remaining upright stem – are the focus of the rest of this post.

This photo (taken a month ago on the 8th of January) shows a cluster of mushrooms that sprout from the trunk at about breast height. Here they are at a juvenile stage looking real slimy.

A fortnight later (on the 21st of January) and they’ve turned a darker shade of orange. I’ve identified them – with 99% confidence – as Flammulina velutipes - the velvet stem, velvet foot or winter mushroom (or, according to MushroomExpert.Com, the “At Least Something’s Out In January” mushroom!) It is one of only a few mushrooms around at this time of year, but F. velutipes is “only likely to fruit during warm spells, so if there is snow covering the ground you’re not likely to find it.” True enough, we’ve had no snow since Christmas.

This photo taken with the zoom shows a massive crack on the aforementioned branch and a cheeky little cluster of F. velutipes growing from the exposed dead wood.

The cluster yesterday.

Flammulina velutipes - things you should be knowing: It’s edible and tasty. It can be frozen solid and still produce spores after thawing out. It’s a common species, fruiting from October to March. It’s a saprotroph, growing on the dead wood of broadleaf species – mainly horse chestnut and elms (Ulmus).

This tree deserves your pity.

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Next month’s FOTT will be hosted by Rebecca of Rebecca in the Woods. Once again the theme is open and the deadline this month is the 27th. You’ll find the call for the submissions at the Festival of the Trees co-ordinating blog.

Posted in Pests and diseases

Daedaleopsis confragosa brackets revisited; the host tree probably has bleeding canker

In the last post I included several photos of bracket fungi on a horse chestnut (Aesculus hippocastanum), which I tentatively IDed as Daedaleopsis confragosa, the thin walled maze polypore or blushing bracket. I am now sure that this is the correct species following a second visit to the tree on Friday afternoon.

My photographs in the last post showed only the upper surface of the brackets, an omission I am making up for with these pictures of the undersides with their maze-like pores.

I found an interesting snippet on a forum earlier: that D. confragosa’s elongated pores (and the random arrangement thereof) are something of an intermediary between circular pores and linear gills. Furthermore: “two years ago we found a collection, where immediately under the older fruitbodies young fruitbodies [had] emerged, so close that they [were] connected. The older fruitbodies had pores, the younger ones gills…”!

I paid more attention to the tree itself on this visit. It’s not in a good way. It has lost at least a couple of its main branches, and at least a couple more are dead or mostly so. Much of the bark is cracked and stained with oozes, symptoms of the nasty bleeding canker disease that may end up doing to British horse chestnuts what Dutch elm disease did to our elms in the 1970s. Regarding the subjects of my photos, D. confragosa grows on dead wood - in this case wood likely killed by bleeding canker.

Higher up the stem, about ten feet from the ground, a different species of fungus is growing on the bark around where two of the tree’s branches have broken off. The tree here is almost certainly dead, and more branches look liable to drop soon. So in summary: much of the tree has been killed by bleeding canker; secondary infections have taken hold in the dead wood; and the tree is starting to fall apart. Crikey.

This particular horse chestnut grows in very narrow strip of woodland on the southern shore of Broomhead Reservoir, separated by a lane from woodland to the south (mainly conifer plantation). A mile to the east, a road running along the northern shore of More Hall Reservoir is lined predominantly by horse chestnut and cherry trees. Virtually every single horse chestnut on this mile-long stretch is in a similarly bad way, and I am quite sure that bleeding canker is the cause. I think I’ll have much to say on this in future posts!

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I will be hosting next month’s edition of the Festival of the Trees. Please send your submissions to mail [at] treeblog [dot] co [dot] uk before the 30th of January, ensuring that you include Festival of the Trees or FOTT within the header. ONLY ONE WEEK REMAINS BEFORE THE DEADLINE!

Posted in Pests and diseases

Bracket fungi on a horse chestnut: Daedaleopsis confragosa?

I was driving alongside Broomhead Reservoir on the third day of the new year when I spied from the corner of my little eye something beginning with B: brackets, and lots of them. I jumped out of the car and moved in for a closer look.

The tree? A horse chestnut (Aesculus hippocastanum) that clearly isn’t in rude health. It has recently shed a branch (both the scar and the detached limb are visible in the previous photograph). This may well be a consequence of the fungal infection afflicting the unfortunate tree, the visible symptoms of which consist of an impressive array of bracket fungi poking from the lower stem.

The fungi? Well I’m not certain, but I think the brackets may be the thin walled maze polypore a.k.a. the blushing bracket: Daedaleopsis confragosa. Am I right or am I wrong? If you can settle this matter for me please don’t hesitate to leave a comment or blast off an email!

Apparently D. confragosa is especially fond of willows - and these brackets look remarkably like the ones I saw on a willow at Worsbrough Reservoir at the beginning of September. Although at the time I came to a different conclusion…

”Small to medium-sized, kidney-shaped bracket, reddish brown with dirty pinkish white pores on the under-surface, annual; arranged singly or in tiers, on dead wood of broad-leaf trees, favouring willow, birch and beech.” – Jordan, M. (2004). The Encyclopedia of Fungi of Britain and Europe. Frances Lincoln.

Occurs “throughout the year, but sporulating late summer to autumn; frequent.” – Jordan’s Fungi.

Beneath the diseased horse chestnut (as well as a few D. confragosa brackets that had dropped off), I spotted a few decomposing examples of a different type of fungus. Whether these grew around the base of the trunk or were fallen brackets I know not, nor whether they were goodies or baddies from the tree’s point of view. Maybe I’ll come back to this tree later in the year and see if more have grown.

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***** I will be hosting next month’s edition of the Festival of the Trees. Please send your submissions to mail [at] treeblog [dot] co [dot] uk before the 30th of January, ensuring that you include Festival of the Trees or FOTT within the header. ONLY TEN DAYS UNTIL THE DEADLINE! *****

Posted in Pests and diseases

Help! Are these oyster mushrooms (Pleurotus ostreatus)?

These mushrooms were growing from the trunk of a barely-alive lime (linden) tree (Tilia sp.) in a northern English city. I’m almost certain that they belong to the genus Pleurotus, but I am unsure of the species. I’m thinking Pleurotus ostreatus, the oyster mushroom, but I’m not confident. Does anybody out there recognise these? Please email me (the address is on the right) or leave a comment below!

Pleurotus ostreatus? Photos taken on the 17th of February.

Posted in Pests and diseases

Pests, diseases, disorders, competing growth and unfavourable conditions (a field trip): the pests and disorders

On Thursday the 21st of January my arboriculture class set out from college on a field trip to see a smörgåsbord of pests, diseases, disorders, competing growths and unfavourable conditions afflicting a variety of trees in the vicinity of York and Malton. Some of them were new to me, most I was already aware of, but it made for a very interesting way to spend a day and we got to see some cracking trees. Here’s a quick run-through of the pests and disorders that we saw:

January isn’t a very good time to see pests in Britain. I think the scene is a lot more banging in the summer months.

Rabbit damage at base of ash (Fraxinus excelsior).

European rabbit (Oryctolagus cuniculus) - The field trip took place in a particularly harsh winter and the poor old bunny rabbits had resorted to stripping bark from trees. The damaged trees we saw formed part of a rough hedge along a field and were growing right beside a few rabbit burrows. The photo shows damage to an ash but we also saw damage to a tiny hawthorn (Crataegus). Bark stripping weakens trees (when trees are ring-barked / girdled all material above that point dies) and opens them to infection. [Other animals such as hares, deer and squirrels will also strip bark.]

Graffiti carved onto a beech tree (Fagus sylvatica).

Humans (Homo sapiens) – The example we saw was disfiguration of the bark by people carving initials or symbols. Graffiti has a negative impact on the aesthetic appeal of the tree and can open it up to infection. [Other examples of direct human damage: vandalism (breaking off branches), vehicle damage (usually inflicted by tall vans and lorries), and butchery by D.I.Y. / cowboy tree surgeons.]


Witch’s brooms on downy birch.

Witch’s broom (Taphrina betulina) – T. betulina is a fungus that causes dense balls of twigs (that look like birds nests from a distance in winter) – witch’s brooms - to form on the branches of silver and downy birches (Betula pendula and Betula pubescens respectively). As far as I was aware* the fungus doesn’t have any significant effect on the health of host tree, although as witch’s brooms grow larger every year, I assume that they eventually become so heavy that the supporting branch will break.

[ * I found this abstract to a scientific paper by Spanos & Woodward (1994) 1 online: The impact of infection by Taphrina betulina on the growth of Betula pubescens was studied in naturally regenerated stands in the North East of Scotland. Infection by T. betulina was associated with a significant reduction in height growth and poorer developmental tendency, vigour, and stem quality of B. pubescens, whereas diameter at breast height was little affected. Tree height was reduced by an average of 25% over all diameter classes, with the greatest effect in smaller trees. For brooms of 100 mm diameter and over, numbers and sizes were strongly correlated with diameter at breast height and tree age. Reductions in height and vigour were not correlated with the number and size of brooms present, but were more pronounced in younger, smaller trees.]

Included bark (to the left of the red line) in the crotch where two main stems meet on a beech.

Included bark - “Included bark forms when the bark of the branch and trunk squeeze together” (Shigo, 1991) 2. “In crotches that have very narrow angles of attachment the branch bark ridge [a ridge of bark in the crotch] sometimes fails to expand outward and is swallowed by the growth of the branch and trunk. Each year thereafter, more bark is enclosed within the crotch. This condition is referred to as included bark… Included bark has long been associated with weakness in tree crotches since it is frequently seen in failed tree forks. It is easy to assume that the included bark prevents the formation of connecting wood between two stems and therefore reduces crotch strength. …branch attachments with included bark are inherently weak and should be removed” (Farrell, undated) 3.

Sphaeroblasts - Disappointingly, I don’t have a photo of these intriguing fellas. You ever seen a Malteser- to fist-sized ball in the bark of a tree? Those are sphaeroblasts, described by Strouts & Winter (2000) 4 as “Bark-coloured spheroid lumps, small or large… woody, bark-covered structures, being an abnormal development of a bud which has produced annually a woody sheath without ever producing a shoot. Harmless.”

A large burr on an English oak (Quercus robur).

Burrs or burls - The burr we visited on our field trip – seen in the photo above – was a huge burr on a good-sized oak. It formed in a similar way to how sphaeroblasts form – the difference I think is that the buds in a burr are on the outside of the bark, not beneath it. The buds seem to multiply like crazy and sometimes produce tiny little shoots that never amount to anything. As far as I know, burrs are harmless to trees. They’re supposed to be highly valued by craftsmen because of the spectacular grain of the wood inside.

A pair of oak marble galls (one not fully developed) [photo taken 15 January 2009].

Galls - We saw a few old marble galls on the oak with the massive burr. I mentioned these in a post in January 2009: oak marble galls [are] caused by asexual Andricus kollari larvae. A. kollari is a member of the family Cynipidae, “whose members are of special interest because most of them induce gall formation on plants and many of them display a marked alteration between sexual and parthenogenetic generations. They are called gall wasps… Most of the European species occur on oaks, although some species attack roses and certain herbaceous plants. There are about 90 British species.” - from Chinery’s Insects of Britain & Northern Europe (Collins Field Guide, 1993). Again, as far as I’m aware, galls don’t significantly affect the health of a tree in normal conditions.

1 Spanos, Y. A. & Woodward, S. (1994). The effects of Taphrina betulina infection on growth of Betula pubescens. European Journal of Forest Pathology, 24 (5), 277-286.
2 Shigo, Alex L. (1991). Modern Arboriculture. Shigo and Trees, Associates.
3 Farrell, Robert, W. (undated) Structural Features Related to Tree Crotch Strength (Thesis submitted to the Faculty of Virginia Polytechnic Institute and State University for the degree of Master of Science in Forestry). Available in PDF format from: http://www.treeworld.info/manualuploads/crotchstrength.pdf [Accessed 11th February 2010].
4 Strouts, R. G. and Winter, T. G. (2000). Diagnosis of ill-health in trees. The Stationary Office.

Posted in Pests and diseases

Pests, diseases, disorders, competing growth and unfavourable conditions (a field trip): the diseases

Last Thursday my arboriculture class set out from college on a field trip to see a smörgåsbord of pests, diseases, disorders, competing growth and unfavourable conditions afflicting a variety of trees in the vicinity of York and Malton. Some of them were new to me, most I were already aware of, but it was a highly interesting way to spend a day and we got to see some cracking trees. Here’s a quick run-through of the diseases that we saw:

Supermassive sycamore (this photo falls a long way short of doing it justice) with decayed Dryad’s saddles. The one on the floor has fallen off the tree.

Dryad’s saddle (Polyporus squamosus) – A bracket fungi. Described in Jordan’s Fungi 1 as “Large, creamy-brown scaly cap with cream pore-bearing under-surface, annual; parasitic on broad-leaf trees, also on stumps, favouring beech, elm and sycamore.” We only saw old and decaying specimens, but the sycamores (Acer pseudoplatanus) they had grown on were jaw-droppingly colossal - easily the biggest I’ve seen! These ancient sycamores formed an avenue along a road near Birdsall House. I’ve got to go back and get some decent photos of them in the summer before they collapse or get felled for safety reasons.

These crazy patterns are galleries produced by elm bark beetles.

Dutch elm disease – The Big Baddie. The current epidemic is caused by the fungus Ophiostma novo-ulmi, spread by elm bark beetles of the genus Scolytus. It is the most catastrophically devastating tree disease ever recorded in British history. On the field trip we saw a dead elm (Ulmus) replete with bark beetle galleries in the wood beneath the bark.

Cankers on a sycamore.

Canker – There are various kinds of cankers and a variety of causes of cankers. Strouts and Winter 2 define a canker as a “clearly defined patch of dead and sunken or malformed bark”. We saw cankers on sycamore and red horse chestnut (Aesculus x carnea) - a hybrid amusingly described in the Collins Tree Guide 3 as a tree of “rather endearing ugliness”.

Huge ivy-covered canker on a red horse chestnut.

Razor strop on silver birch.

Birch polypore or razor strop (Piptoporus betulinus) – Another bracket-producing fungus, it is restricted to birch (Betula). We saw loads of razor strops on dead and drying silver birches (Betula pendula) in a small piece of woodland that has become waterlogged as a consequence of mining subsidence.

Ganoderma on a veteran English oak.

Ganoderma - A genus of bracket- (polypore) producing fungi that is parasitic on broad-leafed species. We saw a large, dead Ganoderma at the base of a huge and ancient English oak (Quercus robur), but we didn’t identify it to species level. The oak was extremely diseased and its days are sadly numbered.

Slime flux on the same oak.

Slime flux or bacterial wetwood – A bacterial infection causing the host to ooze infected sap from wounds or apparently healthy bark. According to Strouts and Winter 2, bacterial wetwood is “common yet rarely results in overt disease”.

Inonotus hispidus on an ivy-clad ash.

Inonotus hispidus - Another polypore-producer that is parasitic on broad-leaves, particularly ash (Fraxinus excelsior). We saw several dead brackets on an ivy-covered ash growing by a stream.

Fomes fomentarius on silver birch.

Hoof fungus or tinder bracket (Fomes fomentarius) – Like you’d expect, the brackets of this fungus look like hooves. It favours birch – its modus operandi is similar to that of razor strop. We saw several of these brackets on the waterlogged birches.

1 Jordan, M. (2004). The Encyclopedia of Fungi of Britain and Europe. Frances Lincoln.
2 Strouts, R. G. and Winter, T. G. (2000). Diagnosis of ill-health in trees. The Stationary Office.
3 Johnson, O. and More, D. (2006). Collins Tree Guide. HarperCollins Publishers.

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Call for Submissions: Festival 44 Returns to the treeblog. Submit!

Posted in Pests and diseases

Two species of caterpillar on the grey alders

Earlier this afternoon I noticed several caterpillars on two of the Set A grey alders (Alnus incana). There were a few colourful caterpillars munching away on grey alder No. 3 and there were several white “snowflakey” caterpillars on grey alder No. 2. These snowflake caterpillars have been on the alders since at least mid-August and they seem to have some kind of magic power that can prevent a camera from focussing on them. They eat in random patches to leave the leaves full of holes like a Swiss cheese whereas the colourful, curly caterpillars eat in a more systematic fashion, devouring neat sections between veins. These caterpillars stand with their tails sticking up into the air; when I got close to them they gave them a little wave.

Last year there were two or three other species of caterpillar on the alders. Have a look at all the posts tagged with ‘caterpillars’ if you’re interested!

By the way, I’ve no idea what species either of these caterpillars belong to. Leave a comment or drop me an email if you know what they are, please!

Update (July 2010): The black and yellow ‘caterpillars’ are actually larval forms of the hazel or birch sawfly (Croesus septentrionalis).

Posted in Pests and diseases + The treeblog trees

One big Ganoderma bracket

Ganoderma sp. bracket at the base of a small cherry (Prunus sp.) tree, not a million miles from York. Seen yesterday - the 24th of September.

This Ganoderma sp. is a saprophytic fungus: it will only attack the “non-living” heartwood and won’t harm the “living” sapwood. Infected trees may appear normally healthy in external appearance, but inside they can be a soft and mushy mess liable to collapse or fall over at any time. In my new job as an apprentice arborist I’ve already seen a few examples of cherries with healthy canopies, but with Gandoderma fruiting bodies (the brackets) growing from the roots or base of the trunk. These roadside trees had to be felled in the interests of public safety, and I’ve seen from the stumps how rotten the infected heartwood becomes: far too soft to provide the tree with any kind of structural support.

Zounds! Some good egg has stuck his hand in the frame for scale!

One small cherry, one large bracket. If you can’t find it yourself, it’s at ground level right at the base of the tree trunk.

Posted in Pests and diseases

Sudden oak death and bleeding canker

There are a few tree diseases in the news at the moment. At least two of the newspapers yesterday ran the story that an avenue of 43 horse chestnuts (Aesculus hippocastanum) infected with the bleeding canker disease are to be cut down for safety reasons. The avenue at Barrington Court in Somerset, a National Trust property, is to be replanted with oaks.

According to Forest Research, parts of Europe are seeing an increased incidence of the disease. From their Bleeding Canker of Horse Chestnut page:

Until recently, such Phytophthora bleeding cankers were considered to be uncommon and were only seen in the south of England. However, over the past four or five years, the number of reports of horse chestnut trees with 'bleeding cankers' has increased markedly... The increased incidence of stem bleeding on horse chestnut is not just limited to the UK; the Netherlands, Belgium, France and Germany are also experiencing a similar upsurge.

Closer investigation of the bleeding cankers on horse chestnut has revealed that Phytophthora is no longer the primary causal agent. Instead a completely different pathogen, Pseudomonas syringae pv aesculi, is responsible for the increase in these symptoms appearing on horse chestnut.

The other diseases in the news are also species of Phytophthora: P. ramorum and P. kernoviae. Both diseases attack and kill many species of tree and shrub - P. ramorum is the cause of the rather frightening sudden oak death, which was the subject of an early treeblog post two years ago. The two diseases are in the news because the British government has allocated £25 million towards their eradication. According to this BBC article, “Rhododendrons, a carrier of both diseases, are likely to be removed in woodland to combat the problem.” Good. Rhododendron (Rhododendron ponticum) is bad news. It is highly invasive in the UK and takes over woodland through the development of a dense covering of nothing but evergreen rhododendron bushes that stifle and inhibit our native flora and fauna. I would like nothing more than to see rhododendron completely eradicated from Britain, but getting rid of it is easier said than done. It is one stubborn son of a bitch.

Uh-oh. Rhododendron at the edge of my beloved Whitwell Moor (23 May 2007).

P.S. Phytophthoras, sometimes referred to as fungal-like pathogens, are actually protists of the order Oomycetes (water moulds). They are more closely related to plants than fungi.

P.P.S. The abbreviation ‘pv’ means ‘pathovar’, “a bacterial strain or set of strains with the same or similar characteristics, that is differentiated at infrasubspecific level from other strains of the same species or subspecies on the basis of distinctive pathogenicity to one or more plant hosts” (definition from Wikipedia).

Posted in Pests and diseases

English oak: galls, lichen and fungi

What is a gall? The British Plant Gall Society’s site has a good definition from Redfern & Shirley’s British Plant Galls:

A gall is an abnormal growth produced by a plant or other host under the influence of another organism. It involves enlargement and/or proliferation of host cells, and provides both shelter and food or nutrients for the invading organism.

British oaks are particularly rich in galls. Here is just a tiny sample.

A pair of oak marble galls (one not fully developed) caused by asexual Andricus kollari larvae. A. kollari is a member of the family Cynipidae, “whose members are of special interest because most of them induce gall formation on plants and many of them display a marked alteration between sexual and parthenogenetic generations. They are called gall wasps… Most of the European species occur on oaks, although some species attack roses and certain herbaceous plants. There are about 90 British species.” - from Chinery’s Insects of Britain & Northern Europe (Collins Field Guide, 1993). This page at hedgerowmobile.com details the life-cycle of A. kollari and has good photos of the wasps and larvae with cross-sections of the gall.

A pair of artichoke galls caused by asexual Andricus fecundator larvae. Another gall wasp, A. fecundator is closely related to A. kollari. I’ve known about marble galls since I was a kid, but my first encounter with an artichoke gall came recently, in September 2007, on a university field trip to Kintyre. How did I ever miss them? hedgerowmobile.com has an A. fecundator page too.

An unknown gall. Perhaps a partially formed marble gall?

An unknown bud gall. If you recognise this gall and can ID the causal agent, please leave a comment.

Enough galls; how about some fungi? This weirdness has taken over the underside of this branch. The top side is covered with leprose lichen.

More leprose lichen. Notice how it only grows on the right side of the branch. There must be a favourable microclimate there, perhaps because of sunlight or exposure.

Another lichen; this one is fruitcose. As for what species it is… I wish I could tell you.

The subjects of all those photographs were found on this one English or pedunculate oak (Quercus robur). Even though it has split in two, both halves continue to grow; in fact, the large branch that goes off to the right is impressively long.

One more thing! A reminder about the next edition of the Festival of the Trees, which treeblog will be hosting. Please submit your blog posts, photographs, poetry, works of art, articles, news pieces, bark rubbings and anything else tree-related to mail [at] treeblog [dot] co [dot] uk, making sure that Festival of the Trees or FOTT is contained in the subject header. Alternatively, you can use the online submission form at blogcarnival.com. You do not need to be the author or artist of the content you submit. Although there is no theme this month, it would make my day if you submit something pertaining to a particular favourite tree of yours! The deadline for submissions is the 30th of January, so that leaves you a fortnight to get them in. Get to it!

Posted in Pests and diseases

Squirrels in Britain: the red v. the grey

Grey squirrel in the Royal Botanic Garden Edinburgh (25th of June 2008).

As you most likely already know, the red squirrel (Sciurus vulgaris) is Britain's native squirrel. But over the last century or so it has been largely superceded by the grey squirrel (Sciurus carolinensis), a native of North America. The greys outcompete the reds, which are then displaced from their territories. According to the Forestry Commission:

Greys can feed more efficiently [than reds] in broadleaved woodlands and can survive at densities of up to 8 per hectare. The density of reds is up to 1 per hectare in broadleaved woodland but can be as low as 0.1 per hectare in coniferous woodland.

The grey squirrel also hosts the ‘squirrel pox’ virus, by which it is rarely affected. But the disease can be passed on to reds to whom it is fatal. The range of red squirrels in Britain is therefore much smaller today than in the good old days before the greys were introduced.

From the point of view of British ecologists and conservationists the reds are the goodies and the greys are the baddies. Of course, individual grey squirrels don't know that they are invasive aliens causing the extinction of a fellow species. They are just doing what comes naturally. But the undesirability of the grey squirrel isn’t limited to its interactions with the reds; greys are predators of bird nests and strip the bark from trees, leading to poorer trees and poorer woodland owners.

Red squirrel at Cluny House Gardens, Aberfeldy (18th of November 2011).

So what is to be done? Land managers with a vested interest in reducing grey squirrel numbers (whether for the benefit of red squirrels, trees, or other wildlife) have a few options. Greys can be trapped, poisoned or shot. The Forestry Commission uses poisoned bait, but doesn’t advocate large-scale culls:

Eradication is not a feasible or desirable option given current methods. Worldwide, the record on eradicating small successful introduced mammals is very poor with research showing that low-level widespread culling has no impact on either grey squirrel numbers or upon damage to trees and priority species. As such it is ineffective and a waste of resources.

Even with new methods and unlimited resources, a successful eradication policy would require the total support of the public. Evidence suggests that a substantial proportion of the public would not support a grey squirrel eradication policy. This policy aims to create a balanced approach to wildlife conservation: controlling grey squirrel populations at a level which does not threaten our native woodlands and priority species.

So, member of the public… would you support a grey squirrel eradication policy? I would. The benefits are greater than one might first suspect: see these stories from the Guardian (quoted) and the BBC!

At Ridley's Fish and Game shop in Corbridge, Northumberland, the owner David Ridley says he has sold 1,000 [grey squirrels] - at £3.50 a squirrel - since he tested the market at the beginning of the year.


Simpson likens the taste to wild boar. Ridley thinks it is more a cross between duck and lamb. 'It's moist and sweet because, basically, its diet has been berries and nuts,' he said.

Both believe its new-found popularity is partly due to its green credentials. 'People like the fact it is wild meat, low in fat and local - so no food miles,' says Simpson. Ridley reckons that patriotism also plays a part: 'Eat a grey and save a red. That's the message.'


'A large squirrel would be enough for one-and-a-half people. The public really are being drawn to it. I think that it's because it is being perceived as a healthy meat. Southern fried squirrel is good. And tandoori style works. It is especially tasty fricasséed with Cornish cream and walnuts. But the one everyone seems to like is the Cornish squirrel pasty.'

Grey squirrel in the Royal Botanic Garden Edinburgh (25th of June 2008).

Posted in Invasive species + Pests and diseases

Chestnut blight (Cryphonectria parasitica)

The first recorded occurrence of chestnut blight disease was in 1904 within the New York Zoological Gardens. Its subsequent rapid spread throughout the forests of eastern North America was an ecological disaster, whereby the American chestnut (Castanea dentata) was reduced to an inconsequential component of the understorey, when once it had been the most important hardwood species in the eastern United States. The spread of the disease encompassed the entire natural range of C. dentata, with 3.6 million ha of the tree dead or dying in 1950. The disease was also discovered on the sweet chestnut (a.k.a. the Spanish chestnut) (Castanea sativa) in Italy in 1938, and spread throughout much of southern Europe. The disease is caused by Cryphonectria parasitica (formerly known as Endothia parasitica), which was unintentionally introduced on chestnut trees imported from East Asia. The economic, ecological, and social consequences of chestnut blight disease in North America were severe.

C. parasitica is a haploid ascomycete fungus producing both asexual and sexual types of spore. The fungus attacks through wounds, be they broken branches, breaks in the bark, or holes made by woodpeckers and beetles. The mycelium grows in the bark and outer sapwood, and spreads outwards from the source of infection until it has entirely girdled the trunk or branch. Necrosis and collapse of bark tissue forms a visible sunken canker, the expansion of which disrupts flow through the phloem and damages the cambium layer, limiting the ability of the host to produce new tissue. Girdling of the stem results in the wilting and death of the distal segment of the tree. The root systems of C. dentata often survive an attack of C. parasitica and continue to generate new shoots; however, these shoots rarely attain sexual maturity due to their remaining susceptibility to the blight fungus. Since two flowering trees are needed for seed formation (cross-pollination is required), sexual reproduction has been drastically diminished. As a result, there has been little chance for C. dentata to develop resistance.

The discovery of hypovirulent strains of the fungal pathogen has allowed highly successful therapeutic treatments of individual cankers. This process is well-suited to intensively managed, high-value orchards and plantations, although not to natural woodland. Natural hypovirulent strains of C. parasitica in Europe are known to be highly variable from one area to another in terms of hypovirulence. Deployment of hypovirulence in eastern North America has been an almost complete failure. Modern molecular approaches have been used to create transgenic hypovirulent strains of C. parasitica which themselves contain the genetic material of the natural hypoviruses. However, at present this new approach to biological control has not been entirely successful. Yet research continues, and in the near future a transgenic approach to biological control of the chestnut blight disease may well help restore the American chestnut to its former glory and ecological status.

[An abridged section of an essay for my third year uni course ‘Evolution and Ecology of Plants’.]

Posted in Pests and diseases

Sudden Oak Death - be on the lookout

Sudden Oak Death is the term used to describe the disease caused by the fungal pathogen Phytophthora ramorum amongst oaks. However, P. ramorum is not limited to oaks: it has the potential to cause disease over a range of hosts in Britain. A DEFRA information leaflet published in 2006 lists known UK hosts:

To date the full range of known ornamental hosts in the UK and Europe include species of Arbutus, Calluna (heather), Camellia, Griselinia, Hamamelis (witch-hazel), Kalmia, Laurus (laurel), Leucothoe, Lonicera (honeysuckle), Magnolia, Osmanthus, Parrotia, Photinia, Pieris, Rhododendron, Syringa (lilac), container grown Taxus (yew), Umbellularia californica (Californian bay laurel) and Viburnum. Most nursery findings have been on container-grown Rhododendron, Viburnum and Camellia plants. However, the main threat is to tree species and other ecologically important plants, such as heathland species.

In October 2003, a southern red oak tree (Quercus falcata), a native American species, was the first tree infected with P. ramorum in the UK. There have since been findings in the UK on several other oak species (holm oak, turkey oak, sessile oak), as well as ash, European beech, horse chestnut, sweet chestnut, sycamore and Winter’s bark. Infected rhododendrons have been in close proximity to all infected UK trees to date. Experimental work has highlighted other tree species that could also be at risk from P. ramorum in the UK. These include Douglas fir, maple spp., Noble fir, Lawson cypress and Sitka spruce.

Symptoms of the disease vary amongst the different hosts, but in general dieback of foliage and bleeding cankers on the trunks of trees are good indicators of P. ramorum infection. Full details can be found in the DEFRA leaflet available here [.pdf format, 873kb]. If you suspect a plant of being infected by P. ramorum, contact DEFRA immediately! (Contact details can be found in the leaflet)

Posted in Pests and diseases

Rhododendron ponticum L. - enemy of the woodland

Rhododendron ponticum is a non-native evergreen shrub species existing in the British Isles as an alien. It is an extremely important species for two main reasons. Firstly, it is superbly adapted to thrive in many British habitats; particularly woodland, of both coniferous and broad-leaved varieties. R. ponticum is ‘thoroughly naturalised’ in many woodlands, particularly on sandy podzolic soils. This prolific invader is a substantial ecological menace to the natural woodland flora and fauna of the British woodland. Its foliage is so thick that it casts a dense shadow that prevents light reaching other woodland flowers. Further, R. ponticum contains poisonous chemicals, thus inhibiting predation, and deters competition allelopathically via the secretion of acids. Secondly, it is a host plant of Phytophthora ramorum, the pathogen behind Sudden Oak Death. For these reasons, R. ponticum must be managed in order to curb its negative effects upon the natural ecology of the British Isles. Unfortunately, R. ponticum is particularly difficult to eradicate; its waxy leaves render herbicides generally impotent, it produces vast quantities of seed, and it requires much labour to chop down or uproot, it being a large plant.

The above passage is an abridged version of the introduction to an essay I am writing as part of my degree in Ecological Science at the University of Edinburgh. I just hope that people realise, if they didn't already, the tremendous damage being done to our woodlands by rhododendrons.

R. ponticum inflorescence.

Posted in Invasive species + Pests and diseases

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