12 posts tagged with

Castanea - the chestnuts

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Autumn at the Royal Botanic Garden Edinburgh

Autumn is once again a receding memory, growing dimmer everyday as we continue the inexorable slide into the darkest depths of winter… But try hard enough and it’s still possible to cast our minds back to a time when the trees still had leaves; when warm shades of gold, orange and red coloured the landscape; when the mercury didn’t sit so low in the thermometer.

I moved up to Edinburgh from Sheffield at the end of September. (I lived here for the best part of four years previously, while I studied ecological science at the University of Edinburgh.) My flat is no more than a ten minute walk from the outstanding botanic gardens, which are one of my favourite things about living here. I paid the gardens three visits with my camera in one week soon after I moved in. How about some autumnal photographs then?


28th September 2013

This towering deodar (Cedrus deodara) grows close to the East Gate. It is a beautiful and imposing tree, one of the finest in the gardens.

Looking up into the hefty crown of the deodar - how many branches? How many growth points? What tonnage of timber?

A neighbouring big, old sweet chestnut (Castanea sativa) seems sadly to be in serious decline. The tree was rather sparsely foliated at the time of this visit, but at that point in the autumn natural leaf loss would have been premature. It’s a shame because it’s another fine tree. A major branch has a few old wounds on it, one of which sported a nice bit of fungus. It’ll be interesting to see how the tree looks in spring.

I came across these Pholiota squarrosa mushrooms growing at the base of a big European beech (Fagus sylvatica). They were also growing around the base of a nearby heartnut, a variant of the Japanese walnut (Juglans ailantifolia var. cordiformis). P. squarrosa is a parasitic white-rot fungus that attacks a wide range of host trees.

This mushroom was growing under a pine tree… …along with its wee pal.

This dawn redwood (Metasequoia glyptostroboides) in the Chinese Hillside part of the gardens was positively radiant. The needles - here so vibrantly illuminated - are now long gone, this being a deciduous species.


29th September 2013

This oak really stood out from the crowd!

A nice cider gum (Eucalyptus gunnii) growing in front of the 1960s glasshouses. At least this tree will keep its leaves all winter long.


3rd October 2013

Autumnal maple leaves.

On this visit I was quite keen to get some photos of the mushroom population. There weren’t so many of a big enough size to stand out as I strolled along, but if I just stopped for a moment to study the mulch that surrounds the base of every tree, there were far more mushrooms to be seen than most people would have realised. Perhaps these are waxcaps of some sort?

The distinctive spiny cupules of sweet chestnut. Apparently the nuts can’t attain their full size in the British climate, so the roasting chestnuts that appear in the shops for winter are imported from the continent. I had a bag of roast chestnuts at Edinburgh’s European Christmas Market last week – they were enormous and very tasty!

I’m fairly sure this is a hare’s foot inkcap (Coprinus lagopus). This mushroom is quite interesting; according to Wikipedia, “As the mushroom matures, the shape of the cap becomes more conical or convex, and finally flattens out, with edges curved upward. The veil is initially whitish, then turns to a silvery grey or grey-brown; it eventually splits up, becoming hairy (fibrillose). ... In maturity the gill edges dissolve (deliquesce) into a black liquid. These mushrooms are evanescent, lasting only last a few hours before death…”


Posted in Gone for a walk





BudWatch (21st March 2010)

I went out for a wander on Sunday and was slightly disappointed to see such little springly progress from the buds on the locally-growing deciduous trees.

Hazel (Corylus avellana) buds and catkins. The catkins – some folks know them as lambs’ tails – are made up of male flowers. A female flower is hiding in the upper-centre of this photo.

Birch (probably downy birch, Betula pubescens).

English oak (Quercus robur). I’ve noticed that the terminal buds are often flanked by a pair of smaller buds, although the terminal bud in this photo has lost one of its two buddies. (It’s the Lonely Oak!)

Larch (probably European larch, Larix decidua) pegs and a ‘bud’ of some sort – maybe a flower very early on in development? I was very disappointed to find that there were no larch roses on this tree at all; this time last year they were out in force!

Goat willow (Salix caprea). On some of the trees catkins were already forming! I noticed that the buds on the trees with catkins were a light green while the trees without catkins had reddish buds (as in the above photo). Is this a way to tell the male trees from the female trees?

Common alder (Alnus glutinosa). Distinctively purply-velvety buds.

Hawthorn (probably the common hawthorn, Crataegus monogyna).

Here’s a wee hawthorn story: I was at college today, being taught how to use Tirfor winches in the context of stump removal. It is an agricultural college, and someone in the equestrian section pointlessly wanted a small section of hawthorn hedge, about five metres long, removing from a little patch of grass next to the stables. It was the remnant of a hedgerow that was mostly destroyed when the stables were built – a hedgerow probably laid down hundreds of years ago. Our instructor, an arboricultural legend (who shares my view that it is a great shame to get rid of something planted so long ago), reckoned it probably dated from the mid-eighteenth century, perhaps from medieval times; possibly, if it was Midland hawthorn (Crataegus laevigata), it may have dated from as far back as the tenth century! The roots were certainly grand old things.

European beech (Fagus sylvatica). The buds are easily identified with their long and pointy ways. ‘Cigar-shaped’, some say.

Sweet chestnut (Castanea sativa). Unassuming, eh?

And of the buds of other locally-growing tree species that I saw up close but are MIA from this post… Common ash (Fraxinus excelsior) buds showed no signs of opening yet, sycamore (Acer pseudoplatanus) buds were green and swelling, and elder (Sambucus nigra) – I saw a couple of elders with closed buds but one growing on a south-facing slope was covered in tiny green leaves, yippee!


Posted in Gone for a walk





The Wigtwizzle Chestnut in the snow

The venerable veteran of Wigtwizzle – a sweet chestnut (Castanea sativa) of considerable age and girth.

These photos are from a couple of Sundays ago when, driving home over the moors, I was ambushed by much snow. I couldn’t resist stopping for a few piccies.

These beeches (Fagus sylvatica) grow in the adjacent parkland that once surrounded Broomhead Hall.


Posted in Notable trees





Set D: sweet chestnuts collected & planted

Three sweet chestnuts sitting in an opened cupule. The dead catkin that held the male flowers, still attached to the base of the cupule (which once was a female flower), can be seen in the background. [Photo: 8 Oct. ‘09]

Last Thursday (the 8th of October) I went on a tree mission to Wigtwizzle with my sister. Mission objective: to collect nuts from the massive sweet chestnut (Castanea sativa) there for treeblog Set D.

The Wigtwizzle chestnut. Beast! [Photo: 7 Jul. ‘07]

While a lot of unripe cupules were stillon the tree, a great many ripe ones were lying open on the ground below. While trying to avoid a nettling, I managed to gather a haul of chestnuts with a total fresh weight of 75 g (2 ½ oz). Mission accomplished!

The Set D sweet chestnut haul. Third time lucky?

I collected nuts from the same tree in 2007 and 2008 for Set B and Set C respectively, but I didn’t manage to grow a single seedling. I now know the error of my bad old ways; I erred by waiting until spring before planting, by which time the chestnuts – which lose moisture rapidly and so are unsuited to storage – would have been well and truly dessicated. This time around, with only two days passing between collection and planting, treeblog might finally produce some baby sweet chestnuts.

Come closer…

I planted one hundred of the nuts yesterday in five forty-individual-pockets-to-a-tray seed trays using a special seeds and cuttings compost from B&Q. I ran out of room (and out of compost), so I had about sixty nuts left over. Until I decide what to do with them, I’ve mixed ‘em with some damp compost and popped ‘em in a plastic bag.

One of the seed trays showing forty chestnuts in forty ‘pockets’: a planting action shot.

The final product: five seed trays with one hundred sweet chestnuts carefully picked and planted. This is treeblog Set D(c) - the chestnut part of Set D. Set D(b) – the beech part – was planted on the 30th of September and the rowan seeds – Set D(r) – have just begun pretreatment and are on schedule for a springtime planting.

The 10th of October 2009 = Set D(c) Day 0.


Posted in Notable trees + The treeblog trees





Raising trees from seed: treeblog vs the Forestry Commission, or Set C mistakes

Before we get stuck in to the main course, would Reader like a starter? Another two seedlings were observed in the birch tray yesterday (Day 50), bringing the total to twenty-six. On top of that, a Set C first: two seedlings were discovered in sweet chestnut territory! But are they really sweet chestnut seedlings or just weed impostors? I’ve never seen a sweet chestnut seedling before, but I had a mental image of them being, uh, beefier. At least they’re not nettles

Anyhoo, I was browsing the internet the other day when I came across a Forestry Commission Practice Guide entitled Raising trees and shrubs from seed (Gosling, 2007). “This could be relevant,” I thought, and relevant it is. As hoped, the guide provides advice on raising all three of treeblog Set C’s species from seed. It would seem I’ve not been going about things in quite the right fashion.

The Set C birch seeds. I collected them from an impressive tree on Whitwell Moor. Those catkins (more correctly “strobiles”) were chock-a-block full of seeds too.

Birch:
According to the guide, birches are fairly easy to germinate. As “orthodox seeds”, birch seeds can be dried and stored for a long period of time. One of the recommended methods of storing birch seed (for no more than one winter) is to “Store in a loosely-tied polythene bag in the main compartment of a refrigerator (approximately +4°C)”. I kept my seeds in a plastic sandwich bag in my bedroom, which is obviously warmer than a fridge. The guide recommends either sowing in Jan-Feb to pretreat naturally or sowing in spring with or without artificial pretreatment. The recommended pretreatment here is to keep the seeds cold (about 4°C) for three to nine weeks (isn’t that just keeping them in the fridge a bit longer?). The guide classes this pretreatment of birch seeds as “Generally effective: a significant proportion of live seeds should germinate

I pretreated my birch seeds by moving them into the shed for a few weeks before planting, and things seem to be going well. Twenty-six seedlings so far, a number I’d be very happy with if I knew for certain they were all birches. I actually sowed several hundred birch seeds, so only twenty-six seedlings looks like a poor rate of germination - but I don’t have anywhere to keep hundreds of birch seedlings!

The Set C sweet chestnuts. I collected them from a magnificent old tree at Wigtwizzle.

Sweet chestnut:
The guide devotes a paragraph to the curious phenomenon of “suicidal” seeds:

…some very small seeds, such as willow and poplar, and some very large fruits, such as oak, sycamore, sweet chestnut and horse chestnut, die quite soon after being shed from the tree – one of the last properties you would normally associate with seeds. The fruits are killed if they dry out and at present there is no known method of doing anything more than slowing down their rate of deterioration. It is therefore only worth collecting seeds of these species if you can sow them fairly quickly, or are prepared to suffer significant losses over, for example, one winter’s storage.

Great. It goes on to describe chestnuts as recalcitrant – highly perishable. One thing you can’t do is to let these things dry out: “if they are frozen or dried, they die”. I didn’t have anywhere humid to store my chestnuts, so I stuck them in the shed all winter. The air in the shed is certainly not as dry as that in the house, but I wouldn’t exactly call it humid. At least I didn’t put them in the freezer.

Still, there is some hope. According to the guide, if you store your freshly-collected chestnuts at low temperatures (3°C to 5°C) – to slow seed deterioration and minimise fungal growth – and high humidity – to retard drying – then you’ll only suffer 60-70% losses over a couple of years. Well, my nuts mightn’t have been kept humid, but they were kept cold (hopefully not too cold) and were only in storage for one winter, so at least some of them ought to still be viable. Later on, the guide warns that “sweet chestnut… will typically decline from 90% to 50% germination over the 10-24 weeks between collection in October/November to spring sowing in March/April”.

The good news is that while they are a pain in the backside to store, sweet chestnut, along with poplars, willows, oaks and horse chestnut, are the “easiest to germinate of all tree species”. No pretreatment is required.

If Set C, like Set B before it, fails to bear treeblog any young sweet chestnuts, then Set D will have to succeed! If it comes to that, then in the autumn, as soon as a new horde is collected, they shall be buried in compost and kept cool and moist all winter.

I collected these, the majority of the Set C rowan berries, from a tree on Whitwell Moor. A further eighty or so berries were collected from a tree near Upper Midhope.

Rowan:
So far it looks like I did okay with the birches, and I might yet scrape through with some sweet chestnuts, but how did I do with the rowans? Ha! terrible!

Rowan berries tend to contain two seeds, although they may hold more. I did not know this when I planted my rowans still in berry form - I thought they only had the one! Something I did think about but failed to act upon is this: rowan berries are eaten by birds; birds digest the berries; birds excrete the undigested seeds; the seeds then grow. How I wished for caged birds to eat my berries in a sort of controlled berry-digesting, seed-cleaning sweatshop. Alas! this just wasn’t practical and I didn’t fancy doing the birds’ job myself (what if I digested both berries and seeds?). In the end I simply planted the berries whole, which was a bit silly:

Fleshy fruits are also some of the most awkward and certainly the messiest to process. …very occasionally a little fermentation can help. However, for seeds such as hawthorn, holly and rowan, fermentation can be significantly harmful or even fatal and is therefore to be avoided. Subsequently, most seeds will need repeated washing not only to remove the clinging remnants of sticky flesh, but also as a means of removing chemicals that have the potential to inhibit germination.

Germination-inhibiting chemicals? Oh no! (At least rowan seeds, like birch seeds, are “orthodox” so can be dried and frozen for storage. My berries experienced the same storage conditions as my birch seeds.) Anyway, once your rowan seeds are nice and clean with no tarrying trace of berry, they can enter pretreatment hell. The guide describes pretreatment as “Only partially effective: even with the longest pretreatment durations and/or several pretreatment cycles”! Still, it recommends 2-4 warm (about 15°C) weeks and 16-30 cold (about 4°C) weeks of pretreatment. Awesome.

I think I’m going to have to exhume my rowan berries, release the seeds from their fleshy prisons, and replant. No time for pretreatment though. Maybe the next winter can be contracted to perform that job if nothing germinates before then?

Level of shame = high.

* * * * *

Raising trees and shrubs from seed is a great little guide. It provides a host of advice on collecting, preparing, storing and planting seed. You can download it free from here: http://www.forestry.gov.uk/pdf/fcpg018.pdf/$FILE/fcpg018.pdf


Posted in The treeblog trees





treeblog Set C planted today!

Day 0 (Set C).

One hundred and one weeks since the planting of Set A and fifty-one weeks since the planting of failed Set B, I planted treeblog’s Set C today in a private garden ceremony. This latest set is represented by three species: rowan (Sorbus aucuparia L.), sweet chestnut (Castanea sativa Mill.), and birch (either silver (Betula pendula Roth) or downy (Betula pubescens Ehrh.)). Whereas a single tree provided me with all my chestnuts, and another with all my birch seeds, my rowan berries were collected from two different trees.


I collected these rowan berries from a tree at the edge of Whitwell Moor. I’ve been acquainted with this tree for a decade now and remember climbing in it during my days at high school. Here it is on the day of berry collection (26th September 2008):



This second lot of rowan berries, which are slightly smaller and more orange than the others, come from a tree near Upper Midhope. I collected them on the 15th of last August, soon after it had sadly collapsed. I paid the fallen tree a visit three weeks ago and was glad to see it still in place and with live buds. Fingers crossed it can go on to see out a few more years. Here’s the rowan as it was on the 24th of August 2006, in all its former glory:



The birch seeds - which kept trying to blow away as I took this photo - were collected on the same day as the berries from the rowan on Whitwell Moor. The bulk of the seed, by the way, is still in the catkins in this photo. They were a pleasure to break up. The seed was produced by a great tree of amazing girth which is either a silver birch or a downy birch. I can’t quite make up my mind seeing as how it appears to have characteristics of both species. My suspicions are that it’s a silver birch that has been roughed up by the elements thanks to its exposed location at the edge of a wood on Whitwell Moor. If only I was in North America… From Wikipedia: “Many North American texts treat the two species as conspecific… but they are regarded as distinct species throughout Europe”.

The great silver/downy birch (26th September 2008):



My sweet chestnuts, the quantity of which gives me deep joy. These bad boys were collected from the Wigtwizzle Chestnut on not one, not two, but on three separate expeditions on the 5th, 9th, and 17th of last October. Primo! The Wigtwizzle Chestnut (seen below on the 7th of July 2007) is one of the most impressive trees in my local area. When you get close, the sheer size of this veteran’s trunk grabs hold of you and slaps your mind. It’s quite literally awesome. Chestnuts from this tree were also planted for last year’s Set B, but none of them germinated. However, I didn’t have that many, and they’d been kept in the house over winter which had probably dried them out beyond the realms of viability. This year the nuts were kept in a garden shed and I’ve got quite a few more.


Another (more aesthetically pleasing) view of those chestnuts.

The planting process was straightforward. I half-filled four seed trays with compost. Into one tray went all the birch seeds, into another went the Whitwell Moor rowan berries, into the third went half of the sweet chestnuts, and into the fourth went the rest of the chestnuts and the Upper Midhope rowan berries. All nicely spaced out likes. Then a light covering of more compost and a good watering.

The trays are now safe in the treeblog compound. Let the germination begin!



Posted in The treeblog trees





Gathering sweet chesntuts for Set C

sweet chestnuts

(The photos in this post can be viewed at a higher resolution – click them to open their Flickr pages, then click the ‘All sizes’ button.)

Sweet chestnuts. I have a desire to see sweet chestnut trees in the treeblog stables, and I know the perfect thoroughbred stallion to sire them. Last year, after jumping the gun a few times, I got hold of a tub of chestnuts from this hoss and planted thirty in March of this year for treeblog’s Set B. Not a single nut germinated. In fact Set B only produced one seedling, and that didn’t live very long. Set B was a massive failure.

sweet chestnuts

2009 will be the year of treeblog’s Set C, and this time there will be sweet chestnuts. I’ve been back to the parent tree twice this week – on Saturday and Thursday – and I have collected a shed-load of nuts. I reckon I must have got about 150. So there is no way treeblog is coming away empty-handed in 2009. Set C is going to be sweet.

pile of sweet chestnuts

The chestnuts I gathered on my forays.


Posted in The treeblog trees





Royal Botanic Garden Edinburgh (27th July 2008) Part 2: Assorted photographs

common lime inflorescence

Inflorescence on a common lime a.k.a. European linden (Tilia x europaea). The common lime is a hybrid of the small-leaved lime (Tilia cordata) and the large-leaved lime (Tilia platyphyllos).

cider gum

A cider gum (Eucalyptus gunnii) tree!

ribbon gum and cider gum

Two eucalypts in front of one of the glasshouses: another cider gum on the right, and in the background a ribbon gum a.k.a. manna gum (Eucalyptus viminalis).

cider gum foliage and seed pods

Cider gum foliage and seed pods. These are the ‘mature form’ leaves, as opposed to the ‘juvenile form’ leaves currently seen on treeblog’s own cider gums.

a grove of six giant sequoias

A grove of six giant sequoias a.k.a. Sierra redwood (Sequoiadendron giganteum). The largest tree in the world, General Sherman, is a giant sequoia.

large sweet chestnut tree

This sweet chestnut (Castanea sativa) tree has a massive trunk, which splits into three huge boughs.

looking up a large sweet chestnut bough

Looking up one of the huge boughs. I saw three different sweet chestnuts in the Garden and all were of massive girth. I bet they are easily some of the biggest trees in the collection.

sweet chestnut male flowers and developing cupules

Male sweet chestnut catkins in full bloom. The bright green spiny female parts will develop into the distinctive spiky cupules which each usually contain three chestnuts.


Posted in Gone for a walk





Thetford silver birch provenance trial (Part 2)

A follow-up post to this one all about my time at the Thetford Forest silver birch provenance trial, replete with photos of trees and bark and twigs and stuff like that.

common or pearl-studded puffballs

The trial was rife with these little horrors: common or pearl-studded puffballs (Lycoperdon perlatum), well past their prime. These little bags are full of spores, so mind you don't step on them. The spores are spikey and will irritate the lungs if enough are inhaled, causing lycoperdonosis.

red bryophyte

A normally-green-but-for-some-reason-red bryophyte. Isn't it pretty?

silver birch seed catkins

A pair of female silver birch catkins. These will be packed with tiny winged seeds!

twisting twig

This is very strange. A twig coming in from the right has made contact with a twig from a different tree and has coiled around it like a vine tendril. Why has it done this? Is it some freakish genetic mutation? Can we cultivate vine birches???

sweet chestnut

A sweet chestnut. There were a couple of big sweet chestnuts and a few big oaks around the edge of the provenance trial. These probably grew up when the land was in its previous use as a pine plantation and were retained when the pines were felled. There were loads of chestnuts covering the ground beneath this tree. I hereby conclude that this part of the forest has a squirrel shortage, and that rabbits don't like chestnuts (there were a lot of signs of rabbit activity).

bark on a Scots pine stump

Looking down at bark on a pine stump. A few of these stumps were scattered about the trial, remnants of the old plantation. Most of the stumps had been removed and were piled outside the trial. I guess the ground was then rotovated to level it off again.

young pines

These young pines, probably Scots pines, were growing right next to the birch trial. One day they might look like...

mature Scots pine

... this. A mature Scots pine plantation, just a couple of hundred metres from the birch trial. These can't be far from being harvested now.

And today I am fresh back from another silver birch provenance trial, this one overlooking Loch Tay up in the Highlands. I shall tell you all about it in the next post!


Posted in Holidays and field trips





Sweet chestnuts (finally)

For some time now I have been waiting to get hold of some nuts from a certain venerable sweet chestnut tree. I visited the tree in July and August, but the nuts where nowhere near ripe back then, and it appears that I totally jumped the gun! As I have been up in Edinburgh since the end of August, I have been unable to visit the sweet chestnut myself since then, so I have been sending my father on nut missions in my place. The tree has finally given up its fruits, and my father was there on the 20th of October (last Saturday) to collect a few for treeblog's second set of trees: Set B.

The chestnuts! These will be planted as part of treeblog Set B next year.

I collected these chestnuts from a tree at the University of Edinburgh's King's Buildings on the 15th of October. They will not be planted for treeblog... they will probably be roasted and eaten instead.

A few of my Edinburgh chestnuts, once removed from their spikey cupules. There are three nuts to a cupule, and the cupules tend to hang in pairs.


Posted in The treeblog trees





Sweet chestnut - waiting for the nuts

Thinking ahead to next years' set of treeblog seedlings (Set B), I have already been out and collected two lots of seeds. Another species I want to grow for treeblog is the sweet (or Spanish) chestnut, Castanea sativa. My Collins Field Guide Trees of Britain & Northern Europe [2nd Ed.], by Alan Mitchell (1978, HarperCollinsPublishers) has this to say on the sweet chestnut's flowers and fruit:

Axillary bunches of cord-like catkins at end of June open whitish-yellow, 25-32 cm long, crowded with small male flowers each a mass of stamens, turn brown and fall in mid-July. Female flowers sometimes on small, separate spreading catkin, 5-6 cm long, 5-6 flowers; usually 1-2 at base of short, 10-12 cm catkin of unopened, yellowish rudimentary female or rarely male flowers, near tip of shoot. Female flower a 1 cm rosette of bright green, minutely hairy spines with a bunch of spreading, slender white styles. Fruit in bunches of 2-3, in light yellow-green 3 x 4 cm husk covered in sharp spines 1.5 cm long, radiating in clusters; interior white with silky, appressed hairs. Usually two nuts: one globose, the other smaller, concave; dark, shiny red-brown, narrowing to a tip bearing dead styles.

There is a huge, old sweet chestnut quite local to where I live, and it is the offspring of this tree that I wish to raise. I visited the tree on the 7th of July, earlier this year. However, the 'cord-like catkins' were not yet in flower.

cord-like male catkins on the sweet chestnut

Cord-like male catkins on the sweet chestnut (7th July 2007).

close-up of male catkins - not yet in bloom

Close-up of the male catkins (not yet in bloom) (7th July 2007).

I visitied the sweet chestnut again a few days ago on the 8th of August, but was disappointed to find that the nuts were not yet ready for harvesting. In fact, the tree was still in flower, despite my Field Guide stating that the male flower-supporting catkins "turn brown and fall in mid-July". Perhaps the unusually wet weather this summer has affected the tree's phenology. treeblog will have to wait a little longer to get hold of some sweet chesntuts.

cord-like male catkins in bloom on the sweet chestnut

Cord-like male catkins in bloom on the sweet chestnut (9th August 2007).

close-up of male catkins in bloom, with female flowers (spiny) in foreground

Close-up of male catkins in bloom, with spiny female flowers in the foreground (9th August 2007).


Posted in The treeblog trees





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





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