Dear Luke,
I don't know whether you've seen Prof. Siegfried Fink's excellent textbook
"Pathological and Regenerative Plant Pathology". It includes authoritative
information about interactions between trees and fungi (including decay
fungi). I've mentioned the publishing-details in a message that I've just
sent to Gerrit Keiser.
The ideas that you've listed below are fascinating. I don't think that I can
add much to what you've already said, but I'll try my best as follows:
1. Secondary metabolites: I agree that any work involving wood blocks (as
opposed to living trees) would fail to take account of the possibility of
defensive substances being transported in order to strengthen reaction zones.
You've touched on a long-standing controversy: are reaction zones just formed
"once and for all", or are they maintained by a supply of sugars, which could
be converted in additional phenolic and other defensive materials? I think
that research would be possible but difficult. For example, radioactive
tracers could be used, but only after preliminary research in order to find
how and where they should be applied (without damaging the tree or
endangering people!). In principle, I can't see much reason for sugars to be
transported towards an already-formed reaction zone, unless the depletion of
sugars creates a local nutrient "sink". On the other hand, other defensive
substances might be systemically translocated. I'm thinking here of induced
systemic defence. Otherwise, I'm not aware of much new work following the
excellent review of tree defences by the late Ray Pearce (New Phytologist:
Vol. 132, No. 2 (Feb., 1996), pp. 203-233).
2. Moisture content as a passive defence: I agree with Boddy and Rayner that
moisture-content (and hence aeration) is of key importance in controlling
fungal activity in woody tissues. Certain brown rot fungi and many
strip-canker fungi colonise sapwood latently or endophytically and then wait
(perhaps for decades) until the wood becomes suitable for them to cause
active decay. I think that the trigger is most likely to be a loss of
wood-moisture. At least some of these fungi produce thick-walled spores or
other resting-structures, which can remain dormant for many years, but I
don't know of any such structures being formed by Ganoderma spp. Amongst the
white-rot fungi have other means of overcoming the lack of aeration that they
encounter in water-filled xylem cells. Some of them tunnel their way within
the cell walls, where they can presumably gain access to oxygen diffusing
from inter-cellular spaces. Others, having colonised a volume of wood, can
pump excess water away. It's obvious that their fruit bodies can provide an
exit-route, either by evaporation or by active exudation of water (as in the
well-known case of various Inonotus species). Even in wet-stored timber,
where wood is "super-saturated", Armillaria can become established. Berthold
Mezler has found that superficial hyphae of Armillaria create their own
oxygen diffusion system, by forming bubbles of carbon dioxide in wood cells
on the surface of the wet logs. (Canadian Journal of Botany (2004) 82,
1338-1345.)
3. Respiration in excised wood blocks: In the work published by Ferner &
Schwarze, London plane wood blocks were heat-sterilised in order to ensure
that only the test-fungus was present during the incubation. The host cells
were therefore not capable of respiration, but there would have been fungal
respiration in the colonised part of the wood-block. I don't think that
sugars could have diffused much in solution across the pre-existing reaction
zones. The fungi would, of course have utilised the sugars, but G.
applanatum would have done so only on the inoculated side of the block, since
it failed to breach the reaction zone. Preservation of sugars would have
contributed to the dry-weight slightly, but most of the weight-loss would
have been caused by breakdown of the cell walls.
Finally, I agree with you about the patterns of fungal colonisation and
fruiting around the bases of trees. In some cases, it looks as though decay
stops when the fungus runs out of available wood as its energy-source.
Otherwise, the fungus survives either by being able to move into fresh
sapwood (arguably as a parasite) or by taking advantage of the natural aging
of sapwood. As you rightly point out, the formation of new increments by
the tree can often keep pace with the outward extension of the fungus. I
think, therefore that we should not assume that a modest amount of
'parasitic' colonisation by the fungus is a problem for the tree. As you
also point out, the long-term co-existence can be shortened as a result of
storm damage, drought or excessive pruning, which cause increased aeration of
the wood, favouring massive extension of the decay.
Kindest regards,
David
-----Original Message-----
From: luke steer [mailto:luketreescapes@xxxxxxxxxxx.com]
Sent: 23 December 2011 21:16
To: UK Tree Care
Subject: RE: Ganoderma applanatum/australe on n.maple - implications
Dear David,
Thank you for taking the time to define terms and describe potential
relationships between living trees and decay fungi.
Thank you for also including the link to Schwarze and Ferner (2003), which I
have read along with the relevant sections in Schwarze (2008). Reading these
has prompted some questions that I'm sure others will have also thought of.
I would be interested to hear comments from the forum about the following.
1. In the experiments described the blocks of wood were excised from
trees and therefore what occurred in them was not influenced by the rest of
the tree. Could it be possible that a tree could mobilise secondary
metabolites, manufactured in other parts, to arrest fungal growth? It would
be interesting to know whether fungistatic and/or fungitoxic chemicals could
be transported in xylem and/or phloem sap, the symplast or a combination of
these. How could we find out whether this is a possibility and, if it is,
its significance?
2. Boddy and Raynor (1983)
(http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1983.tb04871.x/abstract)
proposed that the saturated nature of live sapwood would prevent development
of decay fungi within it. Could it be possible that once Ganoderma adspersum
had penetrated the reaction zone further growth would be arrested, or at
least slowed, by the saturated nature of the substrate? Could it potentially
enter 'hibernation' until the wood becomes aerated? This would create a
situation where it would be 'first on the scene' after the wood becomes
aerated or physiologically dysfunctional - a similar strategy to fungi that
are latent within live trees until they lose vitality.
3. Would the fact that the wood blocks were excised from the tree also
affect the energy status of live sapwood cells? Could, during the life of the
experiment, their carbohydrate reserves become depleted by respiration and,
if so, could this affect the ability of the wood behind the reaction zone to
take defensive action and therefore the weight of the blocks at the end of
the experiment?
Finally, I tend to assume that the majority, if not all, the Ganoderma
brackets that we see in the UK on live beech trees are those of G. adspersum.
These are usually in the flutings between buttresses. I've previously
assumed that the growth of hyphae from a centrally occupied area in a tree,
towards the outside, was primarily to produce the brackets and was confined
to the dead, moribund or slowly growing sapwood found in these areas - they
are rarely seen on healthy buttresses that are growing rapidly. I mused that
wood in these compressed flutings may have died, become moribund or that the
hyphae were growing between the two faces of bark. I also presumed that the
growth of fungal hyphae in these areas was primarily to produce fruit bodies
rather than obtain additional substrate to digest. Do others consider that
there may be some validity with these assumptions? Maybe we need to create a
bank of photographs of recently felled tree stumps with brackets on them and
compare these observations.
If fungal hyphae are restricted to dead or moribund tissues in flutings, and
don't have the capacity to aggressively colonise healthy sapwood while the
tree remains healthy, should it not be the case, bearing in mind the axiom of
uniform stress, that the tree will allocate sufficient resources to growing
additional tissue around colonised areas to compensate for the reduced
mechanical strength of the decayed wood (sorry, that sentence is a bit of a
mouthful!). That is until it loses vitality? If this were to be the case
the decay fungi and healthy tree could live together for many years or
decades until the tree suffered an event that severed/killed roots,
severed/killed crown area or sapwood. In the Lake District, my observations
suggest that drought and storm damage are the weather events that most often
create physiologically dysfunctional sapwood that becomes available for
colonisation by decay fungi.
Please accept my apologies if I'm rambling.
Merry Christmas to one and all.
Luke
-----Original Message-----
From: David Lonsdale [mailto:d.lonsdale2@xxxxxxxxxxx.com]
Sent: 21 December 2011 18:30
To: UK Tree Care
Subject: RE: Ganoderma applanatum/australe on n.maple - implications
Dear Gerritt,
In answer to your question, I define the relevant terms as follows:
1. Biotrophic parasite: a parasite which obtains its nutrients from the
living cells of the host (usually by the penetration of those cells, without
killing them). Examples include rusts and mildews.
2. Necrotrophic parasite: a parasite which obtains its nutrients by killing
cells of the host (usually by the secretion of enzymes and/or toxins).
These parasites are usually able to grow also as saprotrophs (4, below) and
they are therefore often alternatively called "facultative parasites".
Examples include many fungi and bacteria that cause general dieback and/or
decay of host tissue.
3. Hemibiotrophic parasite: a parasite which obtains its nutrients both as a
biotroph (usually when it first penetrates host tissue) and as a nectroph
(usually at a later stage, after an initial biotrophic phase). Examples
include many organisms that cause leaf spot diseases.
4. Saprotroph: an organism which obtains its nutrients from the dead remains
of one or more living organisms.
As far as I know, there are no wood decay fungi in categories (1) or (3)
above. Some of them have the ability to grow into previously living sapwood,
causing it to die (or become "dysfunctional" and then causing decay. I think
that they can be regarded as necrotrophic parasites (2), but I do not like to
use this term without qualification, since many of them live predominantly on
wood that is already dead. This could be sapwood that has been damaged by
injury, or it could be central wood or the tree, which has become heartwood
or ripewood because of aging.
Traditionally, wood decay fungi have been described as "parasitic" if they
are found on living stems, branches or roots. I do not think that this is
correct if the fungus concerned is colonising only wood that is already dead.
According to the above definitions, Ganoderma applanatum has been observed to
be mostly saprotrophic, whereas G. adpsersum/australe has some capacity to
act as a necrotrophic parasite. These observation seem to be confirmed by
some experimental work by my friends Schwarze & Ferner at the University of
Freiburg i. Br., Germany. (see:
http://www.enspec.com/articles/ENSPEC%20Research%20Paper%20-%20Ganoderma%20on%20Trees.pdf
)
Schwarze & Ferner found that G. adpsersum/australe was able to penetrate
defensive barriers (reaction zones), thus growing into functional sapwood.
It does not necessarily harm the tree seriously. Instead, it might be able
to co-exist with the tree for many years, instead of dying out when it has
utilised all the wood that was initially available to it. In some cases,
however, the fungus does enough damage to the sapwood (especially in the
roots of the tree) to cause the decline and perhaps death of the tree. Also,
the decay can, in my experience, become overwhelmingly rapid if the wood
becomes more aerated because of excessive pruning or the storm-breakage of
major branches.
As suggested in the recent correspondence, G. adpsersum/australe appears to
behave differently in different host species. I think that it can be
especially aggressive in species with which it has not co-evolved. I have
seen examples where G. adspsersum/australe (or perhaps a similar-looking
species of Ganoderma) seems to have killed exotic conifers such as Araucaria
araucana. I agree that it can cause extensive decay in the broadleaved trees
in your list. However, in Fagus sylvatica (one of its main hosts in the UK),
the tree and the fungus often seem to co-exist for many years. The
co-existence is probably even longer in species like Quercus robur and Q.
petraea, which have durable heartwood and therefore tend to become decayed
more slowly.
Regards,
David
-----Original Message-----
From: Viper Snake [mailto:snake24@xxxxx.nl]
Sent: 21 December 2011 14:14
To: UK Tree Care
Subject: RE: Ganoderma applanatum/australe on n.maple - implications
David,
1). Tony Croft has been using the term 'biotrophic parasite' in relation to
G. adspersum/australe on here. I questioned him about these labels and asked
him whether he could point me to any research that demonstrated that the
fungus was 'biotrophic' and/or 'parasitic', because this was news to me, but
he hasn't replied yet. Can you point me in the right direction?
1). Tony uses my terms and the results of my in situ research on biotrophic
and/or necrotrophic parasites, which I already have explained on Arbtalk, see
: 2. http://arbtalk.co.uk/forum/529703-post53.html .
2). Similarly, that G. adspersum/australe is deterministically fatal to
Acers, no matter the circumstances, is also news to me. Where does the
research for this conclusion come from ?
2). From my own field research on the effects on the stability and condition
of different deciduous tree species of the biotrophic parasitic G. australe,
of which the mycelium causes a white rot with selective delignification, that
is most detrimental to Acer, Platanus, Populus, Salix, Tilia, Aesculus (Anne
Frank tree) and Quercus rubra.
Gerrit
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