So, there was this story yesterday in the Guardian about how ash trees are at risk from a fungus: http://www.guardian.co.uk/environment/2012/oct/04/deadly-fungus-ash-tree-imports?intcmp=122
This topic has been in the media a fair bit lately, but very few of the stories have gone into the mechanisms and details, so I thought I would write briefly about those, as they are fascinating, and can help with understanding the problem better.
Most of the stories in the media have just said that it affects leaves, which is a very vague description.
So, first of all, to make sure we all know the tree we are talking about, this is an Ash tree, otherwise known as Fraxinus excelsior:
Fraxinus excelsior, the common Ash. Image from Wikipedia
And here is it’s close-up (It doesnt get red-eye like I do, and is always photogenic!)
Close up of the leaves and “keys” (fruit) of the common Ash. Image from Wikipedia
So, now we have met the victim, lets meet the perpetrator (Sorry, I am catching up on CSI episodes at the moment, so excuse me if I go a bit Horatio Caine).
This is where it can appear a bit confusing, because this fungus actually has two names: The one most mentioned in the media is Chalara Fraxinea and it looks like this when it is grown in a lab:
Chalara fraxinea, politely posing in a petri dish. Image from Federal Institute of Technology Zurich
This is the fungus mentioned in the Forestry Commission factsheet about this problem (See further reading for link).
This is what is known as an anamorph, which means it is the asexual reproductive phase of this fungus.
I think the reproduction of plants, fungi and small micro-organisms is really cool, so I am going to explain it a bit here as it can seem a bit confusing (I remember getting tied in knots trying to revise this for functional biology!)
The asexual reproduction of fungi such as this species involves producing spores (from the greek spora, which means seeding, or sowing), which you might know from the puffball mushroom, when you kick it, it gives off a load of dust-like stuff, which is actually the spores for the next generation of the fungus, which looks like this:
Puffball mushroom releasing its spores. Image from wikipedia
Each of those spores is a potential new fungus, provided it lands in a suitable environment for growth. This method of dispersal is very haphazard, and this is why these organisms produce so many spores. It is a bit like closing your eyes and throwing a handful of seeds randomly out on a bit of ground and hoping for the best.
They are formed by mitosis, which is also how our cells in our body are replaced and is in itself a really really cool process (especially when you see slides of it), and which I will cover in depth in a later post.
As I mentioned earlier in this post, this fungus has two names, the asexual form C.fraxinea and the sexual form Hymenoscyphus pseudoalbidus. Now, maybe it is just me, but I found it a little confusing initially to understand how one organism can have two names, or even two life cycles when I first started reading about this.
This image shows the life cycle of an Ascomycete, which is the group of fungi which this particular one belongs to. The asexual cycle is the loop off to the left of the diagram.
General life cycle of an ascomycete. Image from Penn State University
From what I gather from reading several journal articles on this species, it seems that the asexual form is on the leaf litter, and dead wood on the forest floor, and this is not infectious (or pathenogenic to use the sciencey word).
It all goes a bit nasty for our Ash trees when it is in the sexual form, H.pseudoalbidus . It is called “pseudoalbidus” because there is another species called H.albidus which is not responsible for this problem in Ash trees, but appears physically similar.
This is what the fungus looks like:
H.pseudoalbidus on a branch. Image from Institute of Technology, Zurich
This confusion with two different names for the sexual and asexual form of fungi will be less confusing soon, as in 2013 they are changing the naming structure, so that there is one name for a species of fungi, regardless of which stage of the life cycle it is in.
As you can see from the diagram, the asexual form of the fungus only refers to the spores, everything else within its lifecycle is classified as H.pseudoalbidus. Calling this C.fraxinea in the media is quite confusing, but understandable, as many journals refer to this fungus as C.fraxinea.
The cycle of infection appears to be, that the spores remain in the litter, or on dead branches over the winter, and then, in the summer, it germinates, and becomes the white mushroom thingies. These release spores, which are spread by the wind, and some end up on the leaves of Ash trees, and on the branches. These form structures known as mycelium which are basically a mass of threads, and it is these which are responsible for the damage to leaves and branches, if they get into a gap in the bark, they form lesions like on this branch:
Necrotic lesions on a branch. Image from EPPO (European Plant Protection Organisation)
These are also known as cankers, and result from the death of the tissues.
The fungus also damages the leaves, as shown in this image:
Leaf dieback as a result of fungal infection. Image from EOL
The dead branches and leaves then fall to the floor, and the cycle begins again.
This is a relatively new infection in Ash trees, first being noticed in the mid 1990s.
There are ongoing discussions as to why this has arisen, as this fungus has been known since the late 1800s, but as the non-infectious H.albidus. There is discussion about whether climatic stress has weakened the trees resistance to infection, or whether the infectious version of this fungus is better suited to the milder climate conditions over recent years, or whether this new infectious form is a mutuation which has arisen recently.
Whatever the cause, the result is devastating. Denmark has lost around 90% of its Ash trees since the infection arrived, and other European nations are reporting large scale losses of Ash trees. The infection appears to have arrived in the UK (Which is usually protected from these types of infection because of its island status) by importing of young trees which were carrying the fungus.
So far, it seems that the fungus has not managed to infect “wild” trees in the UK, and the government has begun a consultation, which will end on the 26th of October, which could lead to a ban on imports of Ash (and given the severity of the threat, I would hope that a ban is imposed).
Further Reading: (Most are very easy to read, with the exception of the journal article at the end, they are mostly from the Forestry Commission, and similar bodies)
http://www.eppo.int/QUARANTINE/Alert_List/fungi/Chalara_fraxinea.htm
http://www.ethlife.ethz.ch/archive_articles/100408_eschenpilz_per/index_EN
http://www.fera.defra.gov.uk/plants/plantHealth/pestsDiseases/documents/chalaraFraxinea.pdf (Rapid Risk Assessment)
http://www.forestpathology.ethz.ch/research/Chalara_fraxinea/index_EN
http://www.forestry.gov.uk/pdf/pest-alert-ash-dieback-2012.pdf/$FILE/pest-alert-ash-dieback-2012.pdf
http://www.forestry.gov.uk/chalara
http://www.guardian.co.uk/world/2012/oct/07/disease-killing-denmarks-ash-trees
Krautler & Kirisits: The ash dieback pathogen Hymenoscyphus pseudoalbidus is associated with leaf symptoms on Ash species (2012) http://www.academicjournals.org/jaerd/PDF/Pdf%202012/14MayConf/Kraeutler%20and%20Kirisits.pdf
Skeptical Squirrel 