Fungus as a Killer
David Bole | Richard Fortey
The biggest organism in the world is not the blue whale, but a mycelium that spreads across an incredible 2,384 acres in Oregon’s Blue Mountains. It’s called Armillaria mellea, or the honey fungus, and this example is thought to be over 2,000 years old. It’s a mind boggling example of how far a mycelium can grow. But it also reveals just how destructive a feeding fungus can be.
These are clumps of honey fungus. It’s the same fungus that spread inexorably through the forests of Oregon and it demonstrates a very different, some would say sinister, relationship between mycelium and trees.
Like the balanced nutrient exchange that we see between most fungi and their plant partners, honey fungus takes much more from its host than it gives. It consumes all the sugars it needs, but crucially doesn’t give back enough water and nutrients to help the tree grow properly.
As a result, the greedy mycelium of this fungus thrives, while the tree slowly weakens. Honey fungus is a slow killer. It advances from tree to tree on hidden threads. As our tree population ages and some sickens, the rise of honey fungus is inexorable.
But it’s not the biggest threat to our plants and trees. There’s another species of fungus whose hunger is even more deadly. I’ve come to Norfolk to find evidence of a fungus that’s very difficult to see, but whose eating habits are threatening to wipe out one of Britain’s oldest trees.
Ash Dieback Disease
Just a few years ago, a new killer arrived in Britain – ash dieback disease, or Chalara fraxinea, to give it its scientific name. And no fungus better demonstrates the greed of mycelium for nourishment… and if it has its way, maybe, magnificent forest trees like this ash may yet become just a part of history.
David Bole knows only too well just how destructive this fungus has become.
David Bole “And there’s quite a lot of dieback in here, isn’t there? This is one of the first woods where we discovered it. What we’re finding now is that there’s over 500 cases in the wider environment and as we do more in-depth surveys, more and more cases are coming to the fore.”
Take me through the symptoms.
David Bole “Well, the first thing to look for is this, the black leaves, which we’ve got here and we’ve got a really good example on this little, young tree. The leaves have died but they’re black. They really don’t look healthy and they’re hanging onto the tree.”
I notice they die from the top too, so they’re dead up here but still green down here.
David Bole “Yes, you know, it’s called dieback and that’s a good way to think of it – we have the tree slowly dying back. Other symptoms are these diamond-shaped lesions. The fungus lands on the leaves, the mycelia come in, and works its way up and down the cells of the tree and forms these very particular diamond-shaped lesions.”
This process is rather eerily called necrotrophy, which means eating the dead. The feeding hyphae of ash dieback attach themselves to their tree hosts in the same way as other fungi, but they obtain their sugars without providing any nutrients or water in return. It’s all one-way traffic and has a fatal outcome.
David Bole “OK, so let’s just have a look inside.”
Oh, yeah. You can see discolouration. It’s absolutely patent.
David Bole “So the disease has entered here and this is the fungal mycelia, which are starting to work its way inside the tree. The mycelia get inside all the cells that transport the water up and down the tree and stop the water transport and so the tree effectively dies of thirst, if you like.”
It’s a sad end to one of our most beautiful and elegant forest trees.
David Bole “It really, really is, yes. I mean we’ll probably lose a generation of ash but let’s hope we see that coming back.”
Ash dieback demonstrates just what happens when the delicate balance between plant and fungus gets out of kilter… and that’s what allows this disease to spread so far and so fast. It also shows just what a voracious eater fungal mycelium can be. But though this unstoppable appetite can be deadly in the natural world, some scientists are looking to turn it to our advantage.
