In the 1940s, the highly invasive heath star moss was first introduced to the UK, and since then has been damaging the environment by turfing out other plant species. It can grow everywhere from bare soil and rotting tree stumps, to tarmac and thatched roofs. However, four years ago, whilst on the Isle of Wight, George Greiff, who is now based at the John Innes Centre in Norfolk, noticed patches of dead moss on a cliffside and took samples away. George and his collaborators identified the cause of death: a ‘killer fungus’, and he’s been speaking with Rachael Ralph…
George – So we found a moss dieback fungus, so this is a fungus that basically kills an invasive species of moss. This moss is called the heath star moss or tank moss. It’s an invasive species that arrived in the UK in like around about 1941, they think, on military equipment. It’s kind of really spread insanely since then, occupying a lot of different habitats across the country. It’s a generalist species, so it’s able to grow in a lot of different places, but it’s particularly bad on heathland and sand dune ecosystems.
Rachael – When did you first observe this fungus infecting the moss?
George – So it’s quite a long story. I think I’d been seeing it and collecting it since about 2017, but it was quite a tricky fungus to identify. So I kept kind of seeing it and throwing it away. But about four years ago now, I started finding a lot of it, and noticing a lot more of it, and kind looking for it specifically, and realised it really is kind of basically in every heathland at least that I was looking in and started to study it in detail then.
Rachael – And when you examine the infected moss, what do you actually see? Does the moss change colour or is there any patterning at all?
George – Yeah, so this actually forms, well this term seems to be liked in the news article, fairy rings of death. So this is like brown patches of kind of holes in the moss, which otherwise forms these very lush, dark green, thick mats on the ground, doesn’t allow anything else to grow. But the fungus is basically forming these holes in it, which is like these rings that grow outwards; they’re brown, the moss is dying. There’s like a white fluff on the tops of the leaves of the moss in the frontiers of the infection as it spreads outwards. So it’s quite striking actually, and you can see it in well-developed fungal populations at least quite easily with the naked eye.
Rachael – How do you actually know it was the fungus killing the moss? What kinds of methods did you use to determine that?
George – Yeah, so initially we kind of just, you know, take a sample, take it home, look at it under the microscope and a compound microscope and see what’s going on and then kind of identified it was a fungus. But as I mentioned before, it’s quite a tricky fungus to identify and it doesn’t have many features that allow it to be identified just by its appearance. So actually the thing that really helped a lot with this was DNA sequencing. So I was able to take some of the fungus, put it in a tube, sequence it and that revealed that it’s actually part of this genus called Bryoscyphus, which is a group of moss killing fungi, so that kind of helped. And then, yeah, eventually started connecting with other people who’d found the fungus in France and the Netherlands and we really put it all together and sequenced everything, found it was all the same and put a description together.
Rachael – Do you know exactly how the fungus is killing the moss?
George – Yeah, so that’s a good question. The fungus seems to be growing inside the moss cells, so it kills it from the inside. It also grows on the surface of the cells, so it seems to produce structures on the outside as well. So it’s really kind of attacking it in both ways, probably with chemicals and physically with its mycelium as well.
Rachael – So of course this fungus is infecting the invasive heath star moss species, but do we need to be worried that the fungus can also infect other plant species that aren’t causing a problem to the environment?
George – Yeah, so the fungus doesn’t grow on, you’ll be relieved to hear it doesn’t infect humans, doesn’t infect animals. It doesn’t infect other plants as far as we know, except for one other moss, which is a native moss species related to the heath star moss. However, it doesn’t cause as severe symptoms in the native species that we’ve observed so far. So that’s why we think maybe the fungus is native and was growing on that moss just happily living its life, and then when the invasive species came, it hopped onto the invasive species. That’s one idea. It could also be the other way around, which would be a little less thrilling.
Rachael – Yeah, that would be quite concerning. Do we also need to be concerned that maybe some of these mosses have a mutation that protects them against the fungus, and then they spread their spores, then suddenly we have both an invasive and a resistant moss species growing?
George – Yeah, I guess it is a concern, but you’ve got to realise the moss is reproducing the whole time. It has quite a lot of genetic diversity that it is constantly adapting and in theory could become resistant to the fungus, but on the other hand, the fungus is doing it as well. So I think what would probably happen is one of these kind of co-evolutionary arms races where if the moss changes, the fungus will change, and vice versa, and they’ll keep following each other. So I think the more likely thing we’d have to worry about is if the fungus is able to spill over and infect more vulnerable native mosses that are related to the star moss, but not the same one.
Rachael – And is that what you’re planning to look at in the future?
George – Yeah. So I’ve recently moved from working in the kind of museum sector where we’re kind of using museum specimens like a kind of time machine to look in the past to see whether the fungus was on mosses before the star moss or the tank moss arrived in the 1940s. So we have these specimens and we can look, it’s quite difficult and time consuming. We’ve given that a try. What I’m doing now is kind of working in the lab and trying to see how many different species of moss related to the star moss are susceptible to the fungus, try and work out its mechanisms of infection and learn a bit more about it. We can sequence genomes. There’s quite a lot of exciting and kind of cutting edge science we can do on it.