Podcast: Rhododendrons and sudden oak death

Rhododendron

Rhododendron ponticum

9 July 2013 by Sue Nelson

This week in the Planet Earth podcast, Bethan Purse from NERC's Centre for Ecology & Hydrology, and Matt Elliott and Colin Edwards from the Forestry Commission, talk about a new map that could help control the spread of sudden oak death, a disease that threatens trees and plants like oak, beech, larch and bilberry.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, a disease that's killing millions of trees across the world and how a new map can help control its spread across Scotland.

Hello, I'm Sue Nelson and I'm about ten miles outside Edinburgh near Penicuik, and I'm in a woodland surrounded by a lovely variety of trees from sycamore and beech to lark, even cherry a little bit further down this path and there's also, apart from some rather noisy jackdaws in a nest above my head, there are some rhododendron bushes and I'm joined by Dr Bethan Purse from the Centre for Ecology & Hydrology, pathologist Matt Elliott from Forest Research and Colin Edwards who is from the Forestry Commission, Scotland. Matt, this disease, it's called 'sudden oak death', what is it?

Matt Elliott: So sudden oak death was a suite of symptoms that was first noticed in California in the 1990s where millions of trees were found to be dying with no obvious reason why, and then later on in the 90s and early 2000s a pathogen was described in Europe called Phytophthora ramorum and eventually the link was made that Phytophthora ramorum was killing all of these trees in America plus it was in the nursery trade in Europe and this Phytophthora is a fungal-like pathogen. It is spread in wind-driven rain and it gets onto leaves of plants and the spores grow into the plant and kill it eventually by restricting the movement of water and nutrients and the plant just dies back.

Sue Nelson: Now it's called 'sudden oak death' but it's not just oaks is it?

Matt Elliott: No there's currently over 150 host species on the list.

Sue Nelson: Colin, you're an expert in rhododendrons, it's an invasive species, what's the connection then between rhododendrons and sudden oak disease?

Colin Edwards: Phytophthora was first noticed on rhododendron down in Cornwall around the mid-90s. Rhododendrons were being infected by Phytophthora but it wasn't killing it, so it was acting as a host and unlike when it gets onto trees when it actually kills the host it was infecting the leaves which were then producing spores which would go on to infect either more rhododendron bushes or other species.

Sue Nelson: Now, there's a rhododendron bush very close to us here that doesn't look particularly healthy and it has also got an orange tag on one of its leaves. This, I assume, then is an infected bush.

Colin Edwards: Yes. It is quite typical to find rhododendron in woodlands like this. These are called 'policy woodlands' so they are very mixed. They have got lots of shade and damp understory which is perfect for rhododendron growth but it is also a perfect environment for a pathogen because it is a damp micro environment.

Sue Nelson: Can we just go through so I could have a closer look to actually see the effects? We have to go through the nettles. Likely I've got long trousers on and trainers. There we go... now a lot of the branches are bare and some of the leaves that are there are sort of curled up, yellow, dry, brown.

Colin Edwards: You will notice that the leaves droop and the infection either affects the petiole which turns black because that is where the water sits and the loose spores can infect.

Sue Nelson: Petiole?

Colin Edwards: It's a stalk between the wide part of the leaf and where it joins onto the stem, the branch, or the very base of the leaf turns black and that's because the water droplets sits there and again that's where it becomes infected first.

Sue Nelson: Oh yes, I hadn't noticed that. It is right at the pointed tip that it has happened and this is just beneath a sycamore tree.

Colin Edwards: Yes.

Sue Nelson: Its branches are intermingled with the tree there.

Colin Edwards: This is a bush that I have actually had some experience with. I pass it almost daily and it has gone from full health to the condition we have here where about 10% of the leaves are still alive in only a year. So it's a very rapid progression from healthy to unhealthy and hopefully it will soon be removed to prevent transferring the spores onto anything else.

Sue Nelson: Let's go back through the nettles and catch up with Bethan Purse. How does the disease spread from the rhododendron bush to the tree? It seems quite obvious really because they're touching, the branches are interlocking.

Bethan Purse: A lot of the outbreaks in Cornwall there is actually rhododendrons in contact with the trees that then got infected, so it is by direct contact and by rain splash but also Phytophthora as the potential to live in the soil for a long time so you can clear the rhododendrons and Phytophthora is still in the soil and it also has just recently been found to infect the roots of rhododendrons as well, so there's all these ways in which these pathogens can be maintained on a site which makes them really difficult to control. That's really why with the risk maps and the risk framework we wanted to try and figure out where some of these conditions that Colin was talking about that rhododendron like, like the moist soil, they like not too high elevations below 500 metres.

Sue Nelson: And that's what you've been doing isn't it, is making maps of the countryside of where everything is. So what do you do? Do you make note of what vegetation are there, what trees are there, which bits are infected and which aren't over a period of time?

Bethan Purse: I guess there's several components to it, so we're making maps of where the rhododendron likes using geographical information systems and layers of the elevation, the vegetation and disease and the soil conditions, and then we have elements of the Phytophthora risk framework that actually look at the climate suitability for Phytophthora infections. So we have two main Phytophthoras we're worried about, Phytophthora kernoviae and Phytophthora ramorum and we looked at how temperature and moisture conditions vary over time in each square of Scotland on a daily basis and we make a map of where the suitable climate conditions are the best.

Sue Nelson: Matt, this map, when you've got this map, each square is about a kilometre square, what do you get then in the end? What does it show you?

Matt Elliott: The map is just a graphical representation of where the pathogen is most likely to be found. It doesn't mean it necessarily will be found there, the host needs to be there and also the pathogen needs to make its way there as well, so you need these three things.

Sue Nelson: You've got a map that's colour coded - red is the equivalent of danger, practically, or where there's more. Colin, how does an organisation like the Forestry Commission use this information?

Colin Edwards: Well, the first thing we need to do is prioritise where we do any control because rhododendron is throughout Scotland, it's a bigger problem in the west than it is in the east. It's associated with a range of different habitat types, but we don't have sufficient funds to tackle every single bush, every single outbreak of it everywhere. So this mapping will help prioritise areas where we can concentrate effort to remove the rhododendron so we then reduce the risk of a spread from the rhododendron either into commercial species or into the native flora that's on the sites.

Sue Nelson: This is a pretty serious issue isn't it because if you're losing some of the trees you are also affecting wildlife and other species as well?

Colin Edwards: Yeah, I mean, the landscape has been changed forever in certain parts of the country. In 2010 Phytophthora was found infecting larch, commercial larch forests, and a lot of these forests have now been removed or plan to be removed soon. So whole landscapes have changed which have affected many species. Phytophthora ramorum was also found and kernoviae on Vaccinium myrtillus, which is a very important heath land species in Scotland, the open heath land, so that's also a big worry because that has a follow-on effect for insects and birds which feed on the bushes.

Sue Nelson: And that's where you're going next isn't, Bethan, heath lands?

Bethan Purse: Yes. And also looking at the larch plantations themselves, so the implication of it moving to the larch is also that it can be spread much more rapidly than maybe it has done in rhododendron where rhododendron needs to be in contact with the tree. The larch is shedding pathogen from much taller in the canopy, so you now actually have larch to larch spread as well.

Sue Nelson: So we've got a fungus that can spread from tree to tree, a fungus that is spread by rhododendrons... Colin why not just get rid of all the rhododendrons? They are an invasive species anyway why not just remove them?

Colin Edwards: I'd love to get rid of every single rhododendron bush. It's been my desire for the last 20 years but to put it into perspective we did a survey in Argyle in Bute where we identified 4,500 hectors worth of rhododendron and the clearance costs for that would be £12 million. So you then multiple that up across the whole of Scotland and you're looking at somewhere around £70 million to £90 million, it if was available, if the resource was there and if every technique worked first time. So that's completely unrealistic in not just today's climate but any climate.

Sue Nelson: Colin Edwards from the Forestry Commission, Matt Elliott from the Forest Reserve and Bethan Purse from the Centre for Ecology & Hydrology, thank you all very much. And you can check out some pictures of the site on our Facebook page. I'm Sue Nelson and you've been listening to the Planet Earth podcast from near Edinburgh brought to you by the Natural Environment Research Council.