Plant pests and pathogens

From creatures that live in harmony with plants, to plant pests, and as 2020 is actually the international year of plant health there’s no better time to be talking about them. Climate change will affect these pests too, potentially impacting our ability to grow enough food. This is what Sebastian Eves-van den Akker studies, and he’s particularly interested in nematodes, and he spoke to Eva Higginbotham...

Sebastian - So nematodes are for the most part these microscopic worms, and although most people probably haven't ever heard of a nematode, they are incredibly numerous. So if you were just to count all of the animals on the planet, one by one, then nematodes would account for more than half. So, you know, on average, most animals are nematodes. There's this great quote by a famous Nathan A. Cobb. And to kind of paraphrase. He says that if you were to remove magically all of the matter on the planet, but you left the nematodes in place, you would still be able to see faintly the hills and the valleys and the fields and the rivers, simply by the nematodes that used to live there.

Eva - Sounds like an incredibly well kept secret! So if they're everywhere, where do you actually study them?

Sebastian - Most nematodes are, you know, kind of good guys, right? So that is, you know, important parts of many different ecosystems and they carry out many important ecosystem services. So for example, they can eat detritus and things like that. But a few of them have evolved to be parasites. And as you say, this year being the international year of plant health, what we work on is global food security. And some of these parasites parasitise plants, and are a major threat to global food security.

Eva - I see. And so how do they actually cause disease? What makes them parasitic?

Sebastian - The kind of nematodes that I work with are soil born, they live in the soil and parasitise the roots of plants. And what they do is they move inside the roots of the plant and they make the plant make a tumour. Now this tumour is really the wrong word, but it's got the right kind of connotations. This tumour is this new tissue that forms inside the plant that drains the plant resources and that the nematode eats. And so you can imagine if you've got lots of nematodes making the plant make this structure that it doesn't want, and that's draining the resources, ultimately that can damage the plant.

Eva - What kind of crops do they affect, or do they affect plants other than crops as well?

Sebastian - Basically every plant in the world. So there's at least one species of nematode for every major food crop of the world and indeed, most plants of the world. So if you've got a plant there will be a nematode that can, parasitise it, whether it's on your plants or not.

Eva - What might climate change do to them? Are they looking to have a good time or a bad time with what we're expecting to change in the climate, particularly, maybe in the UK over the next few years?

Sebastian - It's really hard to predict. I mean, we can definitely say that it will have an impact. So climate change will have an impact. But saying what that impact is going to be is going to be a challenge. So for the particular kind of nematode that we work on, which are called potato cyst nematodes (un-surprisingly they parasitise potatoes) and in the UK is there are actually two species. And one of which likes it hot and the other one, which likes it cold. And so in this case, we actually know quite clearly that if, for example, the average temperature and in particular, if the minimum temperature in the UK increases, then this one is going to do better. But in most cases actually predicting the dynamics of how that's going to change is actually quite challenging.

Eva - I guess it's something of a complicated relationship between the nematode and the plant in those cases, because they are parasites. Is there anything we can do to stop them?

Sebastian - Yeah. There's number, different ways you can control nematodes or indeed any plant disease. I mean you know, we're all thinking about immunity at the moment, you know, with what's going on the health crisis. And so we're familiar with this idea of hosts being immune to parasites and diseases. And of course, plants have immune systems as well, but their immune systems are quite different from animals. And so on the most part, plants don't have an adaptive immune system. They have an innate immune system. And I find it quite remarkable that, you know, most plants are immune to most diseases, even though they haven't seen them. Whereas for example, with, for an animal to become immune to some diseases they would have to have some experience of that. And so immune plants is one of the best ways that we can, we can combat these, but there are other ways, you know, that involve inputs, for example, pesticides, or by, you know, managing your crop in rotations and things like that.

Eva- I see. So the sort of ideal scenario is just to have a plant that isn't vulnerable to these nematodes.

Sebastian - I mean, that's the golden bullet if you like. But you know, you never count against evolution. And so if you have plants that are immune, ultimately you will get nematodes or other pathogens that can find a way around that. So really a diversity of approaches is the best way.

Eva - So nematodes are everywhere, but what about other diseases that can affect plants? Are they likely to be a problem too?

Sebastian - Yeah, absolutely. I mean, there are loads of different diseases of plants. They can be restricted to different parts of the world geographically. And you can imagine that some of those, either those diseases themselves or their vectors, will like different temperatures. So again, if the climate changes in the UK, then it's likely that this will make the UK more suitable to some of those other pathogens, or less suitable.

Eva - So different viruses, different kinds of bacterial infections that could affect plants?

Sebastian - Exactly. Yeah. So one that people have heard of a lot in the news recently is Xylella. This is a bacteria that infects plants, and this is sort of, you know, on the borders of the UK, if you like, but can't quite make it.

Eva - And that might change with climate change?

Sebastian - Exactly yeah, it's restricted by temperature.