Lesson video

Hi there, I'm Mrs. Kemp, and welcome to today's lesson, all about plant defences and identifying plant diseases.

This fits into the defences against pathogens, the human immune system and vaccination unit.

So, let's have a look at what our learning outcome is for today.

"I can describe physical, chemical, and mechanical plant defences against pathogens and pests, and can describe ways in which plant diseases can be identified." We'll be using some new key terms today, and you can see those on your screen.

And if you'd like to read those in a bit more detail, please do pause the video.

We've got three learning cycles for today.

We've got pathogens and pests, plant defences, and identifying plant diseases.

Of course, we will start with pathogens and pests.

So, communicable diseases are caused by pathogens, okay? Pathogens are microorganisms that can be passed from organism to organism, hence the fact that it is communicable.

A pathogen can be a virus, a bacteria, or any other types of disease, so things like virus, bacteria, fungi, protists.

Pathogens can infect plants as well as animals.

So we often think about communicable diseases like measles that infects people, but actually plants do also have some diseases.

An example of a plant disease is a tobacco mosaic virus or TMV for short.

And we can see on that leaf there of the tobacco plant, that it's got that funny mosaic stippling to its its leaves, and hence that's why it has its name.

Tobacco plants are closely related to things like tomatoes and cucumbers, and therefore they can also have TMV.

We get that discoloration, like I mentioned, but also you can see on the edges, the leaves are starting to curl up, and that is also something that is caused by the tobacco mosaic virus.

How this affects the plant then? Is that actually, it's got reduced photosynthesis.

In those areas where there isn't as much chlorophyll, were also reducing the surface area of the leaves, and it means that the plant cannot photosynthesize and produce as much glucose as it would normally, therefore, it will lead to stunted growth of that plant, and so the farmer would get a much decreased yield of the tomatoes, cucumbers, or tobacco.

Another example of a plant disease is one caused by a bacterium, and this is known as crown gall disease.

It's actually really problematic and very, very difficult to get rid of.

The bacteria we find in the soil, and we can see a little image there that's been seen through an electron microscope, of some of those bacteria.

But we can also see on the other side, what happens when a plant gets crown gall disease.

So it actually forms a big mass of cells called a gall, and it is a type of tumour for that plant.

Now, as that tumour develops then, what happens is it actually sort of embeds itself into the vascular system of the plant stem.

So we're talking like the phloem and the xylem.

It then restricts the movement of water and nutrients around that plant, and obviously that would then contribute to an overall stunted growth, and eventually it can lead to death of that plant.

Another example of a plant disease is one that's caused by a fungus, and this affects a native species of ash in the UK, known as the common ash.

Its real name is Fraxinus excelsior.

So we can see what the common ash looks like there, and what happens when it gets a fungus known as Chalara ash dieback, is that actually just as the name suggests, part of that plant actually dies back, and we can see that the very top canopy there, has actually lost a lot of its leaves.

It is a fungus, and when it infects the plant, at first, you can see the sort of fruiting bodies of it, and we can see that in the first image there where you've got those like sort of little mushrooms. And actually, later, what happens is that brown lesions appear on the trunk of the tree, but also on the leaves as well.

And eventually then we get that ending up of that complete dieback of the shoots.

It actually does kill the tree, and so it is really, really serious and we've got to be very careful not to spread this from our woodland areas in the UK.

Okay, can we match these up then with the type of disease and then the type of pathogen? We've got TMV, which remember is the tobacco mosaic virus, crown gall disease, and Chalara ash dieback.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you get TMV as a virus, crown gall disease was bacterium, and Chalara ash dieback was that fungus? Excellent.

Well done.

So, plants not only can get diseases through pathogens, they can also become infested with pests, and those pests can be insects essentially.

Just like pathogens can be spread from organism to organism, so can the pests.

The pest can also be responsible for actually passing that pathogen from one organism to another.

So it will actually contaminate another plant as it's moved from that plant to another plant.

So, one example that's very common, you've probably seen these in your garden, or may potentially on some plants that you've seen out and about or even in your house, and they are known as greenfly.

And what they do is that they feed on the sap of young plants such as tomatoes, and you can see them there living happily on that stem.

They are parasites that benefit from taking all the nutrients from another organism.

So that organism is not benefiting whatsoever from that aphid living on the plant, and therefore it is a parasite.

They have really pointy mouth parts that are like needle-like.

And what they do is they poke that into the stem, down towards where the phloem is, you can see that in the top image there, and then they can suck that sap out, essentially taking all the sugars and the nutrients that are in that sap.

This will reduce the growth rate and can eventually kill the plant if there are enough of them on there.

Now actually what we can see in the bottom image there is a ladybird.

Ladybirds happily feed on aphids, and so gardeners will often try to encourage ladybirds to move into their gardens in order to keep the number of aphids lower.

Another type of common insect pests then is the mealybug.

And there's one there in the image.

And they're a little bit harder to see, and so you may not have ever noticed one of these before.

I certainly haven't.

They feed on the sap of many common house plants, and they can cause stunted growth, so the plant not being able to grow to its full size, and can often lead to the plant being more prone to fungal diseases.

So, even if the mealybug hasn't led to the death of the plant, actually, it can then get a secondary infection.

They are difficult to get rid of, but gardeners do try washing them off with things just like washing up liquid to be honest, and then sort of spraying on it and hoping that that will remove the mealybug.

Okay, onto a true or false then.

An aphid is an example of a plant pathogen.

Is that true or is that false? I'll give you a moment to think about it.

That of course is false.

Now can you justify your answer for me? A, an aphid causes plant diseases, or B, an aphid is a pest that can spread pathogens.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you get B? An aphid of course is a pest.

Okay? That can spread pathogens.

It isn't a pathogen itself.

Pathogens are our viruses, bacteria, and fungi.

Okay, onto our first task of the day then.

You can record your answers on your worksheet, so please do get that out now.

Number one, explain in your own words the difference between a plant pathogen and a pest.

Number two, the apple tree shows signs of damage and disease.

Use ideas about pests and pathogens to suggest what could have caused this.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so should we go through number one first? So pathogens cause plant diseases.

They include bacteria, viruses, fungi, and other microorganisms. Pests are in insects or other animals that damage plants by feeding on them.

They can also spread pathogens that cause diseases.

For number two, our ideas could be that the damage could have been caused by pests including insects such as aphids and mealybugs.

These pests can also spread pathogens that could have caused the disease in the apple tree.

The disease could be caused by bacterium, virus, fungus, or other microorganism.

There is actually a third part to this task, and this one is all about a plant that is also a parasite.

This plant is called Orobanche reticulata, and actually, it's extremely rare.

It doesn't actually have any leaves with chlorophyll in like normal plants do.

And this species only grows near the woolly thistle.

You can see that purpley flower there is the Orobanche reticulata, and the woolly thistle is just next to it.

Now this species will only grow there, and the thistle will then become much smaller than usual.

What do you think the plant has caused the stunted growth? So what do you think has caused the stunted growth of that woolly thistle? I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so the Orobanche reticulata is a very rare plant that has no leaves or chlorophyll.

This species only grows near the woolly thistle.

This thistle is much smaller than usual.

What do you think has caused the stunted growth? So, actually the Orobanche reticulata, just like we said in the question, is a parasitic plant, and it lives off the woolly thistle.

It is growing on the plant and actually taking its sap.

This means there is less nutrients for the woolly thistle to grow, therefore stunting its growth.

The Orobanche reticulata does not need leaves or chlorophyll as it doesn't need to photosynthesize to make its own food.

So it's a bit like one of those pests that we saw such as the aphid, but in a plant version.

It's very, very rare.

We don't get many of these.

It's a really exciting plant.

So, onto the next one then.

This one is plant defences.

So just like humans, plants do need to defend themselves against pathogen.

They have many physical defences against pathogens.

So things like the tree bark, it's actually a dead layer of cells which forms a physical barrier to infection.

I suppose if we were going to compare it to humans, we might compare it to skin, for example.

They also have waxy cuticles on the top of leaves.

If you feel a leaf, you can usually tell which bit is the top of the leaf 'cause it feels waxy.

Now, that waxy cuticle helps to prevent infection.

The cell walls around each one of their cells is also difficult to penetrate, is made of something called cellulose, which is a long chain of sugars.

Although some pathogens get round this by releasing enzymes, and that can help to break down the cellulose and soften that cell wall so that they're then able to get into the cells.

Plants such as thistles and cacti, we can see those in the image there, they actually have spines that help prevent them from being damaged by pests, because obviously, it's much more difficult to sort of get hold of and actually eat it if it's prickly like that.

The leaves of a sensitive plant called Mimosa pudica, curl up when they are touched, and this helps to protect the leaves from damage.

So if something went along and landed on that leaf, they curl up and the inset would drop to the floor.

We're going to watch that video now to see that happening.

So we saw there, that if something landed on that leaf, just like somebody stroking it there, the leaf would curl up and the pest would drop to the floor.

Okay, which physical plant defence can we see in the diagram? A, cellulose cell wall, B, spines, or C, waxy cuticle.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Of course, it's a cellulose cell wall, which we can see all the way around the outside of that cell.

Okay, so as well as physical defences, just like human beings, plants have actually got chemical defences, which you may not have realised before.

So, for example, the foxglove and stinging nettles, they actually produce poisons that deter herbivores from eating them.

Okay, we actually would stay away from a stinging netter, wouldn't we? Because it would cause irritation to our skin.

So, us knowing that, mean that we actually leave those populations alone to grow.

Clover and mint produce antimicrobial substances that kill bacteria.

We can use these as antiseptics for humans.

So you can often get things like clover oil, for example, which people often use for teeth infections.

So, we have used the fact that the plants produce these chemicals to help ourselves.

Antimicrobial means that it will kill a microbe.

Plants need energy to produce chemicals, so only produce them when they need them.

This is a plant here called maize, and is damaged by a herbivore.

Example of a herbivore, we've got in the picture there, which is a caterpillar.

And actually, what it'll do is it will release compounds into the air that attract wasps.

Now why would it do that, I wonder? Well, actually, that's because if it attracts the wasp over, the wasp will end up eating the caterpillar, that herbivore, therefore saving the plant from any further damage.

It's really, really clever that they are able to draw in the help from nature around them.

Which of these plants produces chemicals that are antimicrobial? A, cacti, B, maize, or C, mint? I'll give you a moment to think about it, but if you need more time, please pause the video.

Did you realise that it was mint? Excellent.

Well done.

Onto our next task of the day then.

This one is Task B.

You can record this on your worksheet, so please do get that out now.

Rainforest leafcutter ants work together as an army to carve up and transport leaves of the bixa tree underground to feed a fungus.

We can see an image there of the bixa tree, and we can also see those leafcutter plants cut up a bit of the tree, and they're carrying that back in order to feed a fungus that they have down in the ground.

This is what we call a mutualistic relationship.

And in return, the ants provided with nutrition from that fungus.

The bixa tree has no direct defence against the ant, but it does produce a fungicide.

A fungicide is something that will kill a fungus in its leaves when damaged.

Explain how this could protect the bixa tree from any further damage.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so did we think that when the leafcutter ants carve up the leaves of the bixa tree, it produces a fungicide in response? The ants transport the leaves which carry that fungicide underground to feed the fungus.

The fungicide could weaken or kill the fungus, meaning there may not be enough food for the ants, the ants may die from a lack of nutrition.

Alternatively, the ants could find another food source of leaves for the fungus, which would mean that the bixa tree is protected against any further damage.

I hope you got all those points down, but if you want to add a little bit more to your answer, please do that now.

So, onto the next one then.

So when a plant is actually infected with a disease, so be that a pathogen, or be that actually a pest, it is really important that we try to diagnose it and find out what is actually causing that disease.

This will help us to treat it very quickly and therefore stop the spread of it to other plants.

Particular symptoms, just like when we are having infections, are things that are associated with particular diseases, and these can help us to identify those diseases.

The things that we might look for then is actually look for particular pests that have infected that leaf, and to find out what pathogens they normally carry and therefore what may have infected it.

We might look for some dark spots on the leaves.

We can see those leaves there are the leaves of a rose plant.

And also we might look for yellow or wilting of leaves.

Wilting is when they sort of bend downwards and droop.

Which symptoms suggest the plant has tobacco mosaic virus, is it A, B, or C? I'll give you a moment to think about it, but if you need more time, please pause the video.

And of course what we have there is C, and that's the discoloured leaves in a characteristic mosaic pattern.

Okay.

Now, if you're a gardener or a farmer, then you want to try to find out what the symptoms are and look for the cause of those symptoms very, very quickly.

And you might use websites, the internet, or even photographs in gardening manuals in order to compare with your plant to other examples of diseases, in order to try to work out what is wrong with your crop.

Symptoms such as yellow leaves then and dark patches can be caused by environmental factors.

So just like humans need to have minerals in their diet, plants do also need a really, really tiny trace amount of minerals as well.

And if they have mineral deficiencies, we might see that as yellowing leaves.

It could also be due to other factors such as pollution.

It's important to make sure that you investigate the environmental factors as a potential cause of disease.

So analysing the distribution of the disease can also help us to identify it.

Some plant diseases can only actually infect specific species of plants.

And so, therefore, if we can work out what specific species it is, then we'll be able to eliminate certain diseases and know probably which one it's going to be.

Fungal diseases such as rose black spot and ash dieback, and also as we can see in the image here, this barley powdery mildew, infect specific species, and they are spread in particular ways, like wind, water, and also infected tools that you might take from one plant to another, therefore spreading it.

It can help us to identify a disease and then stop the spread if we know how it was spread between infected plants.

The same symptoms then can often result from more than one type of disease.

So, sometimes it is very difficult to identify what is actually wrong with that plant, and we might need scientists to therefore help us to identify it.

What they might do is they might come and collect a sample from your infected plant, and grow those in culture.

So we can see in the image there that we've got a Petri dish with some agar, and they would've taken a sample from that plant and spread it across the agar plate and see what's actually grown.

We can use the appearance of the colonies, so every time one bacteria lands on the plate, it will grow into that little circle and that is what we call a colony.

And we can see what that colony looks like and that can help us to identify the pathogen.

The scientists may also use specific growing conditions that only a specific pathogen will grow in, and then we would know that it was that particular type.

We may need to have a look at that pathogen under the microscope.

And when you look at pathogens, bacteria, sorry, under the microscope, you can see that their cells often have different shapes, and the shape of those can help us to identify them.

We can also use particular stains that can also help us to identify which pathogen we had.

Here, we've got that sphere shape, which is the coccus, and then we've got the rod shape, which is the bacillus.

There are also antibody tests, just like we have for human diseases, and that sample from the individual plant can be used to then use one of these simple antibody tests where it will give us a positive or negative result for a suspected disease.

The antibody is actually only complimentary to the antigen of that one specific pathogen.

So it will only tell you whether or not that particular pathogen is there, and therefore only give you a positive result when it is the correct pathogen.

Okay, which is true about antibody tests? A, each antibody can identify multiple pathogens, or B, the antibodies bind to the antigens on a pathogen, or C, a positive result shows that the pathogen is absent.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you realise that it's the antibodies bind to antigens on a pathogen? Excellent.

Well done.

So there are also some genetic tests that can be done, and a DNA probe can be used.

Now, A DNA probe is a short single-stranded piece of DNA that will only bind to the complementary based sequence in a particular pathogen's genome.

Remember, A binds with T, and G binds with C.

The probe can actually be attached to a fluorescent marker, which we can see there in the image, and that can be detected if the pathogen is present in a sample from the plant.

So, it will tell us if that particular pathogen is there, and it will fluorescently mark it so that if we put it under a UV light, we would be able to see that that probe is there and therefore so is the pathogen.

Using genome sequencing to work out the base sequence of the pathogen's entire genome can also help to identify it.

What is a DNA probe complementary to? A, an antigen on a pathogen, B, a fluorescent marker, or C, a base sequence in a pathogen's genome? I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you realise that that is C, a base sequence in a pathogen's genome? Excellent.

Well done.

Okay, onto our final task of the day then.

This one is Task C.

A farmer notices that their barley isn't growing well.

They notice a white fungus growing on the leaves.

What steps could be taken to identify the pathogen? What could be the consequence for the farmer? I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, let's have a look at some of those answers then.

So first, the farmer could see if they can identify the disease themselves by looking at the symptoms and comparing to a manual or website, and analysing the distribution of infected plants.

The farmer could then provide a sample of the pathogen to a scientist.

The scientist could grow the pathogen in culture, observe it using a microscope and stains, use antibody tests, and use genetic testing to identify the pathogen.

If the crop is not growing well, the farmer will have a lower yield and therefore less profit.

They will also have the cost of testing and treating the crops.

So we have come to the end of today's lesson.

I would like to just go through the main key learning points for today.

Plants can be infected by pathogens such as viruses, bacteria, and fungi that cause diseases.

Pests including insects, can damage plants on feeding on them, and can spread pathogens that cause diseases.

Plants have physical, chemical, and mechanical defences against pathogens and pests such as the waxy cuticle, cell walls, and antimicrobial chemicals.

Plant diseases and the pathogens that cause them can be identified by observing the symptoms, growing the pathogen in culture, using light microscopy, antibody testing, and genetic testing.

I hope you've learned a lot today and I look forward to seeing you again soon.

Bye now.