Nematodes

Nematodes

JERRY COLEBY-WILLIAMS: You wouldn’t necessarily think so, but a single teaspoon full of well composted soil like this is a seething metropolis. It can contain up to 200 000 different species of organism and up to half a kilometre of fungal threads. And all of these creatures are living together, competing with each other for water, nutrients and space.

But here in my Brisbane garden, as well as the usual worms, bacteria and fungi that any healthy, organically rich soil plays host to, I’ve encountered what is actually a common summer pest but one you may not know much about. The Root Knot Nematode.

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Nematodes are also known as ‘Eel Worms’ and they’re colourless, microscopic worm-like animals. Scientists have described about 20,000 species and some specialist estimate there could be over a million. Most of these are harmless to plants. Some are even beneficial. But the Root Knot Nematode is a plant parasite.

Recognising you have a problem with nematodes is not always as they work undercover, or more specifically, underground. What happens is young nematodes burrow into the roots and their feeding stimulates the production of tumour like growths and these inhibit the plant’s ability to take up water and nutrients.

So the result is a weak plant, reduced growth and productivity and plants become susceptible to secondary fungal infections and to sudden wilting.

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If your garden is infected with nematodes, you probably won’t even notice their presence during the cool seasons and that’s because their populations peak during summer and by that stage, when you do notice plants wilting, it’s too late to do anything about that particular crop. The difficulty is knowing if they’ve got them at all. And the only way to do that is to dig the plants up, rinse off the soil off the soil like I’ve done with this kohlrabi and check. And I can see that this has got nematode infection. It’s not bad. That’s why the plant has grown really well. But in severe cases, these roots, these fibrous roots wouldn’t be able to be seen at all. It would be just tumour-like growth and perhaps the plant would completely rot and die.

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So which plants are most affected by the Root Knot Nematode? Well, the Solanaceae or potato family and that includes some of our favourite food plants. Things like eggplant, capsicum, chilli and tomatoes, and that’s in fact where I first discovered what nematodes were doing in my garden.

They were growing beautifully one week, and the next week, they were going yellow and wilted, even though the soil was moist. Now there are some other things, some surprises that occurred. Like this Ceylon Spinach, totally unrelated to the potato family and yet that got nematodes as well. Now, there’s one group of plants that we can be comfortable about growing in nematode infected soil and that’s the peas and beans. The legumes.

Like this Snake Bean. These do produce nodules on their roots and you mustn’t confuse them with nematodes. This is natural. This is what they do. They put nitrogen into the soil and if you look closely, you can see they’re clean and rounded. They’re not tumour shaped as the nematodes cause on other plant roots, so you can grow these in nematode infected soil. And that brings me back to the bed.

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Everything I’m growing in here, apart from the Ceylon Spinach, is not going to be affected by Root Knot Nematodes. I’m resting my bed to reduce their population.

Often you can eat crops that that have been affected by nematodes, but if you do have that situation, get rid of the roots. Don’t put them in the compost heap, drop them into a plastic bag and put them into the garbage.

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No the secret to living with nematodes, which I practice here involves a couple of strategies. Firstly, organic rich soil. By putting in compost, manures and mulches, you can encourage beneficial nematodes to attack the pest species. And practice crop rotation. Don’t make life easy for nematodes by planting the same crops in the same soil, year in, year out. Vary your crops and make it difficult for nematodes to really enjoy living in your garden.

There is one other, newly confirmed technique for controlling nematodes and that involves this – sowing mustard seed as a green manure. Once you’ve cleared the crops, you just broadcast mustard seed over the surface, rake it in, water it, grow it until it’s 45 centimetres high and then you dig it in and something very special happens. If the soil is moist, the mustard will decompose and it releases a chemical known as ‘Isothiocyanates.’ We all know what it tastes like. If you’ve eaten mustard or horseradish, it’s the same thing that gives them their hot flavour and it’s that chemical that fumigates the soil controlling nematodes. And we’ve got the CSIRO to thank for that 21st Century solution to nematodes.

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While no infected garden will ever be entirely free of these pests, don’t worry. With some careful observation and some simple organic gardening techniques, you can look forward to abundant harvests for years to come.

STEPHEN RYAN: I’m with Jerry on that one. Looking after your soil is definitely the key to a great garden.

Still up in Queensland, Colin’s found a wonderfully peaceful spot that’s an oasis for native plant lovers.

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