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Posts Tagged ‘nematodes’

OK, I know I have done much for a while but this came up on a forum today so it seemed like a good opportunity.

Nematodes are tiny worm like creatures. They are a particular problem around Perth because we have sandy soils. If you live in one of the older suburbs you probably have them for sure!

How can you tell if you have them?

Plants will be unthrifty, they may simply appear unwell, nutrient deficient or they may be getting a lot of other problems. For example roses or eucalypts with nematodes often have stem cankers as a secondary problem. If you dig the plant up and examine the roots they may have knots on them – or they may not. Most people aautomatically think of root knot nematode when they think of nematodes however many other species of nematodes don’t produce galls or knots. You may just see roots that seem more branched and profuse then normal (not to be confused with proteoid roots on banksias, hakeas etc. Or on leaves, you may see angular blackened sections.

Types of nematodes

Most nematodes can’t really be seen by the naked eye – the commonest species may be up to 1mm long. Some nematodes feed from the outside of the root (dagger, needle or stubby-root nematodes), or they may go inside the plant and either stay in one place (eg root knot nematode) or move around inside the plant and feed along the way (lesion or burrowing nematodes. Some other types of nematodes move around inside the plant but feed on above ground parts such as leaves or stems (such as Aphelenchoides that infect leaves of eg Chrysanthemum or some ferns)

How do you get nematodes?

They can come in on plant material which is already infected or in soil/soils mixes.

What conditions do they like?

Ideal soil conditions vary with species. Moist soil is required by all to reproduce and move. The optimum temperature varies with species. The pore size of the soil affects nematode movement. The small pores of clay soils make movement difficult so the nematodes have to move in the spaces between aggregates. The larger pores in coarse sands may be too big to allow nematodes to gain leverage between particles.

Control

The home gardener has a different range of options to a commercial grower. Nematicides used to be available to the home gardener (Nemacur® granules) but aren’t any more. They are all S7 pesticides so pretty nasty! Over time the microbes in the soil that break the chemical down build up numbers so over a period of a few years the pesticides become less and less effective. There are natural predators of nematodes – such as fungi or other nematodes but these aren’t really commercially available. Method more suited to the home gardener include:

Rotation – the use of a rotation crop that is resistant to the particular nematode. So for root knot, that may be something from the grass family eg a grass or sweetcorn or sorghum. This will not eliminate them entirely but reduces numbers to levels that don’t cause problems.

Fallow – leaving an area fallow has a similar effect to rotating with a resistant crop – numbers fall because they can’t reproduce.

Solarisation is another option. The use of clear plastic laid over tilled moist soil for several weeks during the hottest part of the year.

Bio-fumigation – there are some crops that can be grown and hoed back in that contain chemicals that will help control pests and diseases including nematodes – such as some of the mustards. These crop residues are planted densely and hoed in while in full flower. Castor oil plants and marigolds have root exudates that may kill nematodes – they can be grown as rotation crops and hoed in. The type of marigold matters, not all are useful. Tagetes patula has traditionally been the one to use but some other species also work.

Sugar and molasses – In some Brazilian work, 300g granulated sugar per litre of soil at 7 days intervals controlled root knot.

Work in Australia on field grown tomatoes found 150 m³/ha of sawdust plus urea (600 kg/ha) to be quite effective. Molasses at 375 litres/ha per week for 14 weeks helped reduce numbers but was inferior to the sawdust.

In some other trials, urea concentrations of 4% totally eliminated nematodes but adversely affected plants. A combination of urea and molasses reduced the phytotoxic effects. Some papers mention molasses in water with a final sugar concentration of about 2% reduces nematodes numbers by about half in just over a week.

Other plant extracts – many have been trialled. Things like calendula, rosemary, lantana, onion, fennel, datura and liquorice which are ground up and put in water. What works probably depends on the type of nematode and the crop. Many of these trials have been in vitro and not in field situations.

The APPS website has some good info if you’d like to do any further reading.

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The longer you have a garden, whether it be vegetable or ornamental, the more likely you are to be slowly building up a reservoir of problems. There are several viruses that can take hold and be real problems in the home garden. Tomato spotted wilt and tobacco mosaic virus, for example. There are some other very serious viruses (tomato yellow leaf curl virus) that are not yet in WA. Then there are other problems such as phytoplasma that cause witches brooming (not to be confused with witches brooming from boron deficiency). And then there are root rots and other soil borne diseases such as Sclerotinia and nematodes.

All of these diseases, once contracted by a plant, usually kill it. In low numbers, nematodes may simply stunt plants and largely go unnoticed. Often plants succumb to other problems which are often secondary to the real issue of nematodes. Stem cankers in roses are often the result of undiagnosed nematode problems.

Chemical control of these problems is often only achieved with highly toxic chemicals. Sclerotinia forms highly resistant resting stages that last for years in the soil. Viruses and phytoplasmas cannot be cured, one can only stop them spreading and that is why it is really important to pull out and get rid of infected plants.

Non-chemical measures such as hygiene and rotation become critical in the control of all these diseases. For viruses and phytoplasmas, control of the vectors of the diseases is the only way out. Depending on the individual disease, that is often a leaf hopper or thrips (though Tobacco Mosaic Virus is spread mechanically, not by insects). And the problem with these pests is that they can fly around and hop on and off plants very easily. This is where weed control and control of alternate hosts becomes important. Even capeweed is suspected as an alternate host of some viruses. Alternate hosts are not necessarily from the same family. They may even be other ornamental plants in your garden such as petunias.

So where you are sowing successive crops of tomatoes, eggplant or potatoes, for example, you can expect to have increasing levels of virus building up over the years – assuming your level of insect control is not perfect. Sometimes the only way around it is to have a fallow period where no hosts are present for a period of time – say three months. And of course one problem here is that because these little critters can fly, the alternate host may not be in your backyard! It could be on the front verge or in the neighbours garden.

For nematodes you may need to have a fallow period to get numbers down to low levels. If that is not possible try sowing a non-host for a while such as any grass or sweetcorn. Incidentally, the reason we grow roses on fortuniana ropotstock in Western Australia is because it is relatively resistant to nematodes, unlike many of the rootstocks used in the Eastern states. Further information on some tomato spotted wilt can be found here. This farmnote gives information on Tomato Yellow Leaf Curl Virus and this, on Tobacco Mosaic Virus.

Here is a really good chart, albeit for Victoria, on what to use in various rotations and why.

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