Nematodes

All about Nematodes

Nematodes are microscopic worm-like creatures that live in soil. In fact, they are the most numerous multi-celled creature on earth, found on every continent. The mere mention of them strikes fear into the heart of most gardeners in Perth – however, the reality is that many nematodes are actually beneficial and an essential part of healthy soil biology.

Many nematodes feed on bacteria, fungi, algae, small invertebrates, and other nematodes. However, there are a few varieties which feed on plant roots. They lay their eggs in the roots, and as they hatch and numbers grow, they deprive the plant of moisture and nutrients and the plant shows signs of yellowing, stunted growth, and may die.

Root knot nematodes thrive in sandy soils, and prefer warm conditions. Areas with long, dry summers and short winters suit them well - I'm sure these climatic conditions seem familiar??

Identification

If you are concerned you MAY have root knot nematodes, the only way to be sure is unfortunately to rip out a plant and have a look! There will be warty lumps along the root surface.

Be aware that nitrogen fixing species (peas, beans, legumes) have nitrogen fixing nodules on their roots, which can sometimes be wrongly suspected of being nematodes.

The good thing is there ARE things that can be done.

If you have root knot nematodes in your garden, it is usually a symptom of the soil’s condition, and shows an imbalance in soil biology. Nematodes usually live in the top 30cms of soil, and eggs can survive for a few months in fallow ground. However, if you plant in a crop which the nematodes don’t feed on, and leave it for some months, the nematodes will starve.

The first thing to do is to incorporate more organic material into the soil. Lots of it. Animal manures are beneficial, especially chicken manure, because the nitrogen content stimulates conditions that are not ideal for the nematodes. There is also some evidence that raising pH (which fresh manure will do) will also help with nematode control.

Adding organic material also helps alter the balance of soil biology, and will encourage other nematode species to build up numbers that will then prey on the root knot nematode. Remember the organic gardener’s mantra; feed the soil! Unfortunately this is never a ‘once off’, but your garden will benefit hugely from regular (at least yearly) generous top ups.

Photo to the right is root knot nematode damage.

Crop Rotation

Certain types of plants are particularly susceptible to root knot nematodes. Tomatoes are one. Many types of vegetables are susceptible (carrots, peas, beans, capsicum), as are grape vines, roses, and some stonefruit. The practice of crop rotation is beneficial. Brassicas (cauliflower, broccoli, kale, cabbage, mustard, kohlrabi, turnips, brussels sprouts, bok choi, radish, rocket, mizuna, collards) actually emit a substance into the soil that kills root knot nematodes, so if you have an infestation, plant out members of this family thickly.
Check out our Crop Rotation fact sheet here.

Green Manure Crop

Green manure seeds contain members of the brassica family and have the added benefit of adding a large amount of organic matter once it has been slashed and returned to the soil. We sell green manure seed packs @ GLSC.

Marigolds

African and French marigolds both exude a substance into the soil which will kill root knot nematodes. However, they must be planted THICKLY (one or two flowers dotted around your vegie garden simply won’t be enough) and left in the ground for at least three months to be effective.

Molasses

Using approx. 2 tablespoons of molasses per litre of water (dissolve when the water is warm), apply this to the nematode infected soil with a watering can. (Note: I have read varying quantities recommended – even up to 1:4 molasses to water ratio!) Molasses is available in many forms. Look for the least refined and most organic possible. We do often have it for sale - or you could try health food stores or stock feed suppliers, but you may need to buy larger amounts. Unfortunately most molasses contains sulphur as a preservative which can have a detrimental effect on soil fungi. Using a small amount as a once off is unlikely to cause 'harm' - but it's something to be aware of. Providing other microbial food sources (like fish hydrolysate and frass), or dosing soil with beneficial microbes may be helpful after using molasses.

Molasses works by increasing the food source for bacteria in the soil. It changes the balance of bacteria/fungi/nematodes in the soil biology, and this may not necessarily be a good thing. Earthworms can be adversely affected also. Use this treatment sparingly and as a last resort.

(Note: Molasses is advocated by some as a regular soil additive. For plants that prefer a bacterial dominant soil biology (eg. Turf/grasses) this may be the case. It would be beneficial when planting a new lawn, or when trying to revitalise a struggling one for instance. However we wouldn’t recommend it for shrubs and trees as woodier plants benefit from a more fungal dominant soil biology.)

Photo to the right is nitrogen fixing nodules. (Not nematode damage.)

Solarisation

Leaving bare soil covered with plastic weighted down at the edges will work to ‘cook’ the soil and kill nematodes. Of course, it will also affect beneficial soil biology the same way. For bad infestations, turn through the soil every few weeks to ensure the maximum exposure to sun & heat throughout the top 30cm layer. Areas left like this for a few months, particularly over summer, will have fewer problems in future seasons. Just remember to re-inoculate the soil with lots of healthy biology – use lots of compost to dig through before re-planting.

What to do with infected plant material

Firstly, ensure all roots are dug up and not left in the soil. Very hot composting methods will destroy root knot nematode eggs, but if you are uncertain that your composting methods will do the job, then cut off the roots and dispose of them by burning. The tops of infected plants are fine for composting.

Nematodes don’t move a huge distance on their own – it is thought only a metre or so. Practicing good hygiene (being careful not to distribute infected soil or root material on garden tools or by careless digging) and selective planting, crop rotation, and soil improvement methods, you should be able to control their numbers so they do not have such a devastating effect on your crops.

Products for Nematodes

There are several products from our product range that are perfect for fruit fly control in Perth. If you need help or advice to choose what’s best for you – please contact us today!