Nematode syn. Eelworm Control © Frances Michaels

Nematodes are not insects but microscopic, long, thin worms, which is why a common name for them is eelworms. This soil-borne pest causes stunted, unproductive plants. A common way to identify the problem is infected plants will wilt rapidly in hot weather. When nematodes burrow into the roots they stimulate the development of galls on the roots which become swollen, disfigured and knotty. Root knot nematodes infest a wide range of plants, including roses, potatoes, carrots, tomatoes, cucumbers, lettuces and zucchini. The potato cyst nematode Globodera rostochiensis is very persistent in soils but luckily it is confined to a very small area of Victoria; this area has been quarantined to prevent further spread.

Suggested Organic Strategies

• A healthy soil will have a range of microorganisms that are predators or parasites of nematodes. All the usual ways of building a healthy soil such as adding adding compost, mulching and green manuring will improve nematode control. More information on soil improvement...
• A green manure can be grown specifically for nematode control as there are plants that when chopped through the soil will decompose and release a nematode killing gas; this process is known as bio-fumigation. Plants that contain high levels of bio-fumigant compounds include: rapeseed (canola) Brassica napus, BQ Mulch, marigolds and Indian mustard. BQ Mulch, canola and mustard are cool season crops. Marigold is a warm season crop that when mixed with cowpea makes an effective, warm season, nematode-controlling green manure.
• Practising crop rotation helps as when a non-host crop is grown for a season it can starve the existing nematodes. Non-host plants include cowpea, soybean, oats, wheat and woolly pod vetch. A rotation of 2 years or more between susceptible crops is needed to control a serious outbreak. Vegetable crops resistant to nematodes include broccoli, corn, brussels sprouts, chives and leeks.
• Good hygiene will help limit the spread of this pest as nematodes cannot move quickly through the garden, instead they are often spread on infected plants, muddy boots and garden tools.
• When harvesting infected plants, remove as much infected root from the soil as possible and dispose of well away from garden areas. The infected roots can be used as mulch under native shrubs or trees but do not place them in a compost heap, as it is unlikely to get hot enough to kill the nematodes.
• Solarisation can be a useful remedy for nematodes; it can also help combat stubborn weeds. To be effective do this in summer and first water the soil well. Then cover the soil with clear 4mm thick plastic. Stretch the plastic over the area, get it as close to the soil as possible. Bury the edges by digging a narrow trench, tucking the plastic in and back-filling. The aim is to raise the temperature to between 45°C and 50°C in the top 10 cm of soil. This is high enough to kill disease pathogens but most beneficial soil organisms will survive. Leave the plastic in place for 4 to 6 weeks and then plant as usual.
• Digging fresh chicken manure into a hot, dry soil, something normally to be avoided, has been shown to reduce nematode numbers.
• Drenching with water and molasses or sugar can also kill nematodes, but will have a negative impact on soil life.

Not all nematodes are a problem, a range of beneficial nematodes known as 'entomopathogenic' are used to control plant pests.