Nematodes are very common. There are thousands of species, and they live in virtually every environment. In the Antartic, they are the top of the soil food web. In other soils they are one of masses of microorganisms, but they can still be a huge part of the biomass, due to their large numbers.
Nematodes can be pests of plants, humans and domestic stock, but many are also beneficial. They may play a role in the food chain, where they are consumed by other organisms, or they may control a range of soil pests and boring insects.
One common nematode pest, especially in light soils and glasshouse crops, is Root Knot Nematode, sometimes called ‘eelworm’.
You often see nematodes referred to as worms. In fact they are not related to a worms at all. They are long and slender, smooth and unsegmented. The plant pest nematodes are all very small, but animal or human parasites can be very long.
Adult Root Knot Nemotodes grow to 0.5 mm in length, but cannot be seen with the naked eye because they are thin and translucent/transparent. The females live in the roots of plants, and produce masses of eggs. Males rome around the soil more widely.
When they hatch, the nematodes burrow into fine roots near the growing tip. The root may die if enough tissue becomes clogged with nematode. Otherwise plants may develop large lumpy masses on their roots. Restriction of the feeding and transport tissue eventualy starves the plant, or causes it to wilt, so stunted, easily wilting tops are a clue to the presence of the nematode under the surface. The nematode doen’t usually cause death, but it can severely restrict yield.
Potatoes, carrots and parsnip can become completely lumpy all over. Tomatoes and lettuce are also affected by Root Knot Nematode.
Infected roots are easily attacted by funfi and other diseases.
In warm weather, the life cycle of the nematode can be as short as three or four weeks.
Practice good quarantine. Don’t move infected soil around in pots or on boots, tools and equipment. Bare root transplants, such as roses, can have their roots dipped in hot water (46degrees C) for 16 minutes. Always treat soil around planting holes where the plants have been hot dipped. using one of the methods described below.
Use good rotation practices. Avoid planting successive crops of susceptible plants or varieties for three years when the nematode is known to be present. Sweetcorn, onions and some brassica (cauliflower and cabbage) are known to be restistant. Grasses are not affected. Keeping soil bare for a while will also reduce numbers of the pest. Remember to control any weeds which may be a host to the nematode, during the control phase of the rotation (keeping weeds down in the cropping cycle may help to reduce the population too).
Solarizing is effective. Wet the soil and cover it with clear plastic. Tuck the plastic into a narrow trench and cover the edges with soil. You will have to leave the plastic in place long enough to consistently heat soil to a depth of 15 centimetres. That means at least three or four weeks in the hottest conditions, several months in cooler climates.
Marigolds are often recommended as a companion plant, to protect against nematodes. Unfortunately a few companion plants are unlikely to offer enough discouragement to the nematode, but a thick cover crop of marigolds on an infected garden bed will seriously reduce numbers. Turn the marigolds in at the end of their season. For serious nematode infestations, sow several successive crops of marigolds. Marigolds attract the nematodes to their roots, but after feeding there the nematodes are unable to breed.
Keeping soil organic matter high and digging in green manures may encourage competative and antagonistic species, such as the nematode trapping fungi. This fungi grows little ‘lasso’ loops in the soil. When the fungi detects the movement of the tiny nematode through the gap, it quickly constricts, capturing the nematode, which is then digested.
You could try drenching soil with neem. This method may have some negative effects on other soil organisms, but these can be quickly replaced with additions of organic matter or compost.
Add Chitin to the soil. Chitin is found in some organic fertilisers, especially those fermented from marine creatures and their waste products. It stimulates anatagonistic fungi and soil organisms too.
Burn diseased plants, do not compost them.