By Deryn Thorpe
Root knot nematode abounds in sandy, unimproved soil, like we have in much of Perth and south west Western Australia. It is a parasite that is the bane of gardeners all over the world.
Many of the nematode species living in Perth's soils are not harmful to plants. However root knot nematodes enter the roots as larvae, causing the plant roots to form galls or knots.
Plants fail to thrive and may wilt because the galls block the transport of water and nutrients through the plant.
Vegetable crops like tomato, beans, capsicum, carrot, and eggplant are very susceptible.
Most nematode damage is in sandy soils, which are prevalent in Perth, and is most noticeable when the plants are under stress through inadequate supply of nutrients or water.
Add more organic matter to the soil as increased water and nutrients help plants to tolerate nematode attack. Decaying organic matter also increases the number of organisms that feed on other soil microbes, including nematodes.
To reduce nematode numbers practise crop rotation. Since nematodes continue to feed on and reproduce in root fragments in the soil remove the crop roots as soon as the plants are harvested to diminish the population before the next crop is planted.
Planting cover crops of marigolds and mustard has also been shown to reduce the numbers of root knot nematodes in soil.
Sow mustard seeds densely as a green manure crop and dig it into the soil at flowering. Keep the soil moist and as the mustard decomposes it will release chemicals that fumigate the soil and reduce nematode numbers.
Plant marigold seeds densely at least two months, preferably three or four months, before sowing vegetable crops.
The roots of the living plants produce a chemical that inhibits the hatching of nematode eggs and there is no further benefit by digging the plants into the soil.
French marigolds (Tagetes patula) are thought to be effective against a larger range of root knot nematode species than African marigolds (Tagetes erecta).
Please note that this is intended as general information only. Please ask one of our qualified horticulturists for more specific advice for your situation.