Nematode
Nematoda
Microscopic soil-dwelling roundworms that feed on plant roots, causing poor vigour, stunting and root galling or lesions in turf and horticultural crops.
Plant-parasitic nematodes are microscopic, unsegmented roundworms, typically only 0.3–1.5 mm long, invisible to the naked eye. They are eel-shaped and translucent, with a needle-like mouthpart called a stylet that they use to pierce plant cells and feed. Juveniles resemble smaller versions of the adults and develop through several moults; some species (root-knot, cyst nematodes) swell into rounded, sac-like females once established inside root tissue, while others (root lesion, ring, spiral, sting, stubby-root) remain worm-shaped and move freely through soil water films or within root tissue. All stages live in soil or within roots and are only confirmed by laboratory extraction and microscopy or soil/root testing, not by field observation alone.
Nematodes are a very broad group covering many genera, each with different host ranges and feeding behaviours. Some are endoparasites that enter and live inside roots (root-knot, root lesion, burrowing, cyst nematodes), while others are ectoparasites that feed from outside the root surface (ring, spiral, sting, stubby-root, pin nematodes). Leaf and bud nematodes are unusual in attacking above-ground foliar tissue rather than roots.
They occur widely across Australian turf, nursery, horticultural and broadacre production, with distribution and severity strongly influenced by soil type, temperature and moisture. Sandy soils generally favour higher populations and faster spread than heavy clays. Populations build over the warmer months and cause the most visible damage during periods of plant stress, such as drought, waterlogging or heat, when the root system cannot compensate for feeding damage.
Management is based on prevention and suppression rather than eradication, since nematodes are essentially impossible to eliminate from soil once established. Key strategies include using clean, certified planting material and pasteurised or disease-free growing media in nursery production, practising crop rotation with non-host or resistant species, maintaining good soil organic matter and irrigation management to reduce plant stress, avoiding movement of infested soil or plant material between sites, and fallowing or solarising badly affected soil where practical. Laboratory soil and root testing is the only reliable way to identify which nematode genera are present and at what population level before deciding on management action.
How to identify
- Above-ground signs: stunted, unthrifty or yellowing plants; poor response to fertiliser and water; patchy decline in turf or nursery stock that resembles nutrient deficiency or drought stress; wilting under conditions that would not normally cause it.
- Root signs: stunted, discoloured, poorly branched root systems; small galls or knots on roots (root-knot nematode); dark lesions or rotted patches on roots (root lesion nematode); excessive root branching or a stubby, shortened appearance (stubby-root nematode); reduced root hair development generally.
- Field check: carefully dig up a symptomatic plant with an intact root ball, wash soil from the roots and inspect for galls, lesions or discoloured, stunted roots. Because the nematodes themselves are microscopic, diagnosis requires sending a soil and root sample to a laboratory for extraction and identification — visual root symptoms alone only indicate the likelihood of nematode involvement.
- Lookalikes: nutrient deficiency and root rot fungi (e.g. Pythium, Rhizoctonia) can cause similar stunting and root discolouration without galling; check for fungal lesions with a sweet or musty odour and confirm with a lab test, since nematode damage often predisposes roots to secondary fungal infection and the two frequently occur together.
