Nematodes

Nematodes – an overview:

Nematodes, or ‘eelworms’, are small, non-segmented worms that live in soil, which cannot be seen with the naked eye. They are only 50 microns in diameter and about 1 mm long or less. They feed on fungi, bacteria, and other soil organisms, together with plant cells. In some cases, they also allow fungal rots to enter that destroy the roots. While most nematode species have a beneficial role in the soil, some species are pests and impact agricultural production.

Nematodes can be differentiated by their feeding type or mouthparts. While bacterial feeding nematodes have a tube-like structure to suck bacteria, fungal feeding nematodes have a piercing needle (stylet), which penetrates fungal cells and enables the nematode to suck cell contents. Root feeding (parasitic) nematodes, such as the root knot nematode, also have piercing mouthpart to pierce root cells. Predators feed on other nematodes and small soil organisms.

Parasitic nematodes are pests because they feed on plant roots and slow plant growth. Species such as stem and bulb nematode also migrate to above ground plant parts. Beneficial nematodes attack and kill a range of pests such as borers, grubs, thrips and beetles, with negligible effects on non-target species. Nematodes are found in the top few centimetres of the soil, living in the thin films of water surrounding soil particles.

Factors conducive to nematodes:

Suitable crops, water, adequate organic matter, and minimal disturbance of the soil are factors conducive to the presence of nematodes in the soil. They require water to move and are generally found in well-structured soil with large pore spaces, or coarser (sandy) soil, where food is easily available. Some pesticides may have a detrimental effect on nematodes.

Types of nematodes, hosts and damage:

Some common types of nematodes include potato cyst nematode (PCN), root knot nematode (RKN), root lesion nematodes, and stem and bulb nematode.

Nematode species	Preferred conditions / survival	Host vegetable crops	Symptoms
Potato cyst nematode (Globodera rostochiensis)	PCN cysts can remain dormant in the soil for up to 30 years. The optimum soil temperature is 15-20°C.	Plants of the Solanaceae family (e.g. potato; eggplant; tomato).	Appearance of round pinhead sized white to golden cysts on the roots of infested plant at the time of flowering. Plants show yellowing and wilting.
Root-knot nematode (Meloidogyne species)	Can survive in the soil as egg masses. Prefers warm temperatures.	Carrots; potatoes; beetroot; sweetpotato; parsnip; tomatoes; eggplant; capsicum; celery; onions; peas; beans; spinach; herbs;cucumber; Brussels sprouts; radish	Root swellings or galls are clear on the roots and plants show stunting and yellowing. Water uptake is reduced so plants wilt.

Transmission:

Nematodes are spread by water runoff, infested plant material and infested soil on equipment and people. Potato cyst nematode can easily be spread by the movement of host plants or the soil attached to plants, bulbs, advanced trees and agricultural equipment. Root knot nematode may be spread by planting infested tubers, which do not necessarily show symptoms.

Management:

Nematode control in Australia is heavily dependent on general soil fumigants and non-fumigant nematicides, which can be effective when applied according to label recommendations and with proper soil preparation. However nematacide use should be part of an Integrated Pest Management (IPM) program, together with soil testing. Management methods include:

Prevention:

Measures include weed control, careful examination of transplant roots before planting to ensure they are nematode free, and survey sampling of a planted field to determine the presence or degree of nematode activity. Do not plant in infested soil and use PCN resistant cultivars.

Avoidance:

The following measure can help reduce the presence of nematodes.

Soil testing

This gives an indication of the risk of planting a particular field and the necessity for chemical treatment. As nematodes often have a patchy distribution, it may be possible to split the field into sections, collecting samples for testing from each section.

Rotations and break crops

Rotations and break crops are best chosen with accurate soil counts of particular nematode species. Rotate susceptible crops with non-susceptible crops like cereals and grasses.

Fallow

A period of more than six months can reduce populations of some nematodes, but may not reduce numbers below damage thresholds. It is important to maintain a bare fallow, as weeds can act as hosts and allow nematode numbers to increase. Plough out the crop as soon as harvesting is finished.

Planting

Planting in late autumn or early spring can allow crops to become established when soil temperatures are cooler and less conducive to invasion and production of nematodes.

Monitoring:

It is important to track and properly identify nematode species. Undertake soil testing and routine sampling, to provide information on incidence and distribution.

Suppression:

Includes addition of organic matter and green manure crops to encourage the release of micro-organisms detrimental to pest nematode development or survival.

Restricted movement of potato material:

Potato cyst nematode is a serious pest of potatoes world-wide and is subject to stringent quarantine and/or regulatory procedures wherever it occurs. There are regulations regarding interstate movement of potatoes exported from or imported into Victoria. Control areas have been declared to prevent the spread of PCN from some areas of Victoria.