Root-knot nematode

Root-knot nematodes (Meloidogyne spp.) are minute, worm-like animals that are very common in soil. They have a wide host range, and cause problems in many annual and perennial crops. Solanaceous (e.g. tomatoes, potatoes) and Cucurbit (e.g. pumpkin, zucchini) crops are among the most seriously affected, with the nematodes causing problems in all growing areas.

Although this information is specific to tomatoes, the principles can be applied to most other annual crops.

Scientific name

  • Meloidogyne spp.
  • Meloidogyne javanica and M. incognita are the most widespread species in Queensland.

Immature stages

Root-knot nematode juveniles are active, thread-like worms about 0.5mm long. They are too small to be seen with the naked eye.

Life history

The juveniles hatch from eggs, move through the soil and invade roots near the root tip. Occasionally they develop into males, but usually become spherical-shaped females which set up permanent feeding sites inside the root.

The presence of developing nematodes in the root stimulates the surrounding tissues to enlarge and produce the galls typical of infection by this nematode. Mature female nematodes then lay hundreds of eggs on the root surface, which hatch in warm, moist soil to continue the life cycle.

Continued infection of galled tissue by second and later generations of nematodes causes the massive galls sometimes seen on plants such as tomatoes at the end of the growing season. The length of the life cycle depends on temperature and varies from 4–6 weeks in summer to 10–15 weeks in winter. Consequently, nematode multiplication and the degree of damage are greatest on crops grown from September to May.

Nematodes are basically aquatic animals and require a water film around soil particles before they can move. Also, nematode eggs will not hatch unless there is sufficient moisture in the soil. Thus, soil moisture conditions that are optimum for plant growth are also ideal for the development of root-knot nematode.

Distribution

There are nearly 100 species of root-knot nematodes, though only a few species (e.g. M. enterolobii, M. javanica, M. incognita, M. arenaria and M. hapla) are important in Queensland.

M. javanica and M. incognita are widespread, while M. hapla is common only in areas of high elevation (such as the Atherton Tableland) where it is cooler. M. enterolobii is a recently identified species which is potentially very damaging, but currently has a limited distribution in Queensland.

Root-knot nematodes are difficult to identify to species-level, but accurate species identification is particularly important when resistant varieties and crop rotation are being used as control practices. Most plants are resistant to a limited range of species, meaning that the crops chosen must be resistant to the species (or populations) present in a particular field.

Host range

The common species of root-knot nematodes all have a wide host range and most plants are able to host at least 1 species.

Many important vegetable, fruit, and ornamental crops are good hosts of these nematodes, including:

  • tomato
  • capsicum
  • potato
  • sweetpotato
  • carrot
  • eggplant
  • pumpkin
  • cucumber
  • melons
  • zucchini
  • lettuce
  • beans
  • celery
  • ginger
  • strawberry
  • pineapple
  • papaya
  • banana
  • sugarcane
  • grape
  • stonefruit
  • carnation
  • gardenia
  • chrysanthemum
  • rose.

Damage

Root-knot nematodes do not produce any specific above-ground symptoms. Affected plants have an unthrifty appearance and often show symptoms of stunting, wilting or chlorosis (yellowing). Symptoms are particularly severe when plants are infected soon after planting. However, more commonly, nematode populations do not build up until late in the season and plants grow normally until they reach maturity. Then they begin to wilt and die back at flowering, reducing fruit set and fruit development.

Below ground, the symptoms of root-knot nematodes are quite distinctive. Lumps or galls, ranging in size from 1 to 10mm in diameter, develop all over the roots. In severe infestations, heavily galled roots may rot away, leaving a poor root system with a few large galls.

Monitoring and sampling

Monitoring or assessment of nematode populations is an important aid to nematode management. Monitoring should begin well before problems occur, as it is too late to prevent crop losses once root-knot nematode damage is seen in the field.

Assessment of galling in the field

When tomatoes susceptible crops are planted in the same field every year, a check for root-knot galls at the end of the season provides valuable information on the level of nematode infestation and the likelihood of nematode damage in the next year. A thorough sampling of the field at harvest may provide as much information as having soil samples analysed for nematodes before the planting of the next crop.

Dig up plants from several areas of the field, taking care to retrieve the fine feeder roots, and look carefully for the presence of galls. The number and size of the galls provides an indication of the degree of root-knot nematode infestation.

Nematode analysis

Soil samples can be collected before planting and sent to a laboratory for extraction, identification and a count of the nematodes present. Divide fields into blocks no larger than about 4ha to sample areas of similar soil type or cropping history separately. For each block, collect about 50 small subsamples of soil with a shovel or sampling tube at depths of 5–20cm and place in a bucket. Mix thoroughly, remove a 500mL subsample, seal in a plastic bag and send to the laboratory for analysis.

Do not leave samples in the sun or refrigerate them. The best storage temperatures are 10 to 15oC. Include your name, address, date, location of the field, cropping history, the variety being planted and any information on previous nematicide use or nematode symptoms observed.

Bioassays

Instead of sending soil samples to a laboratory for nematode analysis, a simple bioassay is sometimes useful, particularly for detecting low populations of root-knot nematode. Transplant a nematode-free tomato seedling of a susceptible variety into a pot containing about 2L of your soil sample. Grow plants at temperatures of 20–28oC for about 1 month, and then remove the root system and examine it for galls. At this stage, the galls (if present) will be less than 0.5mm in diameter but their occurrence will indicate the presence of root-knot nematode.

The bioassay method has an advantage over nematode extraction methods because larger samples can be processed. Growing the plants for one month allows ample time for eggs to hatch, and these nematodes to invade the plant and be detected. Its main disadvantage is that samples must be collected well before planting.

Interpretation of results of monitoring data

Nematode analysis will identify all plant-parasitic nematodes species in the soil, including root-knot nematode. Only certain species cause economic damage on each crop, and nematode management decisions should be made on the basis of their presence or absence. Pre-plant risk thresholds for yield losses are available for root-knot nematode in many crops.

Due to sampling and extraction limitations, some nematodes may be missed when population levels are low. The absence of root-knot nematode in these extractions does not necessarily mean that the nematode is not present in the field. Information such as soil texture, previous cropping history and previous occurrence of nematode damage can also help to determine management decisions and whether a block should be re-sampled for further testing.

Control

On-farm biosecurity

Natural spread of nematodes in the soil is very slow, so human assisted movement accounts for most spread. On-farm biosecurity practices help to reduce the risk of new nematode pests entering a farm and spreading from one block to another. Root-knot nematodes can be spread through the movement of infected plant material (e.g. bulbs, tubers etc.), infested soil and other growing media, and farm equipment/tools/footwear contaminated with soil from infested sites. Being mindful of these pathways helps to reduce the risk of spread. Root-knot nematodes and most other soilborne pests and diseases are microscopic. Therefore, it is good practice to assume that anything moving soil and plant material on to your farm could infested.

Crop rotation

Root-knot problems increase and control becomes more difficult when susceptible crops are grown without rotation.

However, crop rotation will not completely eliminate infestations because some eggs in the soil may be viable for up to a year between host crops and most species can feed on a wide range of weeds. Nonetheless, rotation with a resistant crop can significantly reduce root-knot nematode populations in the soil and so reduce losses when a field is again planted to a susceptible crop.

Winter cereals are useful because they are generally poor hosts, and little nematode reproduction occurs during the cold winter months. It is more difficult to find summer crops with good resistance to root-knot nematode, though some sorghum x Sudan grass hybrids (particularly cv. Jumbo) are useful against most populations of the nematode. The legume sunn hemp is another good cover crop option for summer.

Cultivation, fallow and organic amendments

Repeated cultivation kills nematodes in the upper soil layers by exposing them to mechanical abrasion, and the heating and drying action of the sun. However, excessive cultivation is not good for soil health and reduces the population of beneficial soil organisms, many of which are natural enemies of plant-parasitic nematodes. A weed-free bare fallow will reduce the root-knot nematode population in the soil, as the nematodes die of starvation. In warm, moist soils in Queensland, a 4–6 month fallow may reduce root-knot nematode populations by more than 95%. Longer fallow periods are not normally economically feasible, and they increase the risk of soil erosion. Incorporating organic amendments, such as composts and crop residues, has also been shown to reduce root-knot populations by encouraging predators and parasites of plant-parasitic nematodes that naturally occur in most soils.

Resistant varieties

Varieties with root-knot nematode resistance are available for some crops (e.g. tomato, capsicum), but are not always commercially acceptable because of poor agronomic characteristics.

Nematicide treatments

  • Seedbeds – in crops established from seedlings, transplants must be free of root-knot nematodes. Before planting, fumigate all seedbeds with a registered chemical according to label directions.
  • Potting mixes – If peat, sand and other components are obtained from sources free of root-knot nematode and are not contaminated before use, the treatment of potting mixes for nematode control is unnecessary. Treatments for damping-off fungi (e.g. aerated steam at 60°C for 30 minutes) will also kill nematodes.
  • Field – If the management practices above are adopted, nematicides should only be needed in the field as a last resort (e.g. in sandy soils where tomatoes are particularly prone to nematode damage). Even in situations where root-knot nematode problems are usually severe, the use of good management practices reduces the nematode population pressure and gives nematicides a greater chance of providing effective control.

The following information is a suggested decision-making timetable that will assist your management of root-knot nematodes.

If you are growing a susceptible crop, check a sample of roots and determine the level of galling at harvest prior to ploughing out. Maintain a weed-free fallow until a cover crop is planted. Plant a cover crop that is not susceptible to root-knot nematodes, such as winter cereals or a resistant forage sorghum. Two months before planting, collect soil samples and either do a bioassay or get the soil tested for nematodes. If the results of nematode analyses or bioassays, or the previous occurrence of nematode problems, suggest that nematodes are likely to cause damage, either plant a nematode-resistant variety or apply a preplant nematicide.

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.