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. Tomatoes 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.

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.

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 more than 50 species of root-knot nematodes, though only a few species (e.g. 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.

Although root-knot nematodes are difficult to identify, it is not important, for most practical purposes, to know which of the species is present. Species identification becomes particularly important when resistant varieties and crop rotation are being used as control practices because most plants are resistant to a limited range of species. Therefore, 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 fruit, vegetable and ornamental crops are good hosts of these nematodes, including banana, cucurbit, grape, carnations, passionfruit, nectarine, capsicum, bean, kiwi fruit, chrysanthemum, pineapple, tomato, carrot, egg fruit, strawberry, rose, peach, celery, ginger, lettuce, papaya and pumpkin.

In general, members of the grass family are less susceptible than other plants to root-knot nematodes.

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 with 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 (or other root-knot 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 15^oC. 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 20–28^oC 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 at least 2 months before planting.

Interpretation of results of monitoring data

Nematode analysis is likely to show a number of plant-parasitic species. However, root-knot nematode is the only species known to cause economic damage to tomatoes in Queensland and nematode management decisions should be made on the basis of its presence or absence.

Unfortunately, not all nematodes are recovered with the extraction methods currently available and this applies particularly to nematodes in the egg stage. When relatively small samples are processed, 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. In this case, use information about the soil texture, previous cropping history and previous occurrence of nematode damage to decide whether the negative result is accepted or whether the block should be re-sampled.

Tomatoes are very susceptible to root-knot nematodes under Queensland conditions. In the past, control measures were normally recommended if one root-knot nematode was found in a 200mL soil sample taken before planting or a gall was found in the roots of bioassay plants. However, recent research suggests that the economic threshold may be a little higher than previously thought. Thus, well managed crops grown in fields with a preplant density of 1–10 root-knot nematodes per 200mL soil may be heavily galled at harvest but will suffer little yield loss from nematodes.

Control

Crop rotation

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

However, crop rotation will not eliminate infestations because root-knot nematodes can remain in the soil as eggs for at least a year between host crops and most species can feed on a wide range of weeds. Nonetheless, rotation can significantly reduce losses when a field is planted again 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 sorghum x Sudan grass hybrids (particularly cv. Jumbo) are useful against most populations of the nematode.

Fallow and cultivation

Repeated cultivation kills nematodes in the upper soil layers by exposing them to mechanical abrasion, and the heating and drying action of the sun. If the field is maintained weed free, nematodes also die of starvation. In warm, moist soils in Queensland, a 4–6 month fallow may reduce root-knot nematode populations by more that 95%. Longer fallow periods are not normally economically feasible and they increase the risk of soil erosion.

Sanitation

As nematode populations have the capacity to increase rapidly, plough out plants as soon as the crop is harvested to prevent further multiplication. At this time, most of the nematode population is in the roots rather than the soil. Therefore, if these roots are removed from the field and destroyed (e.g. by burning), the nematode population immediately and substantially reduces.

Resistant varieties

Tomato varieties with nematode resistance are available but not always commercially acceptable because of poor agronomic characteristics. Experimental breeding lines with nematode resistance are being tested and may be more suitable. These varieties provide adequate but not absolute protection against common populations of M. incognita and M. javanica. M. hapla and some races of M. incognita are sometimes capable of attacking resistant varieties.

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 nematodes in tomatoes.

If you are growing a crop susceptible to root-knot, check a sample of roots and determine the level of galling approximately 12 months before planting tomatoes. Eight months before planting, destroy nematode-infested root systems and plough out the crop immediately after harvest. 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 forage sorghum. Two months before planting, collect soil samples and either do a bioassay or test the soil 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.