Root-knot nematode-part II
Below ground, the symptoms of root-knot nematodes are quite distinctive. Lumps or galls ranging in size from 1 to 10 mm 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 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. For 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 4 ha 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 5-20 cm and place in a bucket. Mix thoroughly, remove a 500 mL 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 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: nstead 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 2 L of your soil sample. Grow plants at temperatures of 20-28oC for about one month, and then remove the root system and examine it for galls. At this stage, the galls (if present) will be less than 0.5 mm 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 two 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 200 mL 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 200 mL soil may be heavily galled at harvest but will suffer little yield loss from nematodes.