Root-knot nematode-part I

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. Meloidogyne spp. Root-knot nematode juveniles are active, thread-like worms about 0.5 mm long. They are too small to be seen with the naked eye. 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. 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. The common species of root-knot nematodes all have a wide host range and most plants are able to host at least one 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. 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.

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