Root-knot nematode of tomato

North Carolina ranks 10th in the nation for tomato production, with about 3,300 acres of tomato harvested per year and about $52 million in annual sales. Tomatoes are grown in the field, greenhouse settings, and home gardens statewide. Tomato plants can be susceptible to root-knot nematodes, which are microscopic roundworms that inhabit the soil. Root-knot nematodes are parasitic nematodes, meaning they infect and feed on plants.

Besides root-knot nematodes, there are numerous other types of non-parasitic nematodes that inhabit the soil. These non-parasitic types do not infect plants and are an important part of the soil food web. Only a small proportion of all nematodes (like root-knot nematode) are plant parasites.

Because root-knot nematodes live and infect host plants below the soil line, they can often be difficult to detect and manage. Regardless of whether tomatoes are grown in large scale agricultural systems or in a home garden, root-knot nematodes can debilitate tomato production unless effectively managed.

Root-knot nematodes belong to the genus Meloidogyne. Several species of root-knot nematode infect tomato, including Meloidogyne incognita (Southern root-knot nematode), M. arenaria (peanut root-knot nematode), M. javanica (Javanese root-knot nematode), M. hapla (Northern root-knot nematode), and the recently emerging M. enterolobii (guava root-knot nematode). The most wide-spread root-knot nematode in North Carolina is M. incognita. The life cycle of root-knot nematodes takes about 25 days to complete, but this length of time may be affected by soil temperature, moisture, or the presence of host plants.

Host range
Root-knot nematodes have a broad host range, meaning they can infect many other crops in addition to tomato. However, not all root-knot nematode species are able to infect the same crops, although the host ranges of several root-knot species overlap. Among the many host plants are tomato, pepper, sweetpotato, corn, soybean, tobacco, cotton, and numerous species of weeds. Identification of root-knot species may help dictate a suitable crop rotation plan; however, the large number of host plants may limit options for planting in certain situations.

Signs and symptoms
Signs and symptoms of root-knot nematode infection occur below ground in the roots. Roots will display characteristic galls, or swellings, where female nematodes and their egg masses may be found. Using a magnifying glass can help you look for galls on the roots produced by root-knot nematodes.

Different species of root-knot nematodes produce similar symptoms; therefore, it is not possible to differentiate species based on symptoms alone.

Nematodes are rarely evenly distributed in the field and instead are often found in clusters. Therefore, symptoms within a field are usually found in ‘hot spots’ or patches, where nematodes are more concentrated. Above-ground symptoms, when present, are often associated with high nematode populations, and may include stunting, chlorosis (yellowing), and wilting of plants.

In cases of moderate to severe infestation, plants are more susceptible to drought, and their ability to uptake water and nutrients decreases. A decline in yield is often observed with root-knot nematode infection in more susceptible tomato varieties, especially under high nematode population levels. Above-ground or foliar symptoms may not always be present when nematode populations are low. 

Effective nematode management begins with field selection, sampling for nematode populations, and starting with nematode-free planting material. It is difficult to completely eradicate nematodes from a site once they are established; therefore, it is crucial to follow recommended guidelines and sanitation practices to ensure the site remains free of root-knot nematodes and highly productive. If nematode populations establish within the site, the following disease management tools can be used to bring root-knot nematode populations to below damaging levels. Producers are highly encouraged to implement more than one tool for durable and robust management.

Exclusion and Sanitation: Due to their microscopic size, root-knot nematodes naturally have limited movement in the soil; therefore, it is important to limit the spread of nematodes to new areas by exclusion and sanitation. Nematodes can easily be moved in contaminated soil on equipment, shoes, garden tools, or vehicles. To reduce the risk of moving nematodes through contaminated soil, equipment and tools should be sanitized by washing with a solution of 10% household bleach, rinsing with clean water, and air drying before moving to unaffected fields or areas. The use of nematode free transplants and planting media is important to exclude nematodes from the greenhouse, garden, and field.

Crop Rotation: Crop rotation to a non-host crop can be a useful tool to help manage root-knot nematodes. By alternating to a crop that the nematodes can neither feed on nor complete their life cycle on, this tool can promote a decline in root-knot nematode populations in the soil. However, effective use of non-host rotational crops for management of root-knot nematodes is dependent upon the species (and sometimes race) of root-knot nematode present in the field, or if the crop possesses any resistance genes.

Source: North Carolina State.

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