Root knot nematodes—Meloidogyne spp.
Nematodes are tiny (usually microscopic) unsegmented roundworms. Root knot nematodes are the most common nematodes attacking annual and perennial landscape plants, especially in warm, irrigated, coarse-textured soils (sand, sandy loam, and loamy sand).
Many landscape tree and shrub species are hosts of root knot nematodes, as are many herbaceous ornamentals and common weeds. More than one root knot nematode species can occur at the same site. A plant species resistant to one species of root knot nematode may still be susceptible to other Meloidogyne spp.
Identification
Root knot nematodes cause galls or swellings on roots of many broadleaf plants. Some infected plants, especially annual grasses and certain legumes, may exhibit no galls even though nematodes still reproduce on them. When roots infected with root knot nematodes are washed they may appear gnarled and restricted.
Beneficial nitrogen-fixing bacteria often form nodules on the roots of beans and other legumes, but these rub off roots easily, while galls caused by root knot nematodes are truly swellings of the roots. Also, a thumbnail can easily be pressed into a nitrogen-fixing bacterial gall, but not into a root knot gall. There are many other causes of plant galls.
Plant-feeding nematodes develop through six stages: egg, four juvenile stages, and adult. Many species can develop from egg to egg-laying adult in as little as 3 to 4 weeks when the soil is warm and moist.
Root knot nematodes spend most of their active life cycle in galls on roots. First-stage juveniles stay in the egg. The second stage hatches from the egg and moves through soil to invade new sites, usually near root tips, causing root cells to grow into giant cells where the nematodes feed as second-, third-, and fourth-stage juveniles.
As feeding continues, the plant produces a gall around the infected area. Mature females produce eggs in a small gelatinous mass on the root surface or inside the root. Adult females remain in roots and produce an egg mass. Males leave and move through soil to seek and mate with females.
Damage
Boxwood, rose, and most fruit and nut trees can be damaged in landscapes by Meloidogyne spp. Many other species are hosts. Roots infected by nematodes may become attacked by other pathogens, including those causing crown gall, root and crown decays, and certain vascular wilt diseases.
Root knot nematodes usually cause distinctive swellings on the roots of affected plants, which damage the water- and nutrient-conducting abilities of the roots. Galls can crack or split open, allowing entry of disease-causing microorganisms.
Aboveground symptoms of a root knot nematode infestation include
- wilting during the hottest part of the day even with adequate soil moisture
- loss of vigor
- yellowing leaves
- other symptoms similar to a lack of water or nutrients
Infested plants grow more slowly and produce fewer and smaller leaves and fruits. Damage is most serious in warm, irrigated, sandy soils.
Solutions
To confirm a root knot nematode infestation, you must collect appropriate root and soil samples and send them to a laboratory that can provide positive identification of the infesting species.
Nematodes are difficult to manage because in most landscape situations you cannot directly reduce their numbers. The following practical nematode management methods are preventive:
- Use good sanitation to avoid introducing or spreading nematodes.
- Grow plant cultivars or species that are nonhosts or are resistant or tolerant to nematode damage.
- Provide good cultural care and appropriate growing conditions to reduce stress on plants and increase their tolerance of infestations.
To avoid introducing nematodes into landscapes, use only amendments, soil, and plants obtained from a reliable supplier that presumably are free of pathogens. Do not allow drainage or irrigation water from around infested plants to run off onto soil around healthy plants as this spreads nematodes.
Do not transfer soil from around infested plants to healthy plants. Thoroughly wash soil and plant parts from all equipment and tools used around infested plants before leaving that site. Work first in uninfested areas before working around plants or soil suspected of being infested with nematodes.
Consider replacing severely damaged plants and replant with species or cultivars more tolerant of the specific nematodes present; for example, do not replant the same plant genera into the old site. See Pest Notes: Nematodes for more information.
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