Click on any
image for a more
detailed view

Symptoms and signs

Plant Roots

Root-knot nematode symptoms on plant roots are dramatic. As a result of nematode feeding, large galls or "knots" can form throughout the root system of infected plants. Severe infections result in reduced yields on numerous crops and can also affect consumer acceptance of many plants, including vegetables (Figures 1, 2). The degree of root galling generally depends on three factors: nematode population density, Meloidogyne species and "race," and host plant species and even cultivar. As the density of nematodes increases in a particular field, the number of galls per plant also will increase. Large numbers of nematodes penetrating roots in close proximity also will result in larger galls. Meloidogyne hapla (the northern root-knot nematode) produces galls less than half the size of those produced by M. incognita (the southern root-knot nematode) on the same plant hosts. Finally, each crop responds differently to root-knot nematode infection (Figures 3, 4, 5). Carrots typically undergo severe forking with galling predominantly found on lateral roots (Figures 2, 6). Root-knot nematode galls on lettuce are beadlike (Figures 4, 7). On grasses and onions, galls are usually small and barely noticeable, often no more than slight swellings (Figure 8). Depending upon the crop affected and the severity of infection, these symptoms can often result in significant economic losses to growers.


Figure 1	Figure 2

Figure 3	Figure 4

Figure 5	Figure 6

Figure 7	Figure 8

Most root-knot nematodes have a very wide host range. Thus, growers who have a root-knot nematode problem may find it difficult to control the nematode and its damage through crop rotation, although this is sometimes a viable option. Cotton growers who have an infestation of M. incognita can often plant peanuts in subsequent years to reduce nematode populations. Unfortunately, peanut is an excellent host for a race of M. arenaria, which can be found in fields that also contain M. incognita. Growers who have a problem with M. javanica can employ peppers as a rotational crop, but not if they also have M. incognita. These two examples demonstrate the importance of understanding which Meloidogyne species is present. In addition to differences in pathogenicity on a specific crop, there can be an even greater degree of specialization. For example, M. hapla will not reproduce on grasses. In contrast, M. graminis only reproduces on grasses. Thus growers who experience problems with M. hapla can rotate corn and wheat into traditional vegetable production, provided they have the appropriate equipment available.

Above Ground Symptoms

While the most diagnostic root-knot nematode damage occurs below ground, numerous symptoms can also be observed above ground. Severely affected plants will often wilt readily. Because galled roots have only limited ability to absorb and transport water and nutrients to the rest of the plant, severely infected plants may wilt even in the presence of sufficient soil moisture, especially during the afternoon. Plants also may exhibit nutrient deficiency symptoms because of their reduced ability to absorb and transport nutrients from the soil. Additional fertilization will not generally result in remediation of root-knot nematode-induced chlorosis. Stunting is frequently observed on host crops grown in root-knot nematode-infested fields, and crop yields are reduced. In highly sensitive crops such as lettuce and carrots, initial density of 2 and < 1 eggs/cc soil, respectively, are sufficient to cause economic losses. At high densities, root-knot nematodes can actually kill host plants, particularly if the high populations occur early in the growing season when plants have minimal root mass (Figure 1).

Above ground symptoms usually appear on clusters of plants (foci). Because nematodes move slowly through the soil, infestations will gradually radiate outward from an initial point of infection. This can result in large foci of affected plants surrounded by seemingly unaffected plants (Figure 1). Cultivation and other practices that physically move soil and plants will rapidly spread root-knot nematodes.