Root Knot Nematodes in Vegetables

Study Notes

Root Knot of Vegetables

Causal organism: Meloidogyne spp., a type of nematode. Common species responsible for root knot disease include M. arenaria, M. incognita, and M. javanica.
Host: A wide range of vegetables, including potato, tomato, brinjal, okra, cucurbits, carrot, chillies, groundnut, radish, etc.
Symptoms:
- Above-ground symptoms manifest as sickly and stunted growth.
- Leaves turn yellowish-green to yellow, droop, and wilt suddenly.
- Underground roots and tubers exhibit gall formation, which is caused by increased cell division.
- Roots of infected plants often develop spherical or elongated galls.
Environmental Relationship:
- Root knot nematodes thrive in sandy loam or silt loam soils.
- Optimal temperature for infection, multiplication, and gall formation ranges from 12°C to 35°C.
- Temperatures exceeding 40°C are lethal to larvae.
- Survival and movement of larvae are influenced by soil factors such as moisture content, clay content, and aeration.
- Larvae mobility is reduced in soils with excessive moisture, high clay content, and poor aeration.
Host-Parasite Relationship:
- Root knot nematodes represent a group of plant parasitic nematodes.
- Larvae survive in the soil for varying durations and are attracted to the roots of susceptible hosts by root exudates, forming a cluster around them.
- They are obligate parasites, meaning they cannot survive without a host.
- Larvae penetrate the host surface and migrate to the endodermis.
- Female nematodes embed their heads into the intercellular spaces near the endodermis, while male nematodes die.
- Growth of pith cells is arrested, while cortical cells near the feeding site of females enlarge to create giant cells.
- Adjacent cortical cell walls may disintegrate.
- Adult female nematodes remain embedded within host tissues and lay eggs that cling to the root surfaces.
- Eggs are ellipsoidal in shape.
Disease Cycle:
- The pathogen is soil-borne and its eggs survive in plant debris.
- The first larval molt occurs inside the egg, which hatches under favorable conditions, releasing the second stage of infective larvae.
- Infective larvae possess piercing spears or stylets used to penetrate the host surface.
- Stylets apply pressure on the weaker spots of the host surface, aided by the surface tension of water films around the larvae, normal reactions, and friction between soil particles and the nematode body.
- As an obligate parasite, the nematode derives nourishment from living host cells.
- Adult females (fifth stage of larvae) lay eggs that remain enclosed by host tissues.
- Eggs can also hatch on the surface of host tissues.
- Finally, larvae in the soil infect the roots of other hosts, completing the disease cycle.
Control:
- Soil fumigation after the destruction of root residues is effective in controlling the disease.

Figures

Figure 12.30:
- A: Symptoms of root knot disease.
- B: Adult female nematode.
- C: Adult male nematode.
- D: Larva.
Figure 12.31
- A: Egg sac on old gall surface.
- B-C: Single egg.
- D: First stage larva.
- E: Second stage larva.
- F: Second stage larva free in the soil.
- G: Second stage larva attacking rootlet.
- H-K: Second stage larvae infecting and feeding on giant cells.
- L: Female starts laying eggs after the fourth molt, while the adult male leaves the root.
- M: Adult female begins laying eggs.