Root-knot nematodes

 Introduction

The root knot nematode species, M. incognita, is the most widespread and probably the most serious plant parasitic nematode pest of tropical and subtropical regions throughout the world. It occurs as a pest on a very wide range of crops. 

 

Damage

Affected plants appear in patches. Estimates of vegetable crop losses due to Meloidogyne species, mainly M. incognita and M. javanica, have ranged from 17 to 20% for aubergine (Solanum melongena), 18 to 33% for melon and 24 to 38% for tomato. 
Losses of potatoes due to Meloidogyne species, mainly M. incognita, are estimated at 25% or more. 

 

Host range

Root-knot nematodes affect a wide range of crops, particularly vegetables. M. incognita is a major economic pest of food legumes in the tropics and subtropics. Common bean (Phaseolus vulgaris) is very badly damaged by Meloidogyne species in the tropics. Cowpea (Vigna unguiculata) is another very susceptible host crop of M. incognita. Many crops grown as vegetables are susceptible to the nematode particularly tomato, aubergine, okra, cucumber, melon, carrot, gourds, lettuce and peppers. Table I shows susceptibility of various commonly grown crops to root-knot nematodes.

Table 1. Crop susceptibility to root-knot nematodes (after AfFOResT)

Susceptible	Tolerant	Resistant
Bambara nut	Brassicas	Cassava
Beans	Chilli pepper	Garlic
Beetroot	Radish	Leek
Carrot	Sweet potato	Maize
Celery	Turnip	Millet
Cowpea		Rhodes grass
Cucumber		Sesame
Eggplant		Sorghum
Gourd		Sudan grass
Irish potato		Sweet corn
Lettuce
Melon
Okra
Parsley
Peas
Pumpkin
Squash
Sweet pepper
Swiss chard
Tomato
(Source: Dobson et al. 2002)

 

  Root-knot nematodes

Root-knot nematodes (Meloidogyne incognita /M. javanica) Roots of severely attacked (left) and healthy plant (right). Affected plants are  normally stunted and eventually wilt and die. The most characteristic symptom is formation of root galls (knots) and these can  be seen with the naked eye. Affected roots rot.

© A. M. Varela, icipe

Symptoms

Affected plants are stunted and yellow and have a tendency to wilt in hot weather. Very heavily infested plants are killed. Affected plants appear in patches. If infested plants are pulled from the soil, the roots are severely distorted, swollen and have lumps known as galls or root knots. The galls range in size from smaller than a pinhead to 25 mm or more in diameter.
 

Affected plant stages

Flowering stage, fruiting stage, seedling stage and vegetative growing stage. 
 

Affected plant parts

Leaves, roots and whole plant. 
 

Symptoms on affected plant part

Leaves: wilting. 
Roots: galls; reduced root system; swollen roots. 
Whole plant: dwarfing; wilting

There are four species of root-knot nematodes (M. arenaria, M. hapla, M. incognita) and M. javanica) that are capable of attacking vegetables. These species occur in Kenya (National Horticultural Research Station, 1984) and are particularly serious in irrigated fields. Important environmental factors that influence development of Meloidogyne are moist soils and relatively warm temperatures. Some plant penetration by root-knot nematodes occurs between 10 and 35°C, with the optimum at about 27°C depending on the species. No eggs are laid at temperatures lower than 14.2 or higher than 31.7°C. Under average conditions a female produces 300 to 800 eggs. A new generation can arise within 25 days, but under less favourable conditions, the time may be prolonged to 30 to 40 days. All species, with exception of M. hapla are killed by freezing. 

Root-knot nematodes measure about 0.5 mm to 1.5 mm in length. 

Juveniles (young nematodes) penetrate the root tips and occasionally invade roots in the zone of root elongation. Invaded nematodes initiate the development of giant cells in the root tissues and galling of roots occurs. 

Root knot nematodes are soil inhabitants. They are spread by transplanting infested seedlings, or from soil washed down slopes or sticking to farm implements and farm workers. They may also be spread by irrigation water. 

The disease is most serious on light, sandy soils and in furrow irrigated areas. Attack by nematodes may greatly increase the severity of bacterial, Fusarium and Verticillium wilt diseases
 

Conditions that favour development:

• Infected volunteer plants
• Monocultures
• Weeds in fields
• Warm temperatures and moist but well-aerated sandy soils
• Continuous growing of susceptible crops (no rotation)

 

Pest and Disease Management

 

Rotate with onions, baby corn, sweet corn, maize, millet, sorghum, sesame, cassava or Sudan grass.

A rotation system called "STRong" is recommended for management of root-knot nematodes. The system was developed by, African Farmers' Organic Research and Training (AfFOResT), a NGO in Zimbabwe. It involves planting in rotation of a susceptible crop (e.g. tomatoes), followed by a tolerant crop (e.g. cabbage) and then a resistant crop (e.g. onions) before a return to a susceptible crop (e.g. tomatoes). Crop susceptibility of various commonly crops is given in Table 1.

Use trap crops such as marigold (Tagetes spp.) and Indian mustard. (A trap crop is a crop planted to attract a pest and is then destroyed together with the pest). Mixed cropping with marigold can also minimise root-knot nematode damage.
 

Resistant varieties

Use resistant tomato varieties (e.g. 'Caracas', 'Kentom', 'Meru', 'Piersol', 'Roma VFN', 'Tengeru 97', 'Zest F1', 'Star 9001', 'Star 9003'). Tomato varieties carrying 'VFN' label are tolerant to root-knot nematodes. Most of these varieties are commercially available in the region.