6 Major Types of Nematodes Pests of Vegetable Crops

The following points highlight the six major types of nematodes pests of vegetable crops. The types are: 1. Root-Knot Nematodes 2. Cyst Nematodes 3. Lesion Nematodes 4. Reniform Nematodes 5. Lance Nematodes 6. Stem and Bulb Nematode.

Type # 1. Root-Knot Nematodes (Meloidogyne spp.):

Root-knot nematodes are important pests of vegetables belonging to solanaceous (brinjal, tomato, chilli), cucurbitaceous (biter ground, cucumber, pumpkin, bottle gourd) leguminous (cowpea, bean, pea), cruciferous cauliflower, cabbage, broccoli, brussels, sprout), okra and several other root and bulb crops(onion, garlic, lettuce, celery, carrot, radish).

World over these are more than 60 species of this genus, whereas in India about 12 species have been reported to occur.

But, all of these species, predominating populations encountered are M. incognita, M. javanica, M. arenaria and M. hapla. Bio-chemical differences in protein and enzyme content among species have also been used for taxonomic identification. These four species account for more than 95% of the root-knot nematode population world over.

The above ground symptoms: Root knot nematode damage results in poor growth, a decline in quality and yield of the crop and reduced resistance to other stresses (e.g. drought and other diseases). A high level of root-knot nematode damage can lead to total crop loss. Severe symptoms and even death of heavily infected okra plants have been observed by Singh, 2006 at DLRSS, SKUAST-Jammu in J&K (Fig. 28.1 and 28.2).

Nematode damaged roots do not utilize water and fertilizers as effectively leading to additional losses for the grower. Below ground symptoms Meloidogyne spp. cause severe galling, stunting and chlorosis of crops. Other root swellings must not be mistaken for root-knot galls. Nitrogen fixing bacteria cause swellings on the roots of most legumes.

These swellings called nodules are easily distinguished from root-knot galls by differences in how they are attached to the root and their contents. Nitrogen nodules are loosely attached to the root, and can generally be very easily removed, root knot galls originated from infection at the center of root, so they are an integral part of the root whose removal requires tearing the cortex apart.

In addition, fresh nodules should have a milky pink to brown liquid inside them, while root-knot galls have firmer tissues and contain female root-knot nematodes inside the gall tissues, near the fibrous vascular tissues of the root.

The male and female root-knot nematodes are easily distinguishable morphologically. The males are wormlike and about 1.2 to 1.5 mm long by 30 to 36 nm in diameter. The females are pear shaped and about 0.40 to 1.30 mm long by 0.27 to 0.75 mm wide. The life cycle includes egg, juvenile and adult stages. A life cycle is completed in 25 days at 27°C, but it takes longer at lower or higher temperatures.

The female lays about 400-500 eggs per in a gelatinous matrix secreted by rectal glands. The root knot nematode, M. incognita on pea and bean was reported first time from farmer’s field of Jammu.The developmental life stages of the M. incognita in pea has been delineated (Singh, 2010, Fig. 28.3).

Type # 2. Cyst Nematodes (Heterodera spp. and Globodera spp):

The above ground symptoms resemble these associated with root damage and include stunting of shoot, yellowing of foliage and reduced size of various shoot parts. An experienced observer can often see cyst nematode, Heterodera spp. on the roots of their hosts without magnification. The young adult females are visible as tiny white colour.

After a female cyst nematode dies, her white body wall is tanned to a tough brown capsule containing several hundred eggs. The mature female bodies are found attached to roots by their head end embedded almost in the stele. The site of feeding is modified into a syncitium similar to that found in case of Meloidogyne.

On the surface of infected roots, white to brown bodies of females can be discerned with naked eyes. The intensity of body colour depends upon the maturity stage of the young female or cyst. In case of cyst nematode matrix with eggs may also be found attached to the posterior region of the female body.

The male is wormlike, about 1.3 mm long by 30 to 40 µm in diameter. Fully developed females are lemon shaped, 0.6 to 0.8 mm in length and 0.3 to 0.5 mm in diameter. Approximately 21-30 days is required for the completion of life cycle of this nematode.

Type # 3. Lesion Nematodes (Pratylenchus spp.):

The root lesion nematodes are most economically important phytonematodes.lt is having a wide host range. However, more than 350 hosts have been recorded. Assessment of exact loss by lesion nematodes has not been made possible under field conditions due to presence of mixed population of the nematode in the field.

Lesion nematodes have wide host range which can affect the selections of crop used to control the nematode in crop rotation sequences. Soil type tillage operations have also been reported to affect lesion nematodes population dynamics. The plants show chlorosis, stunting and general lack of vigour resulting into wilt.

The plant form patches or zones in the field. The roots show necrosis and lesions which become ideal for infection of other microorganisms. The presence of small brown to black lesions on the root surface is the most important symptoms or damage produced by the lesion nematode. Crop yields are reduced. Both male and female of these nematodes are wormlike, 0.4 to 0.7 mm long and 20 to 25 µm in diameter.

They are migratory endo-parasitic nematodes. The life cycle of the various species of Pratylenchus is completed within 45 to 65 days. Temperature, soil type, moisture and tillage operations are important environment factors which greatly affect the development and reproduction of nematode species as well as disease development. Reproduction is sexual. Females lay eggs singly in roots or in soil.

Type # 4. Reniform Nematodes (Rotylenchulus reniformis):

Reniform nematode has wide host range and associated with large number of vegetables in India. It is considered as pest of great significance after root-knot nematode. The nematode is a semi-endoparasite and remains attached to roots. Infected plants show stunting in growth with reduced and discolored root system. Damage during pre and post emergence of seedlings leads to reduction in germination and crop stand.

The total duration of life cycle is about 4-5 weeks under optimum conditions. Soil pH is also an important factor affecting reproduction of the nematode. Soil moisture and temperature have profound influence on infectivity, penetration and the biology of the nematode.

There was significant reduction in plant height, shoot and root weight of mash at 1000 or more juveniles/ 500 g soil. Cow pea is highly susceptible to damage by the reniform nematode. Growth of cowpea is negatively correlated with R. reniformis population density at inoculums levels from 10 to 10000 per pot.

Type # 5. Lance Nematodes (Hoplolaimus spp.):

Lance nematodes parasitize wide hosts. Among nematodes, which are often too small to be accurately detected by sight, the lance nematode is one of the larger species. Sometimes they feed at a particular site for a long time with nearly half of the body inside the root system (sedentary ectoparasite).

In many cases, juveniles of the lance nematode completely enter the cortical tissue (endoparasite). Damage may show up as patches of yellowing and dying.

These symptoms also can be caused by drought or nutrient deficiency. Hoplolaimus spp. multiplies slowly in comparison to endoparasitic nematodes but they inflict significant crop damage at a lower level of infection.

The availability of feeder roots and temperature are important factors for population build up of this nematode.The nematode acts as a vagrant endoparasite causing root lesions, thickening of cell wall and formation of tunnels in the cortical region.

Type # 6. Stem and Bulb Nematode (Ditylenchus spp.):

This nematode commonly attacks onion, garlic, potato, pea and carrot etc. The symptoms of injury differ from different plants and different parts of the same plant. Infected seedlings become twisted, enlarged and deformed, leading to death of the plant.

The plants become stunted. The nematode feeds on stem leaves and bulbs and is rarely found in soil. The nematode is 1.0 to 1.3 mm long and about 30 pm in diameter. The total duration of life cycle ranges from 19-23 days at 15°C with four moults and four Juveniles stages, the first moult being within the egg.