The science of Nematology is relatively young compared to its contemporary disciplines of Entomology and Plant Pathology. Thus, despite the significant role of nematodes in agriculture, still much is yet to be understood and learnt in this discipline. Nematodes are ubiquitous, present in all moist to watery ecological niches in diverse situations like cold oceans, hot springs, mountain peaks, soil ecosystem, plants as well as animals. Phytoparasitc forms comprise about 15% of all the forms of nematodes that exist in various habitats and have different feeding behaviours.
Phyto-nematodes parasitize all types of plants - from lower thallophytes (algae) to highly developed angiosperms including all plants and trees and cause billions of dollars worth of damage to world food crops as well as tree crops, turf and ornamentals.
There are about 6000 known species of phyto-parasitic nematodes belonging to 197 genera. Being obligate parasites, they must draw their nutrition from plant hosts and in the process, must debilitate the plant. A handful of soil from around the roots of any plant would yield hundreds of plant parasitic nematodes belonging to at least 4-5 genera. A single wheat gall may contain up to 30,000 wheat gall nematodes, one gram of coconut root may contain about 4000 eggs, juveniles and adults of burrowing nematode or about 5000 individuals of coconut ring nematode.
 Figure 2.1 Nematodes by life style, Source: NemaPix
Plant parasitic nematodes present some of the most difficult pest problems evaluated in our agricultural economy, because nematode damage is often overlooked due to mostly non specific symptoms, although the yield losses may equal to those associated with more severe and easily identifiable plant damage. The extent of direct damage by the nematodes to plants depends on several factors. These include-
- Initial nematode population
- Nematode density in soil
- Nature of parasitism (ectoparasite or endoparasite, migratory or sedentary)
- Host susceptibility
- Cropping pattern
- Edaphic factors (soil texture, moisture etc.)
- Ambient climatic conditions
- Maximum temperature and moisture.
Nematodes, by themselves, rarely kill the plants to ensure their own survival. However, in nature, they are involved in all sorts of interactions with other micro-organisms (fungi, bacteria, viruses) leading to disease complexes. Also, their high population build up in soil has been the cause of ‘Soil exhaustion’ that compelled the farmers to migrate to the newer areas since old times. ‘Replant problem’ is another common experience in nematode infested orchards, plantations and forests.
Nematode problems in the tropical and sub-tropical zones are generally more damaging and varied because:
- Higher temperature and longer growing seasons result in more generations per year; higher the population more will be the damage.
- The more number of susceptible crops per year in warm areas results in higher nematode build up.
- Some of the most damaging species like Meloidogyne. incognita are prevalent in warm areas.
- More severe disease complexes occur in warmer areas.
- Some exceptions to these are Globodera rostochiensis (potato), Heterodera schachtii (sugarbeet), Anguina tritici (wheat) etc. which prefer cooler environmental conditions.
Economic losses caused by nematodes
- In the tropical and sub-tropical climates, crop production losses attributable to nematodes were estimated at 14.6% compared with 8.8% in developed countries.
- Overall average annual loss of the world’s major crops due to damage by plant parasitic nematodes was estimated to be 12.3% which is one third of the losses attributed to pests and diseases in general.
- For the 20 major life sustaining crops that serve as man's primary food source, annual yield loss of l0.7% was estimated.
- For another group of 20 crops mainly of commercial importance, a 14% annual yield loss was assessed.
- Developing countries suffer a crop loss of 14.6% compared to 8.8% in developed countries, when estimated losses for all the 40 crops were considered.
- Monetary losses due to nematodes on 21 crops were estimated at US $ 77 billion annually based on 1984 production figures and prices.
- 11% in vegetables ($267 per year fruit)
- 6% in field crops ($110 million/year)
- 12% in fruits and nuts ($225 million/year) and
- 10% loss in ornamentals ($60 million/year).
In Florida, losses due to Tylenchulus semipenetrans were assessed at 50-80% in grapefruit and 40-70% in sweet orange. In UK, loss of potato due to golden nematode has been put to 2 million pounds.
Plant parisitic nematodes of utmost significance
- About 100 nematode diseases are of global economic significance.
- Presently 25 genera of plant parasitic nematodes include species that are economic pests of crop plants.
- Ten most important nematode genera in order of their significance at global level are Meloidogyne, Pratylenchus, Heterodera, Ditylenchus, Globodera, Tylenchulus, Xiphinema, Radopholus, Rotylenchulus and Helicotylenchus. Others may assume significance in near future as their host parasitic relationships are properly understood.
In India, the nematodes that cause most severe damage to horticultural crops include:
- Meloidogyne and Rotylenchulus reniformis in vegetables
- Radopholus similis in banana, black pepper and coconut (c.o. toppling disease of banana, slow wilt of pepper and coconut)
- Pratylenchus coffeae in coffee
- Tylenchulus semipenetrans in citrus (c.o. Citrus decline/Slow decline of citrus)
Similarly, among the cereal crops:
- Ditylenchus angustus (c.o. ufra disease of rice) Aphelenchoides besseyi (white tip of rice), Hirschmanniella spp. and Meloidogyne graminicola in rice
- Heterodera avenae (molya disease in wheat and barley), Anguina tritici (ear cockle of wheat) in wheat are most important.
Some other examples of economically damaging nematodes of specific crops in one or the other region of the world are
- Aphelenchoides besseyi (summer crimp of strawberry)
- Heterodera schachtii (causes ‘beet sickness’ and sickness of cole crops)
- Bursaphelenchus xylophilus (causes lethal ‘pine wilt’ and is responsible for destruction of large areas under pine forests in counties like Japan and USA)
- Bursaphelenchus (=Rhadinaphelenchus) cocophilus (causing ‘red ring of coconut’)
- Radopholus similis (c.o. toppling disease of banana,slow wilt of coconut and pepper)
- Radopholus citrophillus (c.o.spreading decline of citrus)
- Tylenchulus semepenetrans (in citrus causing slow decline).
- Many nematode genera infest tropical forest plants.
Besides, some major nematode problems that have not been reported from India so far are:
- “Spreading decline” of citrus caused by Radopholus citrophilus (in Florida)
- Onion bloat caused by Ditylenchus dipsaci
- Beet sickness caused by Heterodera schachtii
- Cotton disease caused by Belonolaimus longicaudatus.
Some nematodes that cause wide spread problems in wet and cool temperate regions are:
- Ditylenchus dipsaci (stem and bulb nematode on bulb crops)
- Ditylenchus destructor (potato rot nematode)
- Globodera rostochiensis and G. pallida (latter species more important in India) on potato
- Bursaphelenchus xylophilus (Pine wilt nematode)
The annual loss in the country due to :
- Cereal cyst nematode, Heterodera avenae in wheat and barley was estimated to be Rs 32 million and 25 million, respectively, in Rajasthan alone.
- Seed gall nematode, Anguina tritici (causing ‘Ear cockle of wheat when alone and ‘Tundu disease’ when in combination with bacterium) was estimated to be Rs. 70 million in wheat only in North India.
- Pratylenchus coffeae in an area of about 1,000 ha in Karnataka was estimated to be Rs. 25 million is assessed in coffee due to.
- G. rostochiensis is a limiting factor. In Nilgiris, about 3000 ha area is infested with this nematode and total crop failure has been reported number of times.
Gaur and Seshadri (1999) estimated a loss of Rs 24230 million per year due to plant parasitic nematodes on different crops in India. Some of the economically important nematodes in various crops in the country are
- Meloidogyne spp. attack more than 3000 crop plants which include vegetables, tuber crops, pulses, number of fruits, ornamental crops, tobacco etc.
- Aphelenchoides besseyi (white tip), Ditylenchus angustus, Hirschmanniella spp., Heterodera oryzae etc in rice
- Radopholus similis in banana, coconut, arecanut, pepper and many other spice crops in South India.
- Tylenchulus semipenetrans in citrus crop in many citrus growing states of the country.
- Aphelenchoides spp. and Ditylenchus myceliophagous in mushroom are responsible for slow pace of development of mushroom industry in the country as their infestation leads to poor crops or total crop failures in number of mushroom growing states of the country.
Root-knot nematode (Meloidogyne spp.) ranks first as far as damage to crops at global level is concerned because of their world wide distribution , extremely wide host range, destructive nature of the diseases caused by this nematode and their role in many destructive disease complexes.High percent yield losses in solanaceous vegetables by root-knot nematode have been assessed in various parts of the country. Avoidable yield losses to the tune of 28.1, 33.7, 46.2, 43.5 and 28.6 per cent have been assessed in okra, brinjal, tomato, French bean and cowpea, respectively.
Nematodes are excellent bio-indicators for environmental change as once they are present in a habitat and in proximity of hosts conducive to their development, they may rapidly multiply. Indigenous species that have remained in balance may emerge to pest status on agricultural crops with small changes to their habitat, either through changes in cropping practice (crop, cultivars, rotation cycle, etc.) or climate.
A good example of this is illustrated by the rapid and alarming emergence of Meloidogyne minor in Europe .
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