Plant Parasitic Nematodes - Damage to crops and management

Crop losses due to nematodes.

It will indeed be a wonder if any crop is free from plant parasitic nematodes [PPN]. Many a times in olden days, nematodes have caused people to migrate due to soil sickness. It has been estimated by the International Meloidogyne Project that nematodes cause annual looses of 78 billion US dollars in developed countries and more than 100 billion in the developing countries.

Nematode problems are more severe and complicated in warmer than cooler areas, in horticultural than field crops, mono-culture than multi-culture, plantation crops than natural forests and vegetation. Horticultural crops are more efficient producers of biomass and harvestable produce than the agronomical crops. Nematodes pose a constraint to horticultural development and intensive cultivation.

It has been estimated that annually an average 6% loss in field crops, 12% in fruit and nut crops and 11% in vegetables and 10% in ornamental crops is due to nematode infections. Besides causing quantitative losses, nematodes are known to reduce vitamins and minerals in edible plant parts. Nematode damage is less obvious and many a times goes unnoticed. It causes gradual decline in yield. Nematodes cause complex diseases in association with other soil-borne pathogens.

What are nematodes?

Nematodes [Greek nema = thread, eidos = resembling, form] are microscopic organisms which are also called as thread worms, eelworms or round worms. They inhabit the region around the roots in the soil. A few species also feed on the aerial parts of the plants like the stem, leaves, ear-heads, grains etc. Unlike insects, viruses, fungi or bacteria, nematode populations increase rather slowly and the diseases caused by them are generally not of sudden epidemic type resulting in obvious destruction of crop and yield but cause a rather slow decline in yields. Symptoms of nematode diseases are often not clear cut and easy to pinpoint. The damage due to nematodes is many a time a result of interaction between nematodes or bacteria, viruses or fungi.

Types of nematodes

Plant parasitic nematodes are so wide spread and universal in distribution that no living plant is likely to be free from their attack. All cultivated crops are usually attacked by one or more species of nematodes.

PPN are usually microscopic generally ranging from 0.5 to 1mm in length. They are cylindrical, the body tapering towards the head and tail ends. Mature females of some nematodes however assume swollen pouch like, lemon pear kidney or irregular shapes while males remain slender. The most important diagnostic feature of PPN is that they have a stylet or spear at the mouth end. It is a long sharp pointed tube like doctor's hypodermic needle. The stylet is worked back and forth into the plant tissue. A nematode sucks up the plant sap and at the same time injects a small quantity of its gland secretion [consisting of digestive enzymes] into the tissue. The reaction of the plant and the expression of various disease symptoms are attributable to some toxic substances injected and depletion of plant nutrients.

Life cycle of plant parasitic nematodes.

Nematodes have six stages in their life cycle- the egg, four larval stages and the adult. One cycle may be completed in few days or may require several years depending upon a great variety of conditions and circumstances. Molting occurs before each stage.
The emerging second-stage juvenile (J2) is the most common infective stage. Length of life cycle of plant parasitic nematodes can be anywhere from 20-40 days (on average 25 days at 22oC).

Reproductive Potential (Egg production)
Anquina-25,000eggs(2,000/wk)
Meloidogyne-500-1500eggs
Ascaris 2 million eggs/day

Different plant parasitic nematodes have their characteristic feeding habits. The nature of the damage is dependent on the site of feeding and the host response to infection. The nematodes can be grouped according to their parasitic habitats follows:

Type	Nematode Types	Names
1	Migratory Ectoparasite	Root- Ditylenchus, Tylenchorhynchus, Rotylenchus, Helicotylenchus, Trichodorus, Belonolaimus. Stem- Ditylenchus, Leaf-Aphelenchoides
2	Sedentary Ectoparasite	Root- Hoplolaimus, Longidoridae
3	Migratory ectoendo or semiendoparasite	Root- Pratylenchus, Hirschmanniella, Radopholus Stem - Anguina Leaf, Bud underground storage organs and reproductive system - Ditylenchus
4	Migratory endoparasite	Root - Hemicycliophora, Tylenchus
5	Sedentary endoparasite	Root -Meloidogyne, Heterdera, Naccobbus

How nematodes cause injury to plants?

PPN are obligate parasites.

1. During feeding they penetrate the plant cells causing mechanical injury. This is evident in case of a large number of nematodes infecting the plant.
2. Nematodes also inject digestive juices in the plant cells causing hydrolysis of the host components altering the host metabolism.
3. Feeding nematodes deprive the host off nutrients and water necessary for plant growth.
4. The conductive tissues are also blocked leading to impairment of organ functioning.
5. Root knot nematodes may directly be responsible for poor germination and thin seedling stand due to pre and post emergence damage.
6. Root knot nematode, reniform may also interfere with rhizobial nodulation in legumes leading to decrease in nodule number, size and therefore nitrogen fixation.
7. Most important are the secondary infections caused by avenues caused due to penetration. The infections of other plant pathogens like bacteria, viruses and fungi are on the rise and further weaken the plant. Nematodes make the plant vulnerable to damage from pathogens which are otherwise weak.
8. Some nematodes are vectors of soil borne pathogenic viruses.

Symptoms of nematode damage to the plants

The symptoms vary with the types of nematodes, plant age, initial population density and ecological factors.

Some symptoms of damage are as follows-

1.     Loss of vigor of individual plants.

2.     Patches of stunted plants.

3.     General decline followed by reduction in crop yield.

4.     Field symptoms are of generalized nature giving an impression that the crop is suffering from shortage of water and fertilizers even though both of these are in adequate amounts.

5.     General chlorosis.

6.     Die-back

7.     Undersized or premature ripening of fruits.

8.     Leaf drop

9.     small sized foliage

10.   Dead buds.

11.   Leaf distortion

12.   Root galls

13.   Root rots

14.   Root lesions

15.   Excessive proliferation of roots.

16.   Devitalized root tips.

17.   Patchy appearance of field - Patches of stunted and unthrifty plants appear in an infested field permitting over-growth of weeds than in the rest of the field. The patches continue to enlarge if the cultivation of the susceptible host plant continues.

18.   Poor plant stand - Plant density and stand is reduced.

19.   Openness of canopy in bushy annuals.

20.   Soil fatigue, exhaustion or sickness and replant failure- This has been known for centuries but has been commonly attributed [sometimes wrongly] to other soil           pathogens; which is infact a complex interaction of nematodes and soil pathogens.

21.   Decline or die-back of fruits is an outstanding example of nematode infection. Predominantly are the slow and spreading declines of citrus, banana, other fruits and vegetables.

22.   Unthrifty appearance - Plants suffering from nematodes often look pale and unthrifty as though affected by deficiencies of water, nitrogen or phosphorus.

23.   Stunted growth of plants is seen in patches with low and poor quality of fruits, cotton, citrus etc.

Nematode infections in banana cause fewer hands per bunch, lower bunch weight, lower no. of rations and toppling of the whole plants.

24. Chlorosis, yellowing, discoloration and drying of the leaves. Such symptoms can be readily observed in coffee seedlings, potato and cherries. Premature yellowing of the leaves is seen in flowering, ornamentals, groundnut, cotton etc. Severely infected plants show drying of the leaf margins, burning of leaf tips and brittling of leaves that are eventually shed-off. The axillary buds develop into tiny leaves only to drop- off. These symptoms are also indicative of poor nutrient absorption due to root damage.

Wilting - Wilting, defoliation and consequent death may be associated with root knots in hot weather in spite of abundant water. Combination of nematodes and fungi are prevalent in wilts of many plants.

Stems and leaf galls.

Abnormalities in leaves like mottling, leaf discoloration like in potash or iron deficiency, scortching, malformation

Abnormalities in reproductive structures like loss of pollen viability, delayed and abnormal flowering.

      Since many of the symptoms are similar to those caused by bacteria, fungi etc. lab examination of soil and root is necessary to make sure of the role of nematodes in the manifestation of disease.

Nematode management -

All the control methods are directed at bring down the population of nematodes in soil, low enough to raise the crop profitably. Usually a combination of the control methods is needed to raise crops economically. Nematodes are omnipresent and can be transmitted through a variety of methods. Irrigation water is the prime source of transfer followed by use of infected composts, soil,

Regulatory methods-

1. Quarantine- The principle involved is of exclusion, which implies excluding the possibility of entry of a pest in a region where it is not known at the time of regulatory operation but is likely to establish in the absence of the same.

This is needed as the golden nematode of potato has spread to 40 countries in hundred years. Countries have their vigilance and insist on phyto-sanitary certificate. But in India, domestic quarantine is not carried out. Burrowing nematode has spread from Kerala and Tamil nadu to Maharashtra, Goa, Gujarat, Orissa, Tripura and Lakshadweep Islands too.  Some vigilane and cooperative efforts by the farmers and the decision makers are necessary to prevent spread.

Cultural methods-

1. Crop rotation This is one of the important methods of keeping nematode population under check especially when there are distinct host preferences like the cyst nematodes of wheat, potatoes. Susceptible crops should be grown once in few years and rotating them with non-host crops. Vegetables should not be grown repeatedly in the same field but should be rotated with cereals in order to reduce infestation with root knot nematodes.
2. Fallowing and ploughing during summer months.- Hot and dry conditions in India are favorable for controlling the nematodes during the summer months. Nematodes are sensitive to heat and drying action of the sun and the wind. Keeping the land fallow during the summer months and deep ploughing the soil two to three times at intervals of 10 to 15 days is an excellent method to reduce root knot nematodes and others.
3. Use of organic amendments.- Soil amendments with green manure, compost, oil-cakes [ of neem, mahua, mustard, groundnut, cotton, linseed and karanj] sawdust etc. has been found to reduce the nematode populations. These cakes incorporated at the rate of 1 to 1.5 tons/ ha has been found to give good control of root knot nematodes and should be practiced. These encourage the growth of natural enemies of nematodes and the decomposition products of these amendments are also toxic to nematodes.
4. Use of resistant and tolerant varieties- This is one of the most practical and economical methods of controlling nematodes. Though there are practical difficulties the development of multi-resistant strains of plants to nematodes and fungi is the best choice. Though lot of work has been done on developing varieties of tobacco, cotton, soybean, cowpea, capsicum, tomato, sweet potato much more needs to be done. Also the main drawback sometimes is the development of resistant breaking nematodes which make the process futile.

Physical Methods

1. Heat treatment of soil - This is a general practice for soil sterilization against all organisms harmful or beneficial. Nurseries use autoclaved soil [ 30 psi for 30 minutes] for potting.  In greenhouses steam is released through perforated pipes running through the soil. Temperatures go up to 82 ^o C for 30minutes. Dry heating of soil by burning off the standing crop waste is also practiced in some parts of India.
2. Hot water treatment of planting material - It is mainly used for transplanted crops, bulbs, rhizomes, root stock and tubers. Effective time and temperatures vary with respect to the plant parts [ 10 to 60  minutes at 43 to 54 ^oC ]. Usually dormant bulbs of ornamental plants and root stocks of citrus are given this treatment. Hot water treatment of rice seed at 52 to 55oC for 10 to 15 minutes is a very effective method to control white tip nematode which is transmitted through seed.
3. Solar drying - Nematode eggs and juveniles are killed if exposed to
4. Summer ploughing-
5. Soil solarization or pasteurization-
6. Oil - Spraying 15% water soluble hydrogenated fish oil also reduces infestation by nematodes. Corn oil, cotton seed oil, ground nut oil and soybean oil reduce the counts of migratory eco-parasitic nematodes.
7. Floatation of seeds- The control of ear cockle or the 'tundu' disease in wheat is done by removal of nematode galls from seed material and floatation.

Besides these, nematodes are also killed by irradiation, osmotic pressure, ultrasonic and electrical heat. But these methods are uneconomical and not practical.

Chemical Control - This is the most effective method but the main drawbacks are the prohibitive costs and the destruction of normal micro-flora and fauna of the soil. Many of the chemicals are toxic and inflammatory requiring specialized use methods. Many of the chemical nematicides like methyl bromide are now banned in various countries.

1. Soil fumigants- They may be applied to the soils as liquids, emulsions or granules but the active ingredient spreads in the soil in the form of a gas. Halogenated hydrocarbons called DD, EDB, MBr, and DBCP are commonly used soil fumigants. For successful results, it is essential that the soil be treated is well prepared to a seed bed condition, soil moisture is moderate and uniform and the soil temperatures are between 15 to 30^oC at the time of application. Soil fumigants are injected to a depth of 20 to 25cm. Applicators of various types are used from hand injectors to tractor drawn machines which can fumigate 2 to 4 hectares per day. DD and EBD are highly phytotoxic and are applied 3 to 4 weeks before planting. DBCP can however can be used with irrigation water around the roots of plants at recommended doses only.
2. Use of insecticides which are also nematicidal - many of the organo-phosphorus derivatives and carbamates also possess nematicidal properties. Phorate, , Fensulfothion, Thionazin, Dichlohenthion, Disulfoton, Diazinon, Fenamiphos Aldicarb, Carbofuran, Methomyl and Vydate have been tested widely all over the world. These are usually applied as granules in furrows. Some of these compounds have not become popular due to high mammalian toxicity and persistence in the soil.

Biological Methods-

Nematodes have their natural enemies in soil like predatory nematodes, fungi, protozoa, viruses, arthropods.

Use of Bio Control agents like fungi, bacteria, nematodes, mites, arthropods etc

A] Use of nematophagus fungi -Parasites of vermiform [ worm like]  stages-Fungi that form traps- Several species of soil-dwelling fungi produce traps to ensnare nematodes before they infect them.  Traps come in many forms: adhesive hyphae, networks, knobs, rings; and constricting rings.  Some trapping fungi can proliferate in soil in the absence of nematodes while others are more dependent on nematodes as a nutrient source for growth.

• Arthrobotrys spp.
• Monacrosporium spp.
• Dactylella spp.
• Dactylaria spp.
• Geniculifera spp.
• Duddingtonia spp
• Nematoctonus spp.

Fungi that form adhesive spores (or conidia) - As they move through soil pores, nematodes may encounter these conidia.   Some of the fungi are host specific and their conidia will adhere to and infect only a few species of nematode.  Although they can be cultured on media, in the soil these fungi are obligate    parasites of nematodes and other invertebrates. Examples are -

• Hirsutella rhossiliensis
• Drechmeria coniospora
• Tolypocladium (=Verticillium) balanoides
• Nematoctonus spp.

Fungi that form zoospores- Zoospores are motile spores that are propelled by one or two flagella.   When  zoospores locate a host, they attach to the nematode cuticle, often near a body opening (mouth, anus, vulva), shed their flagellum, and become sedentary (i.e., encyst).  The encysted zoospores infect the nematode cuticle by forming a penetration tube that enters through an orafice or directly penetrates the nematode cuticle.When the resources within the infected nematode are exhausted, the hyphae differentiate to form sporangia.  Zoospores are produced within the sporangia and enter the soil via an exit or discharge tube which is formed when the spores are mature.   Zoospore-forming fungi are believed to be opportunistic parasites of vermiform nematodes, attacking or colonizing weakened or dead nematodes.  A further limitation of zoospore-forming fungi as biological control agents of nematodes is their requirement for wet soils.  Zoospore movement is favored in large, water-filled soil pores. Example- Catenaria anguillulae

Parasites of Sedentary Stages - There are a large number of fungi that can parasitize nematode eggs, and the sedentary juveniles and females of cyst and root-knot nematodes.  These fungi are grouped into two categories: obligate parasites are those that can grow only in nematodes; and facultative parasites are those that can grow to some extent in nematodes as well as on organic matter in the soil.

Obligate Parasites - Nematophthora gynophila and Catenaria auxiliaris

   Zoospores of both fungi encyst on the cuticles of young female cyst nematodes, penetrate, and consume the internal contents, including the eggs.   Within the nematode body, zoospores develop within sporangia and are released through exit tubes into the soil.  Both N. gynophila and C. auxiliaris form resting spores which are more persistant than the fragile zoospores.

Facultative Parasites-    Many fungi found within cysts and within egg masses of cyst and root-knot nematodes are common soil saprophytes and are only weak parasites of nematodes.  However, some isolates of these fungi are aggressive nematode parasites.  

Examples of fungi that have been identified as aggressive parasites of sedentary stages of cyst or root-knot nematodes.  Not all isolates of these fungi are aggressive parasites. 

Cylindrocarpon destructans, Dactylella oviparasitica , Paecilomyces lilacinus , Verticillium chlamydosporium , Verticillium lamellicola, Verticillium lecanii , Verticillium leptobactrum , Verticillium suchlasporium ,Dilophospora alopecuri

Seed gall nematodes, Anguina spp., are vectors for bacteria Clavibacter (Rathayibacter) spp. and the fungus Dilophospora alopecuri.   These microbes colonise nematode galls and host grass inflorescences.  Both organisms are antagonistic to the nematode, with the fungus having a greater impact on the nematode population than the bacterium.  The bacterium and fungus compete with the nematode for nutrients, oxygen, and space.  Clavibacter toxicus produces a toxin which cause livestock deaths.

Bacteria- Pasteuria spp. are endospore-forming actinomycetes that are parasites of invertebrates, including nematodes.  The bacterium is an obligate parasite and cannot be cultured outside the body of the invertebrate host.  Spores of nematophagous species adhere to the cuticle of host nematodes that encounter them while moving through soil. The spores form a germ tube and penetrate the cuticle of the nematode.  The germ tube then forms vegetative microcolonies of lobed, septate mycelium.  As the infection progress, the microcolonies break up into daughter colonies that contain fewer, but larger vegetative cells.  These large-celled colonies are referred to as quartets and doublets.  Doublets separate and form single sporangia which give rise to single endospores.  See Life Cycle for a diagram of the different stages in root-knot nematodes. These endospores are resistant to environmental extremes and can persist in the soil for several years.

Pasteuria species are very host specific.   Generally, populations of this bacterium are only efficient parasites of the nematode species from which they originated.  There are four described species of Pasteuria and several undescribed species.

• P. ramosa
• P. penetrans
• P. thornei
• P. nishizawae

Some rhizosphere-inhabiting bacteria (rhizobacteria) are antagonistic to plant-parasitic nematodes.  These bacteria inhibit nematode egg hatch and/or penetration of roots.  The mechanism by which antagonistic bacteria inhibit plant-parasitic nematodes is not known.  However, several hypotheses have been put forth:

1. Production of antibiotics that kill nematode eggs.
2. Degradation of the root exudates that the nematode relies on for host location and to stimulate egg hatch.
3. Induction of systemic acquired resistance (SAR).

Streptomyces avermitilus : The toxin avermectin is produced by the actinomycete Streptomyces avermitilus in fermentation. It is active against insects at very low dosages, and formulations containing avermectin are utilized against insects and nematode parasites of animals and on a number of crops. Although activity has been shown against plant parasitic nematodes, it has never been commercially developed for this purpose.

Predators - Nematodes-Some predatory nematodes have large buccal cavities (or mouths) equipped with one or more teeth.  Prey may be swallowed whole or their cuticle pierced and the body contents sucked out.  Other predatory nematodes are equiped with a hollow spear which they use to pierce the cuticle and ingest the body fluids of their prey.The predatious nematode Odontopharynx longicaudata.   Note the large dorsal tooth in the mouth of the nematode. Head of Aporcelaimellus obscurus showing spear-shaped mouth parts.  This nematode preys on nematodes, mites, and enchytraeid eggs.  Seinura sp. feeding on an unidentified nematode.   The predator has pierced the cuticle of its prey with its spear-like mouth parts and is ingesting the internal body fluids. The predacious nematode Mononchoides (Diplogasterida: Neodiplogasteridae). The predacious nematode Prionchulus punctatus (Monochida: Monochidae); lateral view of anterior, showing prominent labial papillae, dorsal tooth, and subventral denticulate ridge.

Mites, The soil mite Lasioseius subteraneus (Mesostigmata: Ascidae), an aggressive predator of nematodes that colonizes greenhouse cultures of root-knot nematodes.  This mite has a generation time of less than 1 week at 28 ºC and can lay up to 18 eggs per day when feeding on nematodes

 Soil Amendments -

·         stimulate the growth of nematophagous fungi

·         improve soil structure, soil water retention, plant nutrition - (reduces stress on nematode infested plants)

·         produce nematicidal breakdown products

Many, perhaps hundreds, of substances have been applied to soil in attempts to minimize problems from plant parasitic nematodes. In some cases the substance has been a waste product (e.g. coffee grounds, newsprint, crab shells, quinoa bran) from a manufacturing process. In other cases, a crop is grown specifically for this function (e.g. marigolds, vetch, sesame). The mode of action of most of these natural substances is unclear (as is the mode of action of many chemical nematicides) and because of the variability within agricultural soils, activity could be expected to vary from one situation to another.

Some substances such as those containing chitin (e.g. crab and shrimp shells) are thought to stimulate the growth of nematophagous fungi which utilize chitin for food. Chitin has been shown to be present in the eggshells of nematodes and this is thought to be the target of the nematophagous fungi.

The addition of organic matter to soils is known to improve soil structure, aid in water retention, and provide nutrients. Improving soil conditions reduces plant stress which in turn can make the stress caused by plant parasitic nematodes less severe or apparent.

As some plant based products (e.g. marigolds, brassicas, sesame) decompose in the soil, they release chemicals thought to be nematicidal. Brassicas, for example, have been shown to release a compound similar to the active ingredient in metam-sodium containing products.

There is abundant evidence in the literature that this group of substances loosely classified as soil amendments, natural products, or organic amendments, can have an affect on nematode populations or stimulate plant growth in spite of the presence of plant parasitic nematodes. There is less evidence that they perform as consistently as traditional chemical nematicides or provide the same degree of nematode control when tested side by side in replicated field trials. In spite of this, there are probably abundant situations in which such products could be utilized in agricultural situations. Because of the variability in agricultural soils, and the suspected mode of action of many soil amendments, it is likely that growers will need to develop different programs for particular crops and sites.

Suppressive soils:

A suppressive soil can be loosely defined as one which should have a nematode problem but doesn't. It is thought that one or more biological organisms in the soil are suppressing nematode populations. There are indications in the literature that crops grown under monoculture for many years in nematode infested fields will have high nematode populations for several years which will then fall and remain at low levels. It is thought that some biological organisms are functioning to maintain nematode populations at a low level. It is not uncommon in chemical control field trials for chemically treated plots to have significantly higher nematode populations at harvest than untreated controls. At times, this has been interpreted as indicating the field is naturally suppressing nematode populations in the controls. In reality, the difference is likely due to the chemically treated plots having healthier root systems which are able to support larger populations than can be supported by the roots of the untreated controls.

Nematodes - how are they transmitted

Listed above are perhaps the most important principles for preventing nematode problems. The chronic nature of nematode problems should indicate that measures to minimize the spread of nematodes should be a high priority. Unfortunately, this is often not the case.

·         quarantine

·         use certified planting materials

·         check suspect materials before planting

·         nematodes may be present in manure

·         clean equipment before moving

·         avoid contaminated irrigation water

There is a general lack of awareness of nematodes among the general public. This can easily result in the unintentional importation of nematodes on ornamental plants. Growers who do not have nematode problems are often unaware of their existence. This can result in unintentional movement of nematodes in soil, irrigation water, on equipment and on planting stock. In spite of the use of methyl bromide for nursery fumigation, nursery personnel may think this is for weed control which they reason can be done by less expensive means. On farms, nurseries, and in government agencies, there is a continual turnover of personnel, and consequently there is a continuing need for education about the importance of prevention in the management of nematodes.

The terminology associated with government run programs is complex and confusing to those not directly involved. Terms such as sanitation, quarantine, control, and certification have specific legal meanings which are not intuitively obvious and are easily misused as they may be in the ensuing discussion. Plants grown within the state for other than farm planting are not protected by the internal programs.

Historically, the nursery nematode control program for planting stock destined for commercial farm planting was mandatory and publicly funded. Recent budgetary problems threatened the elimination of the program. Currently the program is voluntary and funded by nursery assessments, although stock which is not part of the program cannot be sold within the state.

Nematologists have long cautioned that nematodes can be spread in irrigation water Studies demonstrated that irrigation canals transported a dozen different genera of plant parasitic nematodes. They also showed via greenhouse trials that many of these nematodes were viable. Conclusions were that growers applying an acre inch of water were also applying several million parasitic nematodes. One relatively inexpensive method to combat this problem would be to first pump irrigation water into settling ponds, allow nematodes to settle which should only require a few minutes, and then draw water off the top for irrigation. Other potential methods for spreading nematodes which have not been intensively studied include wind , birds, and other animals. It has been demonstrated, for example, that cysts of the sugarbeet cyst nematode can pass through the digestive system of cattle and retain viability. Manure should be properly composted before using so that it will reach temperatures lethal to nematodes. Also keep in mind that the soil beneath a pile of manure may also be infested with nematodes. Don't be greedy, take the manure but leave the soil behind.

The most striking feature of nematode distribution and damage within a field is the irregularity of infested areas. Damaged crops will appear as irregular patches or streaks that may vary in size, shape, and number. These variations usually reflect the compounding of nematode stress on a plant by such other factors as physical soil differences and irrigation and drainage patterns. Previous patterns of cropping, initial introduction, and soil movement through cultivations are also important factors. Consider a field of uniform soil texture, cropping history, and management practices, such as irrigation and drainage facilities, that is under a regular rotation sequence that includes sugarbeets. At a single site within an otherwise uninfested field, sugarbeet cyst nematode is introduced. Cysts containing viable eggs are introduced by contaminated harvesting or cultivation equipment, tractor or truck wheels, human feet, or animal hooves; by irrigation water; or by wind blown in from an adjacent infested area. Following introduction, at a single site or at separate sites within the field, the nematode becomes established during host crop development and, over a period of years, increases population levels. As the population increases, the sites gradually enlarge in area and, when the nematode population reaches a damaging level, the sites show as unthrifty or damaged plant areas within the field. In later years, under short rotations, these areas enlarge further and coalesce until large areas of the field become uniformly infested. The spread of nematodes within the field from these original sites of introduction is mediated largely through the movement of cysts floating on water used in flood or furrow irrigations. Similarly, cattle grazing on beet tops at harvest can distribute infestation by their hooves. In this example, processes related to initial introduction and subsequent spread of infestation are the major determinants of fieldwide distribution.

Self dispersal- The infective migratory stages of sedentary endoparasitic nematodes and the juvenile and adult stages of migratory parasitic and endoparasitic nematodes can migrate in soil up to about 3 feet, depending on the species. Thus, even with several generations, in 1 year the rate of self-dispersal from a point of initial infection would probably be less than about 3 to 6 feet a season.Detection of nematode problems and adoption of appropriate control methods at the proper time is one of the modern methods of increasing the yields of many of our agricultural and horticultural crops. Chemical control of nematodes has not made a headway in India due to their prohibitive costs and difficulty of operation due to less individual farm area. The judicious use of nematicides and the other eco friemdly methods would be economical in commercial crops like grapes, cotton, citrus, vegetables, horticultural crops etc. This is because lack of use of nematicides here would mean no crop or uneconomical plantation.