Phytonematode, the farmers’ hidden enemy is causing direct and indirect crop losses. Infection of nematode invites soil-borne plant pathogens to get easy access. In India, nearly a dozen of economically nematode species attack several fields and horticultural crops and the crops suffer economical yield losses. The amount of loss in monetary term estimated to be about Rs. 10,204 crores every year. The vegetable crops are most severely damaged particularly in light soils of hot and humid parts of the country. Nowadays, growing crops in polyhouse conditions has become difficult. Playhouse provides congenial conditions to root-knot nematodes for infection, damage and multiplication on continuously grown susceptible crops. Management of nematodes in open-field as well as protected system has been mostly suggested through non-aggressive approaches. Novel approaches like targeted gene silencing, molecular transfer of resistant genes against economically important nematodes, testing molecules of novel chemistry etc. are frontier areas of research.   

Figure  A: Root-knot nematode infected a) bottle gourd

Phytonematode is also known as the hidden enemy of crops, they attack various crops and infestation of crops may cause huge crop losses. Nematodes are microscopic in size (from 0.3 to 10 mm length), invisible to the naked eye and mostly live in the subterranean habitat. The stylet bearing phytonematodes penetrate and feed on the root of growing plants, stealing nutrients vital for plant growth and exposing roots to the attack of other soil pathogens. Nematodes as abiotic constraint have widely been recognized for influencing crop proclivity. Nematode as a crop pest has achieved the status of most devastating pests group responsible for insidious disease symptoms in different crops. The above-ground disease symptoms are often overlooked or confused with nutritional deficiency and other disease symptoms. Among the phytonematodes, root-knot nematodes (Meloidogyne spp.) produce a typical root-knot or root gall symptoms on various crops like bottle gourd (Figure 1a), pointed gourd (Figure 1b), rice (Figure 1c) etc. In addition to root galling, the quality of the harvested produce is also affected, for example potato tuber affected by root-knot nematode (Figure 1d) shows pimples or surface eruptions which render low market price. Similarly, foliar parasite (Aphelenchoides besseyi) of tuberose cause distortion of flower stalk and the affected stalk (Figure 1e) becomes unmarketable. 

Figure Pointed Gourd

Estimation of crop losses due to phytonematodes is relatively difficult because nematode can cause quantitative as wells as qualitative crop losses. On a global scale, annual crop yield losses due to phytonematodes in major 40 crops has been estimated to the extent of 12.3 %, amounting USD 170 billion and the magnitude of crop loss is much higher (ca.14%) in the developing countries. In India, national losses in 30 major field and horticultural crops have also been estimated at Rs. 10204 crores annually. This could be much higher due to escalation of price of produce in the present time.  The yield losses in different crops in India have been estimated; 12 to 52% in cereal crops (rice, wheat, maize), 13 to 68% in pulses (chickpea, mung, frenchbean, blackgram, clusterbean, cowpea), 13 to 15% in oilseeds (ground nut, castor), 19 to 30% in fibre crops (jute, cotton), 14 to 51%  in tobacco, 17 to 50% in cucurbits (cucumber, pointed gourd, bottle gourd), 10 to 42% in solanaceous crops (tomato, brinjal, chilli, potato, capsicum), 34 to 42% in root crops (carrot, beet root), 15 to 43% in fruit crops (banana, citrus, guava, pomegranate, grape),  24 to 33% in spices (black pepper, ginger) and 29% in polyhouse crops ( tomato, cucumber).



Figure Rice

Besides direct damage, nematode serves as a predisposing factor in the development of disease complexes with soil borne-fungi, bacteria, and viruses. In many situations, crop varieties resistant to fungi and bacteria become susceptible in the presence of nematodes. It is beyond doubt, infestation of nematodes either alone or in combination with other pathogens contribute to reducing crop productivity. Intensive and extensive cultivation of crops particularly in irrigated crop production system aggravate nematode problems in many crops. The hidden nature of nematode can cause crop damage out of sight of farmers, scientists and extension officers. Except few, phytonematodes induce non-specific disease symptoms in the above-ground parts of the crops. Therefore, nematode as a crop pest is given so little attention while dealing with field problems of crops.

The most economically important nematode-pests are root-knot (Meloidogyne), cyst (Heterodera and Globodera), seed gall (Anguina tritici), citrus (Tylenchulus semipenetrans), burrowing (Radopholus similis) and lesion nematodes (Pratylenchus spp.). Almost all crops are affected by nematodes. The vegetable crops suffer relatively more damage due to an infestation of nematode of a mode of parasitism (endo/semi-endo and ectoparasites). Among the sedentary endoparasites,    root-knot (Meloidogyne spp.) and cyst nematodes (Heterodera/Globodera spp.), lesion nematode (Pratylenchus spp.) among migratory endoparasites and reniform nematodes (Rotylenchulus reniformis) of semi-endoparasites are most damaging to vegetable crops. Several ectoparasitic nematodes viz. sting (Belonolaimus), awl (Dolichodorus), stunt (Tylenchorhynchus), spiral (Helicotylenchus), stubby root (Trichodorus/Paratrichodorus), ring (Criconemoides), dagger (Xiphinema), lance (Hoplolaimus), needle (Longidorus) etc. are frequently encountered in vegetable crop rhizosphere where they live on feeding surface or subsurface of roots. Among the ectoparasitic nematodes, Tylenchorhynchus brassicae on cabbage, cauliflower, and Paratrichodorus allius on onion are potential yield reducer.

Figure Potato

The polyhouse crops (tomato, cucumber, gerbera, capsicum etc.) are seriously damaged by root knot nematodes (Meloidogyne spp.). Nematode problems in protected cultivation are increasingly become a threat for profitable cultivation.  Whereas farmers are not much aware of nematode problems in open-field cultivation, the dimension of nematode damage in polyhouse conditions created impact on polyhouse growers. This tiny but smart enemy of crops in polyhouse conditions can cause havoc damage to crops and the mere presence of nematode inside polyhouse can reduce the farmer’s profit. The congenial conditions like high temperature, moisture, monocropping with susceptible crops in the contained environment of polyhouse favour nematode to grow and multiply at alarming rate. The population level in the soil goes up many times within a very short span and the standing crop suffers heavy damage. Further the injudicious use of toxic chemicals depletes natural enemies both in soil as well as protected environment. Nematode in polyhouse cannot be completed eradicated once their presence is noticed. Therefore, before establishment of the polyhouse, soil analysis for nematodes is an essential requirement. Very recently, Haryana Government has made it mandatory for the beneficiaries of subsidy for construction of structure in the soil free from pathogenic nematode species. Some good practices like raising nematode free seedlings of polyhouse crops, destruction of previous residues, soil solarization during summer months,   application of organic amendments, farm yard manure enriched biocontrol agent like Purpureocillium lilacinum (a nematode egg parasitic fungus), crop rotation with antagonist crop (marigold) or poor host crops, popular chemical like carbofuran (@ 2kg a.i./ha as basal application) and new chemicals like fluopyram and fluensulfone (both under Government consideration for registration) are useful for reducing nematode damage. Overall hygiene in the polyhouse and around the structure is essential for unwanted entry of nematode inside polyhouse.       

Management of phytonematodes is difficult with the application of nematicides. Moreover, the most effective chemicals have been withdrawn from world market due to their harmful effects on environment. In fact, nematodes are comparatively hardy animals require high doses of insecticide having nematicidal property. With the increasing concern on environment, various alternative pest control methods like cultural, physical, biological and botanical control methods are suggested to reduce the nematode damage of crops. However, judicious use of chemical nematicides could be applied for protection of many crops. Integration of various available practices is one of the current approaches for managing pest problems of crops. Cultural practices are known from time immemorial as a multiple pest control strategy. Biopesticides of botanical origin have also been proved as effective alternative of nematicides. Among the effective nematode management tactics, resistant variety, cultural, biological, botanical, organic amendments and chemicals are nowadays available for integration and adoption for location specific nematode problems. Nematode management in crops includes reduction of initial population densities using low cost inputs, suppression of nematode reproduction and preventing crop damage. In India, out of the twelve most economically important nematode pests, only four nematodes viz., Heterodera avenae, Globodera spp., Meloidogyne spp. and Rotylenchulus reniformis have been worked out for integrated nematode management where only combination of treatments have been evaluated. Therefore, current options for nematode management are cultural practices, physical methods, biointensive nematode suppression, botanicals and sensible use of chemical nematicides.



Figure Foliar Nematode Infected Tuberose cv Calcutta Double

Nematode attributes to field problems of crops and constitutes a potential biotic stress for low productivity of crops. The dimension of nematode damage in different field and horticultural crops is increasing due to change in agricultural practices under the climate change scenario. At present there is no nematicide available in the market for the control of nematode pest of crops. Alternative to chemicals,  non-chemical approaches like good agricultural practices, nematode suppressive crop sequence, botanicals (neem cake, castor cake, karanj cake etc.), organic amendments with decomposed farm yard manure, biofumigation (incorporation of brassica and some non-brassica crops like marigold, sorghum etc.), and fortified biocontrol agent (Purpureocillium lilacinum, a fungal egg parasite of nematodes) are viable options for nematode management. Further investigations on plant-nematode interaction, targeted gene-silencing, evaluation of crop genotypes resistant to major phytonematode species and testing new molecule for management of nematodes are a priority and contextual areas of research.

Matiyar Rahaman Khan - Principal Scientist of Division Nematology
ICAR-Indian Agricultural Research Institute
New Delhi-110012, India
email: drmrkhanbckv@gmail.com 
Mobile No. 9540829296  

&

Ajoy Kumar Ganguly - Former Head, Division of Nematology
ICAR-Indian Agricultural Research Institute
New Delhi-110012, India
email: akganguly1950@gmail.com
Mobile No. 9968254462

 



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