Nematode Pests of Horticultural Crops and their Management (1+1)

Plantation Crops

PLANTATION CROPS
I. COCONUT
The coconut palm has its origin in South East Asia and is cultivated in the warm damp regions of the tropical shores. The largest producers of coconut are the Philippines, Indonesia, India and Sri Lanka. Of the several nematode species reported on coconut, the most important are Radopholus similis and Bursaphelenchus cocophilus (=Rhadinaphelenchus cocophilus).
A. Radopholus similis
Distribution:

• This nematode has a wide distribution in most tropical and sub-tropical areas of the world including Florida (U.S.A.), Jamaica, Sri Lanka and India. In South India 24 per cent incidence of R. similis has been recorded in coconut.

Hosts:

• The isolate of R. similis found in coconut in India belongs to banana race and has a wide host range which includes many weeds, crops, fruit trees, and many palms. Increased incidence of R. similis has been observed in coconut nurseries where banana was used as a shade crop. More than 50% of the seedlings raised in such nurseries failed to establish when transplanted to the main field.

Biology:

• R. similis is a migratory endoparasite and is capable of spending its entire life within roots.
• All juvenile stages and adult females are infective. Males are free living.
• One life cycle is completed within 25 days at a temperature range of 25-28^o C.
• Nematode populations survive under field conditions for six months in moist soil (27 to 36^oC)and one month in dry soil (29 to 39°C). The nematode survives in roots of stumps of felled coconut palms up to six months.
• Females are the most persistent form in coconut roots and soil during summer months and cause annual recurrence of infection.

Nature of Damage:

• Burrowing nematode penetrates the cortical region just below the epidermis behind the root cap by lysis of cells.
  
• The cavities formed in the outer cortex are always surrounded by deeply stained and heavily suberised cells of irregular shape. Maximum number of nematodes and cavities are seen in the outer cortex.
• Consequent to nematode multiplication and lysis of cytoplasm and cell walls, adjacent cavities merge with one another, thus destroying the cortex to a great extent. All stages of the nematode are present in these cavities.

Symptoms:

• Infected coconut palms exhibit general non-specific decline symptoms like yellowing, button shedding, reduction in leaf size and yield etc.
• Root symptoms are more specific in the form of elongated orange coloured lesions on tender and semi-hard roots.
• As parasitization further increases, these lesions enlarge and coalesce to cause extensive rotting of roots. Lesions are not visible on the old, hard, dark brown roots.
• Cracks appear on such damaged roots.
• Tender roots on heavy infestation become spongy in texture.
• Plant growth is restricted due to drastic reduction in the number and mass of tertiary feeder roots.

Management:

• The cultural practices like application of oil-cake, farmyard manure and growing of sun hemp in the basins and inter-spaces, and their incorporation as green manure may help in the inhibition of nematode multiplication.
• Nematode free planting material is the key to successful coconut cultivation and this can be achieved by treating nursery with nematicides.
• The dwarf cultivars like Kenthali and Klappawangi and hybrids like Java Giant x Kulasekharam Dwarf yellow, Java x Malayan Dwarf yellow and San ramon x Gangabondam were found to be less susceptible/ tolerant as they recorded the least nematode multiplication and lesion indices.
• Soil application of phenamiphos or phorate @ 25 kg per ha during September, December and May in infested coconut nurseries completely eliminated R. similis.
• Application of phorate@ 10 g a.i. per palm in June-July and again in October-November increased 30% yield of R. similis infested palms.
• Integrated management is the best approach in which tolerant hybrids should be used. Application of cow dung, farmyard manure, oil cakes and green manure in the basins followed by application of phorate @ 10 g a.i. per palm in June-July and October-November can be used successfully. Crotolaria juncea may be cultivated in the basin and inter-spaces and used as green manure. Growing of banana as shade crop should be avoided in coconut nurseries.

B. Bursaphelenchus cocophilus(= Rhadinaphelenchus cocophilus) (Red Ring Nematode)

C.O. Red Ring Disease of coconut and Little leaf disease of coconut and oil palm in Surinam and Guyana
Distribution:

• This nematode is widely distributed in number of coconut growing of the globe but so far has not been reported from India.

Hosts:

• The primary hosts of B. cocophilus are coconut and oil palm, but date palm and cabbage palm also suffer infection.

Biology:

• This nematodes completes one life cycle in very short span of 9 to 10 days.
• B. cocophilus could survive for a maximum of 15 days in soil.
• The third stage juvenile is the most persistent form.
• Fragments of diseased tissue from contaminated bodies of the palm weevil Rhynchophorus palmarumare are deposited into the bases of leaf axils of healthy plants. The nematode is carried by weevil larvae, pupae and adults both internally and externally. The nematode persists without change or multiplication through the metamorphosis of the insect. On emergence of the adult insect, large numbers of nematodes located in the region of the ovipositor of the female are injected into the soft tissue of the coconut palm when the insect deposits eggs.

Nature of Damage:
In coconut roots, the nematode attacks cortical tissues and occludes xylem vessels, thus impairing the water intake.
Palms of bearing age (5-7 years) are more susceptible to infestation.
Symptoms:

• Yellowing of the lower leaves starting at the tips of distal pinnae which progresses towards the base of the pinnae and leaves succeeded by browning and later by death of the leaf.
• The inner leaves are also affected and the affected trees die within 3-4 months of the appearance of the primary symptoms.
• Internal symptom of a red-ring nematode infested palm is characteristic. At first, about 2.5 cm beneath the stem surface scattered reddish dots of about one mm diameter are seen which later coalesce to form an orange-red ring about 3 cm in width. Hence the name red-ring disease and red-ring nematode. The ring extends the whole length of the stem and roots and in petioles it assumes a crescent-like shape.
• Large number of juveniles is present in the centre of the discoloured areas and adults prefer the peripheral region. Shedding of green nuts of all sizes takes place.
• In advanced stages of infection putrefaction of the soft tissue takes place.

Control:

• The leaf axils of diseased palms should be sprayed with 0.1%Lannate (Methomyl) insecticide suspension when early symptoms appear. Then the palm should be poisoned by adding one tablespoon of weedicide "100" in three holes (2cm diameter and 10 cm deep) bored around the trunk of the tree at a level of 15 cm above the soil.
• In advanced stage the palms should be cut down and the pieces and stumps sprayed with at least 4.5 1 of 0.1%Lannate suspension.
• Guard baskets made of 2 cm mesh wire are used to protect frequent outbreaks of the disease. These baskets are filled with fresh infected tissue and sprayed with 0.1% Lannate suspension. The palm weevils are attracted to the tissues in the basket. After two weeks the tissue in the basket is burnt. One guard basket is used per 0.4 ha of palms.
• The vector weevil is found to be parasitized by several species of entomopathogenic nematodes. these nematodes can be used to put selective pressure against the vector weevils.
  
• Even though no effective method of control is known at the moment, the following methods are being adopted:
• Phyto-sanitation by using arsenic preparations in diseased trees
• Filling of leaf axil with a granular nematicide
• Control of the palm weevil by applying insecticide
• Use of resistant varieties with short internodes
• Plant quarantine measures to check its spread to newer areas

T. S. of coconut trunk showing red ring and Drying coconut palms due to infestation of Bursaphelenchus cocophilus

II. ARECANUT
Arecanut has its origin in the hot damp regions of Asia and the Malay Islands. India is the largest producer of arecanut in the world. Though plant parasitic nematodes of more than 22 genera have so far been reported from arecanut, only Radopholus similis is widely distributed and with proven pathogenicity.
Nature of Damage:
These migratory endoparasitc nematodes penetrate through the cells near root cap and move in inter and intra-cellular positions in the cortex causing extensive lesions.
Symptoms:

• Non-specific above ground symptoms like general yellowing and visible reduction in growth, vigour and yield.
• Root symptoms are conspicuous in the form of small, elongate, orange coloured lesions on young, succulent roots. Later the adjoining lesions coalesce and cause extensive root rotting.
• The thick primary roots produced from the bole region of the palm exhibit large, oval, sunken, dark lesions. Unlike in coconut, the tips of lateral and tertiary roots on infestation become black.

Control:

• Tolerant cultivars like Indonesia-6 (VTL-ll), Mahuva Band Andaman - 5 (VTL-2ge) or highly resistant hybrids like VTLII x VTL17 should be used in R. similis infested areas.
• Application of aldicarb or fensulfothion @ 1 g a.i. per seedling thrice a year for three years controlled R. similis population at the seedling stage.
• Aldicarb@ 10g a.i. per palm, DBCP@ 10ml a.i. per palm or fensulfothion 50g a.i. per palm reduced nematode population and increased the yield 10 fold in adult palms.
• Integrated management of the burrowing nematode infesting arecanut roots can be done by using nematode-free planting materials of arecanut and avoidance of R. similis susceptible inter/mixed crops like black pepper and banana in infested areas.
• Phorate @ 3g a.i.should be applied to the root zones of banana, black pepper and arecanut in arecanut-based farming system.

III. COFFEE

Coffee is one of the most important crop of all the four states of South India. In addition, it is also grown in north eastern states, Orissa, Maharashtra and Madhya Pradesh. Thirteen species belonging to nine genera of nematodes have been found to be associated with coffee. Several Meloidogyne spp. (M. exigua, M. incognita, M. coffeicola, M. arenaria, M. javanica etc.) have been reported from coffee plants but the pest of utmost significance is Pratylenchus coffeae. Of the two commonly grown species, Coffea arabica and C. robusta, former is highly susceptible to nematodes while the latter is either tolerant or resistant to nematode attack.
Pratylenchus coffeae (Coffee root lesion nematode)
Distribution:

• It is one of the highly destructive pests of economic significance in Coffea arabica in all coffee growing nations of the globe involving continents of Asia, Africa and America. In India, it is prevalent in Karnataka, Kerala and Tamil Nadu.

Nature of Damage:

• Nematodes feed as migratory endoparasites and destroy the parenchymatous cortical cells of the roots resulting in the death of young feeder roots.
• The capacity of the roots to absorb water and nutrients is badly impaired.
• Both young and bearing plants are affected.

Symptoms:

• Infected plants give unhealthy appearance as they bear lean and thin stem. Affected plants do not bear sufficient foliage to bear the crop.
• Older leaves become yellow and drop leaving very few undersized, crinkled and chlorotic leaves at the tip of the main stem.
• The branches have shorter internodes, sparse flower buds and poor setting, and beans produced are small sized and unfilled beans.
• Even bearing plants have a lanky stem. Plants become prematurely old and require collar pruning (rejuvenation) or uprooting. Even collar pruned plants either do not produce new suckers and even if some suckers are produced, they are unhealthy with reduced, crinkled and chlorotic leaves.
• Affected plants thus have a poor anchorage and can be uprooted easily.

Damaged caused by pratylenchus coffeae on coffee plants, Photo courtesy: R. Lopez

Control:

• Preplant deep summer ploughing and fallowing of the site for at least one summer season is advisable.
• Preplant soil treatment of coffee nursery with D-D followed by plastic sheet covering or Dazomet application effectively controls P. coffeae.
• Maintaining heavy shade, application of FYM, foliar feeding of nutrients and mulching individual plants help to improve the vigour of the plant.
• Change of nursery site every 6-7 years and use of well dried and sieved jungle soil and FYM for the fresh nursery is recommended. Fenamiphos (5% granules) when applied @ 50 mg per plant controls the nematode for 90 days but application is to be repeated after this period. Weeds should not be allowed to grow on the plantation site.
• Affected plants should be pulled out and burnt.
• Grafting coffee cultivars susceptible to the nematode (Coffea arabica) on resistant root stocks (Coffea canephora var. robusta) has also been successful in the control of P. coffeae.