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

Nematode Pests of Tuber Crops and Management

Potato, sweet potato and yams are the important tuber crops that suffer economic damage due to nematode infestation.

1. Potato
Potato is the fourth most important life sustaining food crop globally, after cereals like rice, wheat and maize. Nematodes are among the most important pest constraint factors which adversely affect the production of potatoes from seed tubers or true potato seed.
Though more than 140 species of nematodes have been reported to be associated with this crop, the major pests of significance at global level are Globodera spp. and Meloidogyne spp. Ditylenchus destructor (Potato rot nematode) though highly damaging, is not present in India.

a. Potato cyst nematode or Golden nematode of potato (Globodera spp.)

• Potato cyst nematodes, Globodera pallida (27 countries) and G. rostochiensis (Golden nematode of potato, 58 countries) comprise one of the most important plant protection problems on global basis.
• The original home for cyst nematodes is considered to be Andes mountains region of Peru in South America from where they spread to Europe.
• Today this nematode has become major potato pest in most of the potato growing areas in the cooler areas of tropical and subtropical zones and temperate regions of the world. The most recent record comes from Indonesia, where potato cyst nematode was reported in 2003.
• In India, the nematode was first reported by F G W Jones in 1961 from a potato field of Ootacamund in Nilgiri hills in Tamil Nadu.It is confined to about 4000 hectares of Nilgiri, Palani and Kodai kanal hills of the southern states due to effective implementation of domestic quarantine as per which the movement of seed potato from infested areas is banned.
• It has recently been reported from Himachal Pradesh also.
• Globodera pallida is more prevalent than G. rostochiensis in India.

Host range

• Potato cyst nematodes are host specific and have a limited host range. In addition to potato, egg plant, tomatoes and a few solanaceous weeds are known to harbour the nematodes, but are not considered as efficient hosts.

Biology :

• Eggs inside the cyst remain viable in soil for a long period of time (up to 8 years).
• They contain the infective second stage juveniles (J2s), which hatch only on getting stimulation by potato root exudates.
• Juveniles become active at 10° C and maximum root invasion takes place at 16° C.
  
• Juveniles penetrate cells near their head to grow and accumulate nutrients; this is called a feeding site or syncytium. Syncytium is formed by the dissolution of cell walls of the cells surrounding the nematode head.
• Male nematodes leave the roots after the final moult, whereas female nematodes moult to become spherical and sedentary adults.
• The mature, enlarged females rupture the root tissue, but remain attached to the root by their heads and protruding necks, which stay inserted in the root tissue.
• The fertilized females become large and sub-spherical and go through a sequence of colour change prior to dying on roots of potato and becoming cysts.
• The life cycle is completed in about 5-7 weeks under Nilgiri conditions where a second generation may also occur.
• When females die, they become cysts, and their cuticles become brown or leathery, and contain as many as 500 eggs.
• Females of G. rostochiensis go through a yellow golden stage, while G. pallida females turn brown directly to become cysts.

Golden cyst nematode on potato Photo by: A D Cushmon, Nemapix

Pathotypes

• In India, out of five known pathotypes of G. rostochienesis only two i.e., Ro 1 and Ro 2 and all the three pathotypes of G. pallida i.e., Pa 1, Pa 2 and Pa 3 are found as mixed populations.

Symptoms

• Symptoms are not evident until appreciable build-up of population in the field. Stunting of plants in nematode infested patches is seen.
• Plants lose their lower leaves soon after sprouting; later shed younger leaves as well. Only a tuft of green leaves remain at the top.
  
• Yellowing of older leaves is followed by new leaves.
  
• Wilting of the foliage during hot sunny hours from which plants recover during cooler part of the day.
• Flowering is sparse or absent. Early senescence and proliferation of lateral roots are often associated with nematode infection. Root system is poorly developed.
• Tuber formation is drastically reduced in count and size.
• Small immature females of white and yellow stages can be observed on the roots at flowering. Females can sometimes be observed on the tuber surface.
  

Healthy and Globodera rostochiensis infested potato plant, Photo by: Christopher Hogger, Nemapix

Management

• Since, the nematode is host specific, rotation with non-host crops like radish, carrot, turnip, cabbage, cauliflower, beet root, strawberry etc. is recommended. However, since potato is a cash crop, farmers normally are reluctant to accept these recommendations.
• Clean planting material is the best way to control potato cyst nematodes and to restrict infestations of new land.
  
• Use of resistant varieties is the best means of managing them as they can reduce potato cyst nematode populations in the field. In India , Kufri Swarna has been released as a resistant variety and this occupies more than 40% area in Nilgiris.
• Strict domestic quarantine barring transport of seed tubers from infested southern hills of the country has confined the nematode to those areas only. However, recently, the nematode has been reported from Shimla hills (H.P.)
• Consumer and environmental concerns are making farmers look at non-chemical alternatives; however, in many areas, chemical control is still a key control measure. A massive chemical control campaign was launched under the Indo-German Nilgiri Development project during 1971-1975. Nematicidal treatment was made mandatory under the Tamil Nadu Pest Act 1971 and all the infested fields were treated with fensulfothion @ 30 kg a.i./ha in the first year and 15 kg a. i. /ha in the next year. Later, cabofuran @ 2 kg a.i./ha was found effective for the economic management of potato cyst nematode under Nilgiri Hills.
• Utilization of these various measures in an integrated management programme can help in keeping the populations below the damage threshold and reduce dissemination, as well as the emergence of new pathotypes.

b. Root knot nematode (Meloidogyne spp.)

Of the 90 species described, only seven have been associated with potato. Of these, only five species are of global importance; most widely distributed being M. incognita, prevalent in the tropics followed by M. javanica and M. arenaria. M. hapla, M. chitwoodi, M. fallax and M. thamesi are found principally in the cooler temperate regions.

Biology

• Both roots and tubers are infected; however, the first generation occurs mainly on the root system, while the succeeding generations attack tubers.
• The second stage juveniles hatch out from the egg masses and infect the young roots wherein they form giant cells through which nematodes extract nourishment from the plant cells. The female juveniles enlarge gradually and undergo four moults to become pear-shaped.
• The male nematodes retain their thread-like appearance and come out freely in the soil from the root system.
• The females are sedentary in nature and deposit about 300-400 eggs into a gelatinous matrix which is usually found adhering to the root galls.
• Depending upon the suitable temperature conditions, five to twelve generations may be completed on the susceptible host.

Symptoms

• Non-specific above-ground symptoms are evident in the form of stunting of infested plants, yellowing of leaves, and tendency to wilt under moisture stress.
• Infected roots have galls or knots of various sizes and shapes. Galling incidence and size are dependent upon nematode density, and the nematode species. M. incognita produces large and distinctive root galls which give a warty appearance or can become deformed on the surface.
• Under favourable environmental conditions, tubers of all sizes can be infected. Pimple-like outgrowths are observed on tuber surface.
• The nematode females are usually found 1–2 mm below the skin. All species produce necrotic spots in the region between tuber surface and the vascular ring.

Fig. 12.1 Bump like pimples on potato tuber due to Meloidogyne chitwoodi, Courtsey:

Management

• Deep ploughing and drying of soil in the summer months desiccates the infective juveniles, thereby reducing initial inoculum in the soil.
• Growing of trap crops like Tagetes patula and T. erecta in between 2 or 3 rows of potato improves the crop performance and also reduces the root-knot formation.
• Seed tubers from root knot nematode infested fields should not be used. The movement of the infested soil and water from the infested fields should be avoided.
• The use of resistant cultivars is probably the most economical means for controlling Meloidogyne species. Advanced clones arising from careful breeding of resistant Solanum sparsipilum show no galling.
• Good control of the nematode has been achieved by applying carbofuran @ 3 kg a.i./ha or ethoprop @10 kg a.i./ha . The efficacy of these pesticides was more when these were applied in two equal split doses i.e., once at planting and other at earthing time.
• Rotation with tomato and sweet potato varieties resistant to M. incognita.

c. Root lesion nematode (Pratylenchus spp.)

• A few important species are P. crenatus, P. thornei, P. scribneri, P. andinus, P. penetrans, P. coffeae and, P. vulnus that damge potatoes in the temperate, tropical and subtropical regions.

Nature of damage

• Damage is often caused by direct feeding usually confined to cortical tissue.
• Nematode survives in infected tubers which act as source of inoculum.

Symptoms

• Patches of poorly grown, less vigorous plants which turn yellow and cease to grow.
• extensive lesion formation on roots
• Infested tubers have small lesions on their surface.

Management

• Hot water treatment of infected tubers at 50°C for 45–60 min reduces nematode spread.

2. Sweet potato

• Sweet potato a native of tropical America.Of all the world’s root and tuber crops, sweet potato is second only to potato in importance. and Meloidogyne incognita and Rotylenchulus reniformis are the important nematodes of sweet potato.

a. Root knot nematode (Meloidogyne spp.)
Meloidogyne incognita is the most important species of the genus attacking sweet potatoes and has a wide global distribuion.
Symptoms

• Infected plants exhibit general symptoms of damage associated with poor root growth, such as yellowing, stunting and the tendency to wilt during the warmer periods of the day.
• Meloidogyne spp. attack both roots and storage roots, causing swellings or galls of different shapes, but galls are not as prominent as on many other crops.
• Pruning effect is observed when the initial nematode population is high.
• They also cause general root tip necrosis in susceptible cultivars.
• Longitudinal cracking during development and swelling of the storage roots occurs.
• Females can be observed on sliced storage roots and are usually associated with brown, necrotic cells around them.

Management

• Crop rotation and intercropping for reducing nematode populations are difficult with Meloidogyne species because of their extensive host range. A crop highly susceptible to root knot nematodes should be avoided in the cropping system.
• Interculturing of antagonistic plants like Crotalaria juncea and C. spectabilis have shown some success against M. incognita and Pratylenchus coffeae) in sweet potato.
• In India, three high yielding cultivars, Sree Vardhini, Sree Nandini and H268, and two short duration cultivars, Sree Rethna and Sree Bhadra, are highly resistant to the local populations of M. incognita.
• Pasteuria penetrans, an obligate bacterial parasite of nematodes, has been used to control M. incognita on sweet potato.
• Hot water treatment of 65 min at 47° C and hot air treatment of 4–8 h at 50° C is effective in eliminating Meloidogyne from root propagative material.
• Chemical dip treatment of the propagation material in a solution of oxamyl or side dressing with nematicides at the time of planting provides early protection against nematodes.
• Since sweet potato cultivation is generally done on a low cash input, the application of chemical control measures is usually cost prohibitive.

b. Reniform nematode (Rotylenchulus reniformis)

Rotylenchulus reniformis is the most predominant nematode on sweet potato in Kerala, India.

Symptoms

• Infestation by R. reniformis may cause cracking of storage roots. The induced cracks are deep and the exposed surfaces are healed over by formation of callus and periderm. The population level necessary for cracking may be very low and is probably less than that for yield reduction.
• Root necrosis occurs and becomes more pronounced as the numbers of the nematode increase.

Management

• Nematicides in the organophosphate and carbamate group also showed good control of nematodes, resulting in improved quality and yields of sweet potatoes.
• Selection P-104 is reported to be resistant to cracking.

3. Yams

• Yams are probably one of the oldest food crops known to man. Yams are normally vegetatively propagated from small whole tubers, portions of tubers (setts) or bulbils. The nematodes of particular importance in yam roots and tubers are endoparasitic forms like Scutellonema bradys, Pratylenchus coffeae, P. sudanensis and Meloidogyne spp. All these are known to cause serious damage by mainly reducing tuber yield and quality.

Scutellonema bradys (The yam nematode)

• The yam nematode, S. bradys, is the cause of a decay of yam tubers known as ‘Dry rot disease of yam’. It is found in many yam-growing areas of the world, which include West Africa, the Caribbean, Brazil, Venezuela, Korea and India.

Host Range
Cowpea, sesame, green gram, pigeon pea, okra, tomato and melon are susceptible to S. bradys.

Nature of damage

• Scutellonema bradys is a migratory endoparasite present in yam soils, roots and tubers.
• All active stages are infective.
• It invades the young, developing tubers through the tissues of the tuber growing point, alongside emerging roots and shoots, through roots and also through cracks or damaged areas in the tuber skin.
• Nematodes feed intra-cellularly in tuber tissues, resulting in rupture of cell walls, loss of cell contents and the formation of cavities.
• They are mainly confined to the subdermal, peridermal and underlying parenchymatous tissues in the outer 1–2 cm of tuber.
• S. bradys continues to feed and reproduce in yams stored after harvesting.

Symptoms

• Dry rot of yams occurs in the outer 1–2 cm of tubers.
• The initial stage of dry rot consists of cream and light yellow lesions below the outer skin of the tuber showing no external symptoms at this stage.
• As the disease progresses, it spreads into the tuber, normally to a maximum depth of 2 cm but sometimes deeper. In these later stages of dry rot, infected tissues first become light brown and then turn dark brown to black.
• External cracks appear in the skin of the tubers and parts can flake off exposing patches of dark brown, dry rot tissues.
• The most severe symptoms of dry rot are seen in mature tubers especially during storage, when it is often associated with general decay of tubers.

Fig. 12.2 Scutellonema bradys infected and healthy yam tubers, Courtsey: flickr.com

Survival and dissemination

• Infested seed tubers are the main source of nematode inoculum in yam ?elds.

Management

• The use of nematode-free propagative material is by far the most appropriate means of preventing nematode damage. The presence of dry rot in tubers without external symptoms can be determined by scraping away sections of tuber skin, or by the use of tuber pieces rather than whole tubers, enabling the grower to examine for dry rot symptoms before planting.
• Crops susceptible to S. bradys should be avoided. Crops such as Tagetes spp. and groundnut (peanut) have been recommended for use to lower nematode populations and restore fertility for yam production.
• Mulching and exclusion of weed hosts has also been reported to reduce nematode populations.
  
• Hot water treatment of 50–55° C for up to 40 min can reduce or eliminate S. bradys from tubers.
  
• Complete resistance to S. bradys has not been found and all the main food yams (D. alata, D. bulbifera, D. cayenensis, D. esculenta and D. rotundata) are susceptible to damage by S. bradys.
  
• Chemical control of S. bradys on yams has some a success. Granular nematicides like oxamyl, carbofuran or isazophos applied as post-plant treatments in yam mounds two weeks after planting @ 2 kg a.i./ha reduce soil populations of S. bradys with remarkable yield increase.