How to Control Nematode: Chemical, Physical and Biological Control? Many accounts have been published of the attempts to control plant nematodes in different countries and climates.
These are discussed in brief here:
Chemical Control:
Chemicals with nematocidal properties are known as nematocides. Many such chemicals are recommended for nematode control. Nematocides are diverse in their chemical and biological activities and in their behaviour in soil.
There are two general types of nematocides – fumigants and non-fumigants. Chemically they can be categorized as halogenated aliphatic hydrocarbons, methyl isothiocyanate liberators, organophosphates and organocarbamates. The common pesticides of nematodes are given in Table.
Nematocides (or nematicides) have been discussed in detail by Allen (1960). The types of nematicides, their biological action in soil and the ecological repercussions of nematocides have been recently reviewed by Van Gundy and McKenry (1977).
Kuhn (1881) attempted to control the sugarbeet nematode Heterodera schachtii Schmidt with a number of chemicals and concluded that the most promising was carbon disulphide. However, the results produced by this chemical were not entirely satisfactory and it also proved very costly.
It is apparent that until 1940 numerous chemicals were investigated as possible nematocides for the control of nematodes. Chloropicrin appears to have been the only satisfactory chemical tested prior to 1940. There are a large number of nematocides available today.
Nematocides used as soil fumigants are available as liquids, emulsifiable concentrates or granules. Application of nematocides in the soil is made either by applying the chemical evenly over the entire field (broadcast) or by applying it only in the rows to be planted with the crop (row treatment).
Highly volatile nematocides should be immediately covered with polyethylene sheeting and should be left in place for at least 48 hours. The most convenient method of fumigating small areas is to inject the chemical with a hand applicator or to place small amounts of granules into holes 15 cm deep and 15-30 cm apart, and immediately cover the holes with soil.
In all cases of pre-plant soil fumigation with phytotoxic nematocides, at least two weeks must elapse from the time of treatment before seeding or planting in the field to avoid plant injury.
More intensive research is called for to determine situations where nematicidal application is necessary and worthwhile, methods to reduce the cost of treatment and the economics of chemical control. Increasing attention has to be devoted to the systemic nematicides, their efficacy and suitability under our conditions.
Treatment of nursery beds, bare root dip of transplants and seed treatment with some of these chemicals offer possibilities of obviating field applications, thus reducing the cost.
Fumigants:
1. Chloropicrin (Trichloronitromethane):
Chloropicrin was first reported by Mathews (1919) to be an effective nematocidal chemical for use in the soil. Johnson and Godfrey (1932) reported excellent control of root knot nematode in pine apple soils.
This chemical is now widely recognized for its nematicidal, fungicidal and herbicidal properties. Chloropicrin has a fairly high vapour pressure and it is most effective when it is confined to the soil by covering the soil with covers, such as traps that are impervious to it. Due to its high cost, its use has been limited in the control of nematodes.
2. Methyl Bromide:
Methyl bromide has been exported to be an effective nematocide. The chemical has been used restrictively as a nematocide except for special crops of high value. Methyl bromide has a high vapour pressure and a low boiling point. It is highly toxic to man and extreme care must be exercised to avoid breathing the vapours.
3. D-D Mixture:
Carter (1943) demonstrated the nematocidal properties of 1, 3-dichloropropene-1, 2- dichloropropane mixture (D-D mixture). Unlike chloropicrin and methyl bromide, D-D mixture, because of its lower vapour pressure, does not require impervious covers or water seals.
4. Ethylene Dibromide (1, 2- Dibromoethane):
In 1945 Christie reported that ethylene d bromide had given excellent control of root knot nematode in the soil. D B C P (1, 2 dibromo-3-Chloropropane) was reported by McBeth and Bergeson (1955) to be anematicide. Allen et al. (1955) reported the chemical to be an effective nematocide.
Non-Fumigant Chemicals:
The search for new systemic insecticides led to the development of a series of organophosphate and organocarbamate chemicals which are potential chemotherapeutants for nematode control.
These are generally not phytotoxic at concentrations used for field control. A major drawback with them is that they are highly toxic to mammals.
These chemicals are dispersed in the soil by water. Nematicidal action is limited to the narrow root zone. Nematicidal activity is usually due to narcotization and behaviour modification rather than the killing of the nematodes. Interference in the nematode infection, development and reproduction can temporarily slow or halt an increase in the number of nematodes.
Chemical control of nematodes has not made much headway in India mainly due to the prohibitive cost of the nematicides, difficulty of application over large areas and the lack of awareness of the nematode problem.
In spite of high initial costs of nematocides, there is increasing evidence that a judicious use of these chemicals would be economical on selected crops, such as cotton, sugarcane, citrus, grape vine, potato, vegetables and even wheat and barley in heavily infested fields.
Cultural Practices:
They result in partial or complete control of nematodes and include:
Crop Rotation of Non-Host Plants:
Several nematode species can infect only a few crops. Since all plant pathogenic nematodes are obligate, the absence of susceptible hosts from the soil for two to three years will result in the elimination of nematodes from that area through starvation and inability to reproduce. The value of crop rotation has long been recognized in the control of Heterodera spp.
Thus, continuous cropping with potatoes increases the cyst population of B. rostochiensis more than does three-year rotations with corn, green beans, red clover or perennial ryegrass. Brown (1961) gives some idea of the effect of crop rotation on the yields of potatoes growing in soil infested with H. rostochiensis.
Potato yield after three years of continuous cropping was 1631 Kg/hectare, and with a non- host in the second year, it was 4,515 Kg/hectare. The role of cropping systems in the management of the nematode population has been reviewed by Nusbaum and Ferris (1973).
In the early days of the sugarbeet industry in Great Britain, beet was grown without rotation. The first control measure against Heterodera schachtii was the introduction of a clause into contracts forbidding the growing of beet after beet or marigolds.
Crittenden (1956) states that the best results for controlling Meloidogyne incognita were obtained by growing non-host crops for two consecutive years followed by a host crop. Similarly, infestations of Pratylenchus leiocephalus can be reduced by growing a non-host crop of peanuts in rotation with maize.
The influence of crop rotations on population of several species is still imperfectly understood. With some nematodes, for example – Trichodorus spp., crop rotation may even be impracticable because of their very wide host range. Rohde and Jenkins (1957) could only find Jimson-weed, asparagus, poinsettia and crotolaria to be non-hosts of these species.
Some plants appear to be antagonistic to nematodes because they reduce populations to a greater extent than do non-hosts. Crotolaria is a good example of this. It has been suggested that the Crotolaria root system has a toxic effect on nematodes, and planted in advance of a crop, it may reduce subsequent damage.
There is evidence from Dutch nematologists that Tagetes spp. may reduce the populations of some plant parasitic nematodes. Oostenbrink and his colleagues (1957) tested 16 varieties of Tagetes patula and Tagetes erecta placed between the rows or around other plants.
They found that all varieties suppressed the populations of Pratylenchus in the roots of other plants as well as in the soil, resulting in a better growth of perennials in the second year of the succeeding crop. The Tagetes had to be grown for three to four months and it was suggested that the roots had a nerllatiddal action.
In spite of many criticisms, the use of cover, trap or enemy crops provides a potentially useful control measure in crop rotations. Such an opinion is supported by the evidence that Asparagus exudes a nematicidal chemical from its roots.
Trap crops may be useful when conventional crop rotations have apparently failed to control a nematode attack, for example, Heterodera gdttingiana. Nevertheless, Shepherd (1962), who has reviewed the subject, maintains that there has been little success in the control of Heterodera spp.
Although crop rotation is at present probably the most successful control measure, it is by no means completely effective.
They include the cleaning of all machinery thoroughly before moving into an uncontaminated area, taking care not to bring nematodes into a field by means of contaminated nursery stock, seed, containers, etc., keeping soil follow free from host plants which deprive nematodes of roots on which to feed.
Some species of nematodes have become adapted to living in the normal soil with ordinary moisture content and depend on a certain amount of aeration present in it. Flooding the land for a period of several months results in the death of these nematodes and thus frees the land of these pathogens. However, it is possible or practical to flood the land for long periods in very few fields and, therefore, the applicability of this control method is quite limited.
Although simple observations have shown that predaceous species of nematodes occur in the soil and can be utilized to eat away the parasitic forms, so far very little work along these lines has been done.
Nevertheless, it has been found that many nematophagous species of fungi are effective means of destroying the nematodes and these can be encouraged in the soil through organic soil amendments, such as the addition of decomposable vegetable matter.
Physical Methods:
Plant parasitic nematodes can be easily killed in the laboratory by the application of heat, irradiation, osmotic pressure, etc. It is more difficult, however, to employ such methods on large quantities of soil, especially if they are under cultivation.
There is strong evidence that nematodes are killed or seriously affected by irradiation. Fassuliotis and Sparrow (1955) demonstrated that irradiation of potato tubers with X-rays inhibited sprouting and Heterodera rostochiensis.
Cysts of this developing nematode exposed to 20,000 roentgens contained only brown and dead eggs, at 40,000r, the eggs lost their contents completely. It is, however, difficult to irradiate nematodes. In fact it is, impracticable because of the length of time needed to irradiate even small areas and because damage to plant roots occurs at levels of irradiation below those required to disrupt the nematodes life cycle.
Heat:
Heat treatment is probably the most successful physical control measure developed so far. It is widely used for the killing of nematodes within plant tissues before planting and has proved useful on nematode infested bulbs and tubers, and roots of plants such as chrysanthemums, strawberries, bananas and citrus.
This process involves determining the correct time and temperature for killing the nematode but not the host. This entails immersing the nematodes (contained in as small a volume of water as possible) into thermostatically controlled water baths at different tempt ratures for different lengths of time.
After such treatment the nematodes are examined for activity. These are stained to see if they are dead or introduced into soil containing host plants to assess their ability to invade, develop and reproduce. Blake (1961) suggested that banana sets less than 13 mm in diameter should be immersed in water at 55°C for 20 minutes in order to kill Radopholus similis.
A new approach to heat treatment is the application of dry heat. Sweet potato seed roots were treated at 45 °C for 30 hours and this completely controlled root knot nematodes, with 60 per cent of the roots surviving.
The white tip nematode of rice Aphelenchoides besseyi is of widespread occurrence in India in most of the rice growing areas. Hot water treatment of the seed at 52-55 °C for ten minutes is the best way to ensure the complete destruction of the nematodes.
More than 100 fungus species have been designated as nematophagous, a term that they have applied to fungi that “feed” on nematodes either as predators or parasites. These nematophagous fungi, members of either the phycomycetes or the class Moniliales of the Fungi Imperfecti, are abundant throughout the world and are frequently encountered in soil samples.
Biological Control:
According to Garrett (1965), “Biological control in any condition under which, or practice whereby, survival or activity of pathogen is reduced through the agency of any other living organism (except man himself), with the result that there is a reduction in incidence of the disease caused by the pathogen.” Trap crops, resistant varieties and antagonistic plants that release nematocidal root exudates may be considered useful or potential biological controls for nematodes.
Biological control aims at increasing the parasites and predators of nematodes in the soil, to increase the mortality of plant nematodes. This can be done by changing the environment, adding organic amendments or introducing other organisms, directly.
To increase the concentration of nematode destroying organisms in the soil it is necessary to alter the, environment so that some of the forces restraining the multiplication of the organisms are weakened. The only serious attempt to biologically control plant nematodes has been concerned with the use of nematode trapping fungi.
Oil cakes of margosa, castor and peanut at 0.2 per cent (W/W) three weeks before planting significantly reduce infestation of M. javanica on okra and tomato. In almost all these experiments a soil amendment was used, thereby altering the environment in favour of nematode trapping fungi.
In the late 1930s Linford and his group successfully reduced the incidence of root- knot of pineapple by incorporating green plant material into the soil. It is generally assumed that the addition of this material stimulates the population of organisms antagonistic to nematodes, which in turn, reduces the parasitic nematode population.
Duddington and Duthoit (1960) obtained statistically significant reductions in the mean number of larvae of Heterodera avenae on 60 cm square plots to which 675 gm of chopped cabbage had been added as compared with untrefited plots.
Duddington, Everad and Dutboit (1961), its later experiments obtained further evidence that Dactylaria thaumasia reduces the infection of oat seedlings by Heterodera avenae. There is of course some doubt as to whether or not the fungi are entirely responsible for such reductions.
Resistance to nematodes may be due to the production of toxic root exudates as found in asparagus, lack of an attractant or a hatching factor in the exudates, barrier to penetration or failure of nematodes to develop within plant tissues.
Varietal resistance is, perhaps, the best measure against nematodes. Thomason and Smith (1957) derived a line of tomato, HES 4875, from a cross of Lycopersicon esculentum and Lycopersicon peruvianum.
This line was highly resistant to Meloidogyne incognita acrita and resistance was governed by a single dominant gene. Furthermore, a back cross of HES 4875 and a commercial tomato was resistant to M. javanica as well. Winstead and Barham (1957) also developed a tomato line, Hawaii 5229, resistant to M. incognita, M. incognita acrita, M. javanica and M. arenaria.
Numerous other workers have succeeded in breeding resistant lines to Meloidogyne in other crops. The chief difficulty lies in obtaining resistance to several species. Ellenby (1945, 1954) screened many Solanum spp. and found that S. vernei and S. tuberosum andigena were resistant to Heterodera rostochiensis. Mai and Peterson (1952) confirmed these results and in addition stated that S. sucrense also had resistant properties.
Oats are severely attacked by Ditylenchus dipsaci, and Goodey (1937) showed that some varieties are resistant to attack, resistance being due to the inability of the nematode to develop in the plant. Jones and his colleagues (1955) tested oat varieties for resistance to D. dipsaci in heavily infested soil.
They found several suitable varieties which, they suggested, made possible the cultivation of oats on infested land and the breeding of resistant oat varieties. In a study of some 250 forms of oats, Griffiths and his colleagues (1957) found new sources of resistance in cultivated and wild species.
Ditylenchus dipsaci has been controlled by the cultivation of resistant plants. This success is largely due to the absence of biotypes, which enables continual resistance wherever the crop is grown. Ford and Feder (1958) devised a technique for screening seedlings of certain varieties to Radopholus similis and later Ford et al. (1959) produced several varieties which were tolerant or resistant to this nematode.
Hardly any work has been done on the breeding of lines resistant to ectoparasitic plant nematodes. This is perhaps not surprising because the evidence for the pathogenicity and the amount of damage they cause is somewhat tenuous for many species.
A rice variety, TKM 6, has shown resistance against the root knot nematode, Meloidogyne graminicola, which appears to be confined to the State of Orissa where it may be a problem in the nurseries in well drained soils.
Cowpea has received some attention from the point of view of breeding varieties resistant to root knot nematodes. Three varieties, viz., Miss Crowder, Purple Hall Pink Eye and Brown seeded local have been recorded to be resistant to root knot nematodes, Meloidogyne spp.
In vegetable crops, tomato has received the most attention so far and a resistant variety, SL 120, has been released by the Indian Agricultural Research Institute, New Delhi. Further efforts at locating sources of resistance in tomato as well as other vegetable crops are still in the investigational stage.