Mainly from Ferris et al (1992) Beyond Pesticides - Biological Approaches to Management in California, based on material from H. Ferris.
An obvious approach to
avoiding problems of nematode parasitism is to choose a geographic area
unsuited to the pathogen. This may involve land purchase, lease, or
realistic crop selection for that area and biogeographic region. At a
more local level, the planting site should be chosen after taking into
account the known distribution of nematodes on the farm (resulting from
varying environmental conditions), and the cultural and cropping history
of each field. The tactic involves selection of conditions to favor the
crop of choice and to disfavor its nematode pests.
Planting dates should be selected to
put the pest population at a disadvantage, for example, when climatic
conditions are unfavorable. Cultivars of potatoes are available in
Scotland that sprout at 4 °C; this allows early planting and avoidance
of the potato-cyst nematode (Globodera
rostochiensis) during the early stages of crop growth.
There are similar examples of avoidance in winter-grown carrots and potatoes in California. Sugarbeets grown in the San Joaquin Valley are harvested earlier and damaged less by the sugarbeet-cyst nematode (Heterodera schachtii) if planted in January and February than if planted in March. In the Imperial Valley, sugarbeets are planted in late summer and harvested from March-June. Early harvest in March avoids two additional reproductive cycles (generations) of sugarbeet-cyst nematode that occur in April-June. Carrots in the Imperial Valley are less damaged by the needle nematode (Longidorus africanus) if planted after soil temperatures have declined below 18 °C, around late September.
Early harvest may limit the time available for production of an
extra generation of a nematode species and limit damage to a root crop when the damage is caused by the late generations of the nematode in a growing season. An example is the production of potatoes in the Tulelake region of northern California where soils are infested with Meloidogyne chitwoodi. Here, early harvest effectively shortens the reproductive period for the nematode pest and reduces nematode blemish on the tubers; however, it also shortens the growing season for the potato crop, reduces yields, and may lessen the shelf-life of potatoes. Another example is the early plowdown of cotton at the end of the growing season in the San Joaquin Valley; this removes roots from the soil that are food sources for plant-parasitic nematodes, particularly Meloidogyne incognita. Consequently, the population level present for the next growing season is much lower.
The use of non-infested planting stock is
fundamental to avoiding a pest problem (i.e, by not introducing
nematodes into the field initially). State- or industry-supported
certification programs are invaluable in this regard.
In California, inspection, certification, and quarantine of plant
material is the role of the CDFA Division of Plant Industry. Known
nematode pests are assigned pest status ratings; the level of response
to a detection at a port of entry depends on the pest status rating of
the nematode species detected. The legislated response ranges from
allowing entry of the material to exclusion and eradication, depending
on the threat posed to California agriculture. Certain types of nursery
stock are monitored and certified by regulatory personnel before sale
and shipment throughout the state. Necessary research support for these
important regulatory activities will include development of molecular
and immune assays to identify species and races of exotic nematode
Grower and pest-control advisor education is the role of Cooperative Extension in the University of California Division of Agriculture and Natural Resources. The education includes providing information on the nature of organisms, probability of spread, nature of damage to crops, and available management tactics.
In addition to inspection and education, treatment of infested plant material can also protect fields from introduction of pests. Hot water dips of banana rhizomes for Radopholus have been used extensively in tropics, and hot water dipping of bulbs, garlic, and strawberries in California can be effective for controlling foliar (Aphelenchoides) and stem nematodes (Ditylenchus).
Restriction of spread by equipment, animals, water, and wind may also be important. There are many well-documented examples of the spread of nematode pests caused by adherence of infested soil or plant material to machinery and equipment, including commercial harvesters, tillers, and pesticide application equipment. A classic example is the arrival of the golden nematode (Globodera rostochiensis) in Long Island New York associated with military equipment returned from Europe after World War I. Importing contaminated top soil for leveling purposes may also contribute to the spread of soil pests in a field. Many nematode species are carried by animals, and some are vectored by insects; e.g., the palm weevil vectors a nematode, Bursaphelenchus (Rhadinaphelenchus) cocophilus, which causes red ring disease of coconut.
Other insects carry the nematode that causes pine wilt (B. xylophilus).
When cattle and rodents are fed roots infested with the root-knot nematode, Meloidogyne incognita, viable eggs survive passage through the digestive tract. Irrigation systems can be a vehicle for nematode spread, especially where "tail-water" from one field is used to irrigate another. Irrigation systems designed after the construction of the Aswan dam in Egypt were, unfortunately, very efficient in introducing damaging populations of plant-parasitic nematodes into the reclaimed desert agricultural areas. The use of settling ponds for water drawn from rivers for irrigation has been shown to be effective in reducing nematode population levels. The New Polders, reclaimed from the sea in the Netherlands, became infested with nematodes through the forces of wind and, perhaps, through material carried on the feet or in the digestive tracts of migratory birds.
A period of 6-18 months without a food source is required
to deplete populations of most plant-parasitic nematode species;
however, if the nematodes are in a reduced state of biological activity they may survive considerably longer. Eggs in cysts of some species of
Heterodera and Globodera will survive as long as 14 years; other species of nematodes
will survive in an anhydrobiotic state for long periods. Maintaining a
field in a fallow condition includes eliminating woody roots that
survive in the soil for extended periods of time (and, thus, constitute
a food source for nematodes), and maintaining the field weed-free. However, fallow may also cause problems with respect to soil
conservation by allowing wind and water erosion to occur.
Flooding promotes anaerobic conditions in the soil and
results in the death of many nematode species. Considerable time
periods may be required; for example, 12-22 months is needed for control
of root-knot nematodes. This practice also requires a plentiful water
supply, and the field must be level in order to avoid expensive
engineering problems. Soil flooding is expensive, as the field is out of production before, during, and for some time after the flooded period. Jacq and Fortuner (1979) showed that sulfate-reducing bacteria produced H2S in soil of flooded rice fields in Senegal under anaerobic conditions, which reduced population levels of Hirschmanniella spinicaudata.
They were able to enhance the effect by incorporating organic material and adding sulfur to the soil; however, the sulfides produced in the process also proved toxic to rice.
Cover crops grown in fields during
the winter months may help reduce nematode population levels by
promoting greater biological activity in the soil and inducing egg
hatch. Nematode control is enhanced by associated soil manipulation and
green manuring through the incorporation of cover crop residues. These
residues release organic acids, including butyric acid, that are
detrimental to nematodes. There may also be a trap-crop effect, as some
plant-parasitic nematodes enter the crop root system, but do not
complete their life-cycle due to low soil temperatures. The trap crop
is then destroyed in the spring before production of a new generation is
Crop rotation is a very effective approach to nematode management. It is easy to select alternate, non-host crops when the host range of the nematode species is narrow, but this is often not the case. The length of rotation needed will depend on the biology and survival capabilities of the target nematode; a major consideration is the relative economics of production of the alternate rotation crops. There is also a danger of selecting for new nematode pests or other problems associated with the rotation crops.
This tactic has already been introduced in terms of shortening the length of exposure of a nematode to its food source. It can also include the removal of infested plants as sources of infestation for other plants, for example, the roguing of infested plants in greenhouses to reduce population levels of the foliar nematode, Aphelenchoides. A major, if controversial, campaign in Florida was aimed at eliminating or minimizing the transmission of "Spreading Decline" of citrus caused by the burrowing nematode, Radopholus similis. It involved pulling out and destroying infested groves and treating the soil with high rates of nematicides. Cotton stalk destruction after harvest reduced continued reproduction of Meloidogyne incognita in Georgia (Lu et al., 2010).
This topic is reviewed in the section on Chemical
Ecology. Some plants in the Compositae produce chemical products detrimental to nematodes, such as thienyl compounds in
marigolds. Often, these compounds are effective against endoparasites,
but may not leach into the soil or, perhaps, must be ingested by a
nematode feeding on the plant in order to be effective. Interplanting
with marigolds has generally been ineffective in protecting a crop from
nematode damage; however, McKenry (1988) showed that the use
of water extracts of residues of these plants can be applied through
drip irrigation and reduce some nematode populations. Asparagus
contains a glycoside toxic to nematodes; several species of grasses,
mentioned previously, contain nematoxic compounds, or create an
unfavorable soil environment for certain nematodes by reducing the
available oxygen. Extracts from many plants purported to have
anthelminthic effects in Chinese Herbal Medicine are effective against soil
nematodes. However, some of these materials are also phytotoxic (Zheng and
Ferris, 1999. Journal of Nematology).
Goodell et al. (1983) showed that the population of root-knot nematode, Meloidogyne incognita, was reduced by approximately 40% (within the tilled zone) for each plowing, following destruction of a cotton crop. Rototilling of sandy soil killed 80% of Paratrichodorus sp. in The Netherlands.
Nematodes which contaminate containers and equipment can be killed by flaming and steaming, or by heating surfaces to 50 °C for 30 min. The cost of heating the soil for nematode control may be prohibitive; it requires 1 calorie to raise 1 g water through 1 °C; 1 acre of sand at 5% moisture contains 136,000,000 ml (= gm) soil solution requiring the equivalent number of calories for each degree C. One acre of clay soil at 5% moisture contains 3.6 million liters of soil solution (= 3.6 billion calories per degree rise). Therefore, an inexpensive source of energy is required to kill nematodes with heat.
Soil solarization may have potential in some cases, particularly for the upper 30 cm of soil in areas where there is high solar output, sandy soil, low soil moisture, shallow root depth of previous crop, and limited acreage, e.g., backyard situations. Solarization has proved very effective for sterilizing soil in containers raised on pallets above the ground when tented with a double layer of tarp. The procedure has potential in the nursery industry (Stapleton, 1998).
Heating the soil by burning
brush or leaf litter has little effect on nematodes; its efficacy is
generally limited to killing weed seeds and pathogens at the soil
surface. Burning the brush in cherry orchards in New York state did not
kill the lesion nematode, Pratylenchus
penetrans, in the upper 15 cm (6
in) of soil (Mai and Parker, 1972).
While steam is useful for sterilizing soil for greenhouse use, there may be undesirable side effects. These include the formation of toxic breakdown products and release of minerals at toxic levels from organisms, lethal effects on beneficials resulting in a biological
vacuum and an increased foothold for opportunist pest species.
Consequently, the use of aerated steam is recommended (about 70 °C), but is not often used. Penetration of the soil mass may be a problem and may involve installation of an expensive, permanent manifold system.
Nematodes in tissues of infested planting stock are frequently killed with hot water dips. Specific temperature and time requirements must be determined to create lethal effects for the nematode population while minimizing damage to plant material. Examples include control of root-knot and lesion nematodes in grape roots by dipping rootings in water at 51.5 °C for 5 min, and control of Ditylenchus dipsaci by dipping bulbs in water at 44 °C for 1 hour.
Electricity has occasionally been reported to
be lethal to nematodes, but success has been variable. A considerable
body of opinion regards lethal effects of electrical currents to be
caused by heating of the soil solution. There is also an indication
that direct current is more effective in killing nematodes than
alternating current. The mechanics of passing electrical current
through the soil on a field-wide scale are problematic, however (Caveness and Caveness, 1970; Daulton and Stokes, 1952).
Tests thus far with microwave energy have been unconvincing. The soil is an effective buffer, and microwave energy does not penetrate it well. Control of nematodes with microwave energy would be energy- and equipment-intensive.
The ability of a plant to tolerate nematode stress may be enhanced
by nutrition and water management, and the reduction of other sources of
plant stress, including abiotic limiting factors, may be effective.
Soil profile manipulation, for example, by backhoeing of tree replant sites or by ripping a hardpan, provides a greater volume of soil for root growth and a greater pool of minerals and nutrients for plant growth.
Cultural approaches to curing a plant that has been
subjected to plant-parasitic nematode damage, or stress infestation,
apply mainly to planting stock. Approaches include hot water dips
already mentioned for killing nematodes in living plant tissues;
surgical removal of infested tissues, such as the paring of banana corms
to remove infestations of the burrowing nematode,
and the use of systemic nematicides-an approach that has potential in
perennial crops, but poses problems in identifying appropriate
formulations or conditions of plant physiology that are conducive to
downward translocation of the materials to the root system.
Important areas of short-term research in the area of cultural
manipulations include cropping systems design and development, and the
development and use of damage thresholds. A fundamental principle of
crop and pest management is that control is unnecessary unless the pest
population reaches a level expected to damage the crop. Therefore,
rational management decisions will require a greater understanding of
the precise relationship between plant growth (or yield) and nematode
population levels; establishment of appropriate levels of sampling
intensity for the crop in question; determining the seasonal effects on
nematode population levels; and development of improved instrumentation
and methodology for obtaining nematode count data. Research activities
directed toward satisfying these requirements will aid growers in
optimizing management decisions.
Ferris et al (1992) Beyond Pesticides - Biological Approaches to Management in California.
LLu, P., Davis, R.F., Kemerait, R.C. (2010). Effect of mowing cotton stalks and preventing plant re-growth on post-harvest reproduction of Meloidogyne incognita. Journal of Nematology 42:96-100.