Introduction to Cyst Nematodes
The cyst nematodes are a major group of plant-parasitic nematodes and are of great economic importance in many countries throughout the world. They cause yield losses to many important crops, including cereals, rice, potatoes and soybean, with the most economically important species occurring within the genera Heterodera and Globodera. Heterodera contains by far the largest number of species, although several other cyst nematode species have been described within other genera. Cyst nematodes were originally considered to be largely a pest of temperate regions but many species occur in tropical and subtropical regions.
Management and Control
The cyst nematodes that cause major damage to cultivated crops are mainly those species within the genera Globodera and Heterodera and these present a unique problem in their management. Many or all of the eggs are produced inside the female body that, upon death, becomes a cyst with a hardened protective wall. This structure protects the eggs inside from rapid desiccation, enhancing their ability to remain viable for many years. In many cases (e.g. Globodera spp.) substantial hatch will only occur in the presence of an HF produced by a potential host, so that any management strategy must be effective over a period of years or usable year after year. However, unlike root-knot nematodes, the majority of cyst nematodes have a relatively narrow host range, making appropriate crop rotation a viable option in certain situations.
Where host range is limited, crop rotation has proved an important component in managing cyst nematode levels. Alternative non-host crops can safely be cultivated, during which time a combination of spontaneous hatch and natural mortality will reduce the field population to below threshold levels. Cyst nematodes that have only 1–3 cultivated host plants include G. rostochiensis, G. pallida (potato, aubergine andtomato), H. avenae (oat, barley and wheat), H. zeae (cultivated and wild maize) and H. carotae (cultivated and wild carrot). Even those cyst nematodes with broader host ranges, such as H. schachtii and H. glycines, have relatively few cultivated hosts, facilitating the potential for control by use of rotations.
Fundamental to the prevention of cyst nematodes spreading into uninfested regions is the use of certified planting material, and strict legislation for those commodities being traded both internationally and locally. This tactic has been the mainstay for controlling several major pests such as G. rostochiensis, G. pallida and H. schachtii. Efficient management and containment of an infestation may be compromised by the ease with which cysts can be dispersed by, for example, wind, in small aggregates of soil, on small roots attached to other parts of plants, by flood water runoff or by adhering to machinery or animals passing through infested land. General hygiene practices should be adopted in higher risk situations when the pest is known to be present in the locality. Such measures would include cleaning machinery both before and after use, restricting movement of soil outside the field boundary and construction of natural wind breaks.
Cyst nematodes would appear to be the perfect target for the use of biological agents in their management. Eggs of cyst nematodes are contained either inside the female’s body/cyst or in a gelatinous sac, so they should be very susceptible to parasitism by fungi or bacteria in the rhizosphere (Riggs and Schuster, 1998). Numerous empirical studies using nematophagous fungi and bacteria have been made against economically important cyst nematodes but detailed research has been conducted on only a few organisms. The interactions between cyst nematodes and their natural enemies are complex and most organisms tested have provided inadequate control; there are no commercial biological control products in widespread use. However, the best example of suppressiveness of soils towards cyst nematodes is in the control of H. avenae by the fungi Nematophthora gynophila and Pochonia chlamydosporium.
Nematicides have been very effective for controlling cyst nematodes but several of the most effective have now been withdrawn due to health and safety concerns and others are under threat. Another factor in the effectiveness of nematicides is biological degradation by soil organisms, which may be increased by the multiple use of a nematicide. Nematicides have been used extensively as a management strategy for cyst nematodes that produce only one or two generations a year, such as G. rostochiensis and G. pallida on potatoes and H. goettingiana on peas. However, those species that produce several generations in a year, such as H. glycines or H. schachtii, appear to be more difficult to control.
The repeated use of a single control measure is likely to fail, sooner or later, from selection of virulent biotypes, accelerated microbial degradation of nematicide or possible selection of more persistent populations of the nematode; in general, selection of individuals unaffected by any control measure that may be applied. The potential for managing cyst nematodes by combining two or more control strategies in an integrated programme has been widely demonstrated. Usually a crop rotation is practised, alongside additional measures. The advantages of this approach include the use of partially effective strategies and protection of highly effective ones that are vulnerable to nematode adaptation or environmental risk; examples include integrated control of G. rostochiensis and G. pallida in Europe, and H. glycines in the USA.