Articles
ECOLOGY OF NEMATODE-BORNE VIRUSES AFFECTING VEGETABLE CROPS
The viruses are primarily pathogens of wild plants and become economically important when susceptible crops are grown in infected soil.
The ecology of the viruses is intimately bound up with the ecology of their vectors and that of their wild host plants.
The viruses are each transmitted by only one or a few vector species and their geographical and local distributions depend largely on those of their specific vectors.
Nematode populations are influenced by soil type and may also fluctuate, depending on such factors as rainfall and cropping history.
Most of the viruses and their vectors have wide natural hostranges; this ensures the survival of the viruses in many different field situations and also makes them important in a wide range of annual and perennial crops.
Vegetable crops, being mostly annual or biennals, present special features that may affect the incidence of the viruses and determine which of them are important in such crops.
Incidence of a virus also depends on whether there is a history of susceptible crops at a site and on the prevalence or proximity of wild hosts such as arable weeds or hedge-row plants.
The viruses, like their vector nematodes, are often patchily distributed in fields.
Lateral spread of the nematodes is slow so that outbreaks of disease extend only slowly.
Dissemination over a distance must occur in infected planting material or infected seed.
Significantly, most nematode-borne viruses are efficiently seed-transmitted in many crops and wild species.
However, although spread in commercial seed is a potential hazard, it has been reported only rarely.
The viruses do not multiply in their vectors but may be retained by them for several weeks or months.
Infected weed seeds provide an important reservoir of infection in soils, especially of those viruses that are retained for only a few weeks by the vector.
Both groups of nematode-borne viruses have bipartite genomes and it is noteworthy that at least in nepoviruses the determinants for two functions essential in the ecology of the viruses, vector specificity and seed transmissibility, are on different genome segments.