Endoparasitic and free-living plant-parasitic nematodes present a different challenge to plant health services. Endoparasites spend most of their life inside bulbs, corms, roots or tubers and hence can be unknowingly spread and escape detection, whilst free-living species are found in soil residues that need to be processed to confirm their presence. There is more potential for unlisted species in these groups to be assigned quarantine status because international trade is facilitating the spread of species that were not known to have potential to become economic pests when most international plant health legislation was first implemented; it usually takes several years to collect evidence with a Pest Risk Analysis (PRA) that a species should be listed as quarantine. The challenge to plant health services is to detect and identify not only listed species but also new or unusual species that might pose a threat to agriculture should they be allowed to establish and spread. Nematologists specialising in identification largely rely on morphological characters for identification, but molecular techniques offer potential for a new range of tools to facilitate this. This is especially important when frequently samples only contain a few or immature specimens that cannot be identified by morphological means.
The root-knot nematodes, Meloidogyne species, are endoparasitic species widespread throughout the world and usually have a wide range of host plants. Meloidogyne chitwoodi was first described from the Pacific Northwest of the USA in 1980 (Golden et al. 1980), but it is not clear if this is its area of origin. It was causing damage to potato tubers and a few years later was also detected in potatoes in The Netherlands, although there is evidence that it may have been present in Europe for some time (OEPP/EPPO 1991). It has a wide host range including other economically important crops such as carrots and other root crops such as salsify. M. chitwoodi lowers the market value of such crops as a result of internal necrosis and external galling and yields are reduced. A PRA (Tiilikkala et al. 1995; Braasch et al. 1996) led to the pest being listed as a quarantine organism in the EU in 1998. This meant that plant health service inspectors were alerted to look specifically for symptoms of this pest, particularly on potato tubers, and samples were submitted to quarantine laboratories for detection and identification.
It is important to distinguish M. chitwoodi and M. fallax, listed species in Europe and elsewhere, from other nematodes, particularly related, unlisted species, that might also be found in potato tubers and roots. In addition, survey work has demanded that large numbers of samples are dealt with quickly so prompt action on control could be taken. This has led to the development of molecular tools to assist identification, most recently using TaqMan technology. The EPPO standard on M. chitwoodi and M. fallax gives additional information (OEPP/ EPPO 2009b).
The free-living pine wood nematode, Bursaphelenchus xylophilus, is associated with pine wilt disease but depends on bark beetles (Monochamus spp.) to spread from tree to tree. It is native to North America and is thought to have been carried to Japan at the beginning of the twentieth century on timber exports. In Japan, it is causing massive mortality of native pine trees. In 1999, B. xylophilus was found in Europe for the first time, in Portugal. Quarantine nematologists were already researching the identity of this species in order to be able to distinguish it from the many species of Bursaphelenchus that inhabit wood. Unfortunately there is a variation in characters between species in the Bursaphelenchus group, which makes morphological identification particularly difficult, so biomolecular tools are highly recommended (OEPP/EPPO 2009c).
The majority of plant-parasitic nematodes are free-living species that feed ecto-parasitically on roots; this group contains thousands of species, some well known to science but others not. Quarantine is concerned with the detection of any species that may pose an economic risk to agriculture, horticulture and, increasingly, the native biodiversity of recipient countries. This demands specialist skills in nematode identification in order to distinguish such species from native ones. Nematodes in the EU-listed group, Xiphinema americanum sensu lato, are listed in many countries because of the ability of some species to transmit viruses. The identification of this group is in a state of flux, but recent work in Europe has led to the development of an EPPO Diagnostic protocol (OEPP/EPPO 2009d). It is hoped that research into molecular tools will provide further tools to facilitate the harmonisation of protocols worldwide.
Control programmes may be aimed at eradication or suppression of intercepted pests, but in the long term the development of certification schemes to produce nematode-free material is important. The development of hosts with resistance to quarantine species is a prolonged and expensive process but may be justified for economically important species. Sustainable methods of control are receiving more attention as chemical controls become scarce and this stimulates research into biological control methods. Combined with knowledge of the biology and life-cycle of the species such research offers the potential to minimise the effect of plant-parasitic nematodes spread in trade.
Was this article helpful?