Nematodes: an increasing threat for the potato crop in Europe?

Introduction
Plant-parasitic nematodes associated with potato all have this in common: they infect below-ground plant parts, primarily the root systems, although some nematode groups also enter developing tubers. These small round worms damage the potato plant by feeding on the content of cells, or by migrating through root tissues, and some species aid pathogenic micro-organisms to infect the already weakened plant. This results in reduced plant vigor, less growth and hence yield reduction, which is not very obvious to allocate to nematodes as symptoms are not specific. Even infection of tubers (and roots) is not easily recognized, except for distortions caused by certain species of root-knot nematodes, stem nematodes, and trichodorids.

Less nematicides
Nematodes have always been present in agricultural fields and their effect on crop yields was clearly demonstrated when nematicides were developed in the 50s and 60s. Differences between treated fields and untreated fields easily convinced growers to apply these products to increase yields. However, due to their high toxicity and negative impact on the environment, many of the very effective broad-spectrum chemicals (e. g. methyl bromide, aldicarb) have been restricted or forbidden in the last 10-20 years, at least in Europe. Now farmers can only choose between very few nematicides and the search for alternative management tools has been intense in the last two decades. This search lead to success stories for some economically important nematode species (e.g. Globodera). On the other hand, restriction of nematicides gave the opportunity to several nematode groups, which were previously kept under control with the broad-spectrum chemicals, to multiply and spread: “old acquaintances” are here again (Pratylenchus, Ditylenchus, Trichodorus). Today, farmers are hoping for new nematicides and some products are indeed under development.

Ignorance
Extension and education are important to make growers and traders aware of nematodes (“the hidden enemy”). Ignorance of the ways in which nematodes are spread (planting materials, soil, machinery and equipment) is one of the reasons that nematodes are widely distributed. When nematodes reach a suitable plant host in a favorable climate, they are able to multiply in the field without being noticed, until damaging levels are attained. The slow build-up of large populations of some nematode species is also a reason for not taking immediate actions, as the consequences of nematode introductions only show up after several years.

Quarantine measures
Potato cyst nematodes (Globodera rostochiensis and G. pallida) are quarantine organisms, yet they are present in all EU countries. The extent of their presence of both species and of their pathotypes (virulence groups) is still not well known however. Their quarantine status prevents spread of virulent populations; especially the required pre-plant soil sampling for seed potato production is a very effective tool. The success of an integrated management approach is demonstrated in the Netherlands where Globodera spp. are widespread, but kept at very low population densities through a combination of control measures. The key to successful management of potato cysts in the field in the Netherlands is a combination of intensive field sampling to detect very low infestations, followed by species identification and the use of appropriate resistant potato cultivars or, rarely, nematicides. However, spread of potato cysts between fields worldwide is still a problem for which practical solutions are under investigation.
In the last few years, we have been facing increasing problems with root-knot nematodes (Meloidogyne species). For potato this is particularly true for M. chitwoodi and M. fallax, which are quarantine nematodes in the EU on seed potato. Their quarantine status is justified because of their wide host range, allowing a fast build-up of populations and increased chances for survival in the field, and also because of their ability to infect (seed) tubers. Hidden in the peel of the tubers, M. chitwoodi and M. fallax travel along with the potatoes and can even develop further during storage. In the last 10-15 years these two species have shown up in new regions on vegetables and on potato, despite the quarantine legislation and monitoring of seed potato by inspection services. We will learn if a quarantine status is indeed slowing down nematode spread by keeping an eye on M. minor, a more recently detected species very similar to M. chitwoodi and M. fallax, mostly found on grasses and (until now) very rarely infecting potato, but without quarantine status.

Globalization
The possibilities for spread have increased dramatically in the last decade due to intensified international trade of plants and associated soil, as well as more frequent exchange of farming equipment between farms and regions. This allows distribution of nematodes over large distances, with the possibility of introducing new species or new pathotypes. Spread through seed potato tubers as well as with soil adhering to ware potato tubers is also possible for non-quarantine nematodes. This is clearly demonstrated for Pratylenchus spp. (= root-lesion nematodes), which are present in the upper layers of potato tubers. Although already wide-spread, new nematode populations which multiply faster and cause more damage (more virulent populations) can be introduced from fairly large distances in this way. Adhering soil can contain many different nematode species but their survival in dry soil and over longer periods still needs investigation. These aspects of nematode spread with soil only recently received attention and risk analyses might result in more strict regulations of movement of soil in the future.
It is assumed that global warming can favor the establishment of certain nematode species in regions that were previously too cold to allow their survival, allowing a wider distribution of certain species in existing potato cropping areas. But also potato is spreading: it is promoted in Africa and other developing countries as a new food-security crop and growing (seed) potato receives a lot of attention. The host status of potato for certain nematode species typical for those regions is not well known and can pose a new threat to potato production, especially if those nematode species find their way into the EU where climate conditions in the southern countries could allow their establishment. Recent research warrants for possible threats by, for example, M. enterolobii and also Scutellonema bradys, the yam nematode.

Challenges for the future
To cope with the increased threat of nematodes in potato, monitoring international trade, but also in the field, is an excellent tool. Fortunately, more molecular methods are becoming available for detection, identification and quantification of nematodes in samples of soil and plant parts. This will make it possible to have routine samples analyzed in a fast way by people that are not necessarily trained in morphological identification. The lack of resistance to most plant-parasitic nematodes, especially Meloidogyne spp. and Pratylenchus spp., is a threat to keep up with the current potato productivity and to secure the supply of the potato chain in a sustainable way. Nematode resistance is now restricted to certain pathotypes of G. rostochiensis and G. pallida. Breeding for resistance to other pathotypes of potato cysts and molecular tools to identify these virulence groups are important challenges for the future. Based on data available from sequencing the genome of potato and nematodes, we expect more potato cultivars with (multiple) nematode resistance in a few decades.
Valuable information is also expected from research on the role of soil amendments and soil microbial fauna in reducing plant-parasitic nematode populations. Without waiting of such new knowledge for an integrated nematode management in potato production, we need to apply rigorous and regulated sanitation measures. Such an approach requires involvement of all participants in the potato chain; not only growers, but also traders, processors and authorities are responsible for a sustainable way of producing potatoes.