Quarantine Nematodes of Global Importance 221 Potato Cyst Nematodes

Cyst forming nematodes, or cyst nematodes, are one of the most specialized and successful plant-parasitic nematode pests of agriculture. These nematodes usually have a very narrow host range. In the case of the potato, the potato cyst nematodes (PCN), Globodera rostochiensis and G. pallida are able to infest potato crops and some other Solanaceae such as tomatoes and woody nightshade (bitter sweet). These nematodes belong to the family Heteroderidae and originated in South America and were probably introduced to Europe along with potato breeding material around 1850 (Turner and Evans 1998).

The life cycle of cyst nematodes is well adapted towards the host and they can survive in various environments. The cyst, the dead body wall of the female, contains the eggs which hatch in the presence of the host. Root exudates from Solanaceae activate these juveniles and this can cause hatch of up to 80% of the nematodes under suitable environmental conditions. The juveniles enter the roots near the root tip and induce a feeding cell or syncytial "transfer cell" (See Chap. 4). The juveniles become sedentary and feed from the syncytia, until their development is complete after four moults. After the 4th moult the female is round and swollen and protrudes from the root. The males are slender, leave the roots, mate and fertilize the females. After mating the female forms eggs and when the female dies the cuticle tans to form a protective cyst with 200-500 eggs within. The life cycle is complete and may take up to three months. The cyst (pinhead size) falls off the roots, waiting for the next suitable host plant.

Damage is related to the level of nematode infestation. Infested plants show retarded growth and heavily infested fields normally show badly growing patches, especially under dry conditions, and as a result yield can be decreased by up to 50%. Apart from the yield reduction, the financial benefits of growing potatoes are also reduced by the costs of control measures and reduction of marketable products.

Sampling soil on land prior to planting of seed potatoes is one way of minimizing spread of these nematodes and forms part of the statutory measures for the production of seed potatoes of all countries. However, when the cysts are present at low density they are often overlooked, or below detection level, whilst the population is spread, mainly by machinery. The difference between the status "not known to occur" and "known not to occur" is not always clear.

When PCN is present in the field there are several ways to minimize the population to such an extent that growing potatoes is feasible although complete eradication is not possible due to the persistence of the cysts in the soil. Available management tools include growing resistant potato varieties, using a lengthened crop rotation, applying chemicals, using biological methods such as solarisation or bio-fumigants or growing catch (trap) crops. The most reliable method in recent years has been the growing of resistant potato varieties.

Potato varieties can differ greatly in the extent to which the nematodes can multiply on them. On a fully susceptible cultivar the nematodes can multiply freely on the roots, stolons and even on the tubers. On resistant cultivars no multiplication can take place and partially resistant cultivars give intermediate multiplication, thus reducing the rate at which the nematode population builds up in the soil. The multiplication of the nematodes depends on the resistance genes present in the potato and on the virulence genes present in the nematodes. When the same resistant potato variety is grown successively, selection for virulent nematodes that can overcome the resistance source may occur. A prime example of this has occurred in the UK since the introduction of commercially viable cultivars carrying the H1 gene. This gene gives complete control against the pathotypes of G. rostochiensis present in the UK and wheras G. rostochiensis was the prevalent species present in the UK in the past, repeated use of cultivars containing H1 has led to an increase in the prevalence of G. pallida, which was previously rarely encountered (Minnis et al. 2002).

Outbreaks of potato cyst nematodes have now occurred in most of the potato growing areas of the world. PCN remains rare in some countries with extensive potato acreages, most notably Australia, Canada, USA, India and, probably, some parts of the former USSR. Most outbreaks involve G. rostochiensis or both species. The relative scarcity of commercially acceptable varieties with full resistance to G. pallida makes the control of this species much more difficult to achieve. Identification of the species, generally based on morphology, is now being taken over by molecular methods as these techniques are easier to learn and adapt than the more specialised knowledge of taxonomy. The EPPO Diagnostic protocol for identification of potato cyst nematodes (OEPP/EPPO 2009a) gives an overview of the more recent identification tools and this topic is also covered in Chap. 21.

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