Potato cyst nematodes (PCN) have been known to occur in the UK for nearly a hundred years. They are the most problematic pests of potatoes and can cause severe yield losses. Previous work has shown the two species, Globodera rostochiensis and G. pallida, to be distributed throughout the UK. This paper reports the results of the first structured and statistically unbiased survey undertaken to assess their occurrence and distribution in the potato growing land of England and Wales. The survey showed that PCN were present in 64% of sites sampled. Of the populations found, 67% were G. pallida, 8% were G. rostochiensis and 25% contained both species. The results show an increase in the incidence of PCN since previous studies were completed and confirm the perceived shift towards G. pallida as the predominant species. Of the infestations found, 62% had a population density of less than 10 eggs g -1 soil.
Potato cyst nematodes (PCN) are the most problematic pest faced by potato growers in the UK. Both species of PCN were brought to Europe from South America, but one, Golobodera pallida, now predominates because it is more refractive to control measures. Despite sometimes poor results, considerable reliance is placed on nematicides for the control of PCN, but this situation may have to change if potato retailers in the form of supermarkets continue to insist on ever tighter restrictions on the use of nematicides.
Plot\ w i t h dii average 01 8 cgg\/g 5011 Ploh with an acerage of 105 eggc/g 5011 C II I t i c a r hlaris Peer 24.8 Pentland Dell 46.3 Pentland Crown 55.9 Cam 5 9 . 4G. rostochiensis populations (almost half of all known infestations ; Howard, 1970). Repeated use of such cultivars in land with mixed populations of G . rostochiensis and G . pallida has selected G.paIlida, which is an increasing problem in the UK (Whitehead, 1986). It has taken longer to incorporate high levels of broadbased resistance from S. vernei into new cultivars.These are now becoming available and will add an extra weapon to the armoury of control measures which growers can use against potato cyst nematodes. However, many of these have proved intolerant of attack by potato cyst nematodes (Gurr, 1987). It is important that new cultivars with partial resistance to G. pallida are also tolerant and Haydock (1990) records three such cultivars as being more tolerant than the non-resistant controls. Painter (195 1) distinguished three main types of plant resistance to insects : non-preference, antibiosis and tolerance. Insects are more reluctant to colonise non-preferred plants and antibiosis retards the growth of individual insects and decreases their rate of reproduction. Tolerant plants can better withstand the effects of pest attack than can more sensitive plants. This classification confuses the effects of the plant on pest reproduction and the effects of the pest on plant growth under the general term 'resistance'. It is better to consider these effects as two independent aspects of the host-parasite relationship and to quantify each: a resistant host is often an intolerant host (indeed, some resistance mechanisms depend for their effect upon a hypersensitive response in challenged tissue) but a resistant host may also be tolerant. Fox & Spasoff (1976) showed that the resistance and tolerance of tobacco to the cyst nematode G. solanacearum (Miller and Gray) were independently inherited, and that appropriate crosses yielded progeny with all combinations of degrees of resistance and tolerance. Much evidence is now available to show that British potato cultivars differ in their ability to tolerate attack by potato cyst nematodes. Evans (1982~) showed that in heavily infested plots the yield of Cara was reduced by 4.8 t/ha compared with 24.6 tjha for Pentland Crown and 32.9 tjha for Pentland Dell (Table 1); Trudgill (1986a) listed other examples.Interest in tolerance of plants to nematode attack became more marked in the 1970's and this is shown by the numbers of publications relating to tolerance of 15 major genera of plant parasitic nematodes, or of potato cyst nematodes alone (Fig. 2). The numbers of publications were maintained at a high level for several years but have recently declined.In assessing cultivar reaction to nematode attack, the most obvious effect of potato cyst nematodes on potato plant growth is the stunting of root system development and the water stress that this causes. Compared to other plant species the potato is drough...
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