M. S. Phillips

The Effect of Initial Population Density On the Reproduction of Globodera Pallida On Partially Resistant Potato Clones Derived From Solanum Vernei

The reproductive rates of two populations of Globodera pallida over a range of initial population densities (Pi) were compared on a non-resistant and two partially resistant potato clones derived from Solanum vernei. The final number of eggs/g soil (Pf) increased with Pi, as did the differences between clones. In contrast the reproductive rate (Pf/Pi) progressively decreased as Pi increased. The form of the curve relating Pf to Pi for the partially resistant clones was similar to that for the non-resistant clone but the rates of response to Pi and the final maximum densities were less on the partially resistant clones and related to their levels of resistance. Each population ranked the clones consistently for degree of resistance at all inoculation levels. This observation is related to other published results and it is concluded that Pf/Pi values are too sensitive to changes in initial nematode density and other environmental factors to be used either to define and separate categories of resistance in the plant or to identify pathotypes in the nematode. A new scheme for assessing the resistance of clones is proposed in which partially resistant clones are used as controls to delineate resistance classes.

Variations in the ability of Globodera pallida to produce females on potato clones bred from Solanum vernei or S. tuberosum ssp. andigena CPC 2802

The estimated multiplication rates of six Pa2/Pa3 populations of Globodera pallida were compared in a pot test on partially resistant potato clones, 12 derived from Solanum vernei and six derived from S. tuberosum ssp. andigena CPC 2802. Although the populations differed in their virulence, they all tended to rank the clones in a similar order of resistance. The clones did not, however, rank the nematode populations with the same consistency. The response of the clones used to differentiate pathotypes, whilst indicating the reactions of some of the breeding clones, were not necessarily representative of all clones. These results are discussed in relation to a potato breeding programme and to the utility of the current International Pathotype Scheme.

Pot and field assessment of partial resistance of potato clones to different populations and densities of Globodera pallida

The effect of initial population density on the rates of multiplication of Globodera pallida on a non-resistant potato cultivar and on three genotypes with resistance derived from Solanum vernei was compared in a field trial and a pot test. On all the cultivars and genotypes the multiplication rate of G. pallida decreased as the initial population density increased. The results of the field and pot experiments were similar except that they differed in magnitude; the mean rate of multiplication in the pots being greater (c. x 2.5) than in the field. Both field and pot experiments ranked the four hosts in the same relative order of resistance and joint regression analysis showed that for each host the two sets of results formed a continuous series; as the overall mean rate of multiplication on the four hosts increased the mean multiplication rate on each of the individual hosts increased linearly whether the results were from the pot or the field. Therefore, it is suggested that pot tests can be used to assess clones with partial resistance provided they include partially resistant standard clones whose performance in the field has been well established and to which the results can be related. The different initial population densities of G. pallida used in these experiments had been produced over 3 years by cropping plots with hosts having different degrees of resistance. The virulences of the resulting populations were examined in a further pot test which indicated that there were differences between them. However, there was no evidence for markedly specific selection of virulence. The differences detected were largely non-specific with the populations reared on S. vernei hybrids producing more females on all clones included in the test.

Assessing potato cultivars for resistance to tuber soft rot (Erwinia carotovora subsp.atroseptica) at four test centres in the UK

SummaryTuber slices of 15 cultivars were inoculated withErwinia carotovora subsp.atroseptica on two occasions at four test centres and the results compared. Correlations in mean lesion diameter between inoculum concentrations, between the same test on different occasions, between centres and between cultivars in different tests were mostly significant in spite of small differences in procedure at each centre. Satisfactory agreement was reached between centres, particularly concerning the most resistant (Cara, Pentland Hawk) and most susceptible (Record, Wilja, Pentland Crown, King Edward) of the cultivars tested. The implications of the findings are discussed in relation to results published elsewhere and to other methods of inoculation.

The use of single, spaced potato plants to assess their tolerance of damage by potato cyst nematodes

SummaryPotato clones and cultivars differ in their ability to tolerate damage by the potato cyst nematodes:G. rostochiensis andG. pallida. This paper reports a simplified field assessment whereby single, spaced plants are grown in heavily infested soils. These trials identified those clones which are either very sensitive to damage or those that grow and yield well in the presence of nematodes. The use of such a screening procedure in a breeding programme is discussed.

Environmental interactions in the Assessment of Partial Resistance to Potato Cyst Nematodes

The multiplication rates of six populations of Globodera pallida on two susceptible cultivars and six clones with partial resistance derived from Solanum vernei were compared at two centres. Differences were obtained between populations but the results at each centre were similar. Two of the populations multiplied significantly less well at both centres on one of the susceptible cultivars and, with the exception of cv. Darwina which was much less resistant in both absolute and comparative terms at one of the centres, the populations ranked the resistances of the clones in a similar order. The results were also used to compare virulence differences between populations. Because of differences in overall multiplication rates the susceptible cultivars were used as a basis for calculating percentage virulence. Although the actual values obtained differed between centres, the populations were ranked in a similar order. An analysis of further sets of data confirmed that, provided the average derived from tests on a number of clones was used, the ranking of clearly distinct populations was consistent. The results support the view that certain clones and populations should be used as standards in tests to determine categories of resistance and virulence.

Resistance to potato cyst nematodesGlobodera rostochiensis andG. pallida in the British Neo-Tuberosum potato population

SummarySurveys of Neo-Tuberosum clones have revealed continuous ranges of reaction to bothGlobodera rostochiensis andG. pallida with a few clones being highly resistant. Reactions of clones to the two species show some correlation but correlations between reactions to differentG. pallida populations are closer. Tests on progenies withG. pallida show the resistances to be heritable.