MONOXENIC TEST FOR THE RESISTANCE OF POTATO GENOTYPES TO THE POTATO CYST NEMATODE GLOBODERA-PALLIDA .3. USE AND EXAMINATION OF THE TEST

A monoxenic in vitro test, which was developed for selecting resistance in potato genotypes to Globodera pallida (Stone) Behrens, can distinguish small differences in the resistance reaction, presumed to be polygenically controlled. To establish meaningful criteria for resistance, a number of characters which could be assessed rapidly were determined (number of necrotic root areas, number of invading juveniles but not yet sexually differentiated, number of juveniles developing as males and females, number of juveniles with arrested development). Assessments were made 21, 35 and 49 days after the introduction of 1000 freshly hatched juveniles of G. pallida Pa3- population 'Frenswegen' per test Petri dish. Because of differences in levels of juvenile invasion per genotype (number of necrotic root areas) and differentiated individuals (Table 1), to make genotypic comparisons, it was necessary to consider other resistance criteria along with percentage juvenile invasion. The ratio of males to females was shown to be the most reliable criterion of resistance. There was a higher proportion of females than males on susceptible, and G. rostochiensis resistant genotypes than on G. pallida resistant genotypes on which males predominated (Table 1). A further subdivision of the G. pallida resistant genotypes was possible using the increased occurrence of necrotic root areas, and juveniles with arrested development. A scheme was constructed for rating the effect of genotypes on the characteristic development of the nematode populations. It contained four groups: susceptible reaction (Figure 1), weakly expressed resistance (Figure 2), expressed resistance (Figure 3) and strongly expressed resistance (Figure 4). Female counts from pot tests with particular genotypes were in general agreement with the results from the monoxenic resistance test (Table 1).