MECHANISMS OF TOLERANCE DIFFERENCES OF POTATO TO DAMAGE BY POTATO CYST-NEMATODE (GLOBODERA-PALLIDA)

The growth and yield of three genotypes of potato were compared in replicated plots containing a range of population densities of Globodera pallida, prepared by growing two potato genotypes with different levels of nematode resistance. Plant samples were taken on three occasions, two during the growing season and one to assess final tuber yields. Percentage ground cover was measured on the two mid-season sample dates. The final yields showed that the partially resistant, second early cv. Glenna was much more tolerant of damage by G. pallida than cv. Pentland Dell. In heavily infested plots, the growth of Glenna was reduced initially as much as (in proportional terms), or more than (in absolute terms) that of Pentland Dell. However, because Glenna was more vigorous and produced large haulms, the mid-season reduction in percentage ground cover was much less for Glenna than for Pentland Dell. The third genotype, partially resistant clone 12380, was less vigorous than Pentland Dell, but was not as intolerant as previous reports had indicated. Overall, its intolerance appeared to be due largely to its lack of vigour. Damage by G. pallida slightly reduced the number of stems and the leaf area per unit of dry matter. The factors responsible for the large vigour differences between genotypes in the absence of G. pallida were complicated although differences in number of stems, utilisation of dry matter to produce leaf area and, by implication, photosynthetic efficiency were involved. At low initial population densities (P(i)) susceptible Pentland Dell increased the final population density (P(f)) much more than partially resistant Glenna and 12380. However, due to competition and root damage, above a P(i) of c. 40 eggs per g soil, the P(f) tended to decrease with Pentland Dell. In spite of their moderately high levels of resistance the P(i) at which 12380 and Glenna decreased population densities was apparently higher at c. 50 and 70 eggs per g soil, respectively.