We studied changes in pathogenicity, virulence, and aggressiveness of Globodera pallida populations over time. As a measure for pathogenicity, the reproduction factor on a partially resistant host was used; for aggressiveness, the reproduction factor on a susceptible reference host was used; and, for virulence, the ratio pathogenicity/aggressiveness was used. The G. pallida populations were reared in a glasshouse for four generations on potato cultivars with different levels of resistance. The cultivar Elkana did not increase pathogenicity significantly, but the more resistant cultivars Karakter and Darwina did. This increase in pathogenicity was caused by an increase in virulence, whereas aggressiveness generally was not altered significantly. The increase in virulence appeared to be caused by an enhanced ability of eggs to develop into cysts, and not by an increase in egg production per new cyst. The observed changes in virulence could be predicted reasonably well by a simple numerical model. The rate of selection depended strongly on the nematode population. Rearing a mix of two different populations on a susceptible host decreased the virulence strongly, as predicted by the Hardy-Weinberg equilibrium, but increased the aggressiveness because of heterosis.
Apple powdery mildew caused by Podosphaera leucotricha: some important features of biology and epidemiology
