SELECTION BY RESISTANT POTATO CULTIVARS IN POPULATIONS OF GLOBODERA-ROSTOCHIENSIS, PATHOTYPES RO-1 AND RO-3

Potato cv. Alcmaria (resistance ex andigena CPC 1673-20 against pathot ype Ro 1 of Globodera rostochiensis) selected a population similar to pathotype Ro 3 of this species from one of pathotype Ro 1 in three yea rs. The rate of selection was consistent with the assumption that fema les produced on cv. Alcmaria were all homozygous for the virulence gen e of the selected population and that the proportion of pathotype Ro 1 and heterozygous males that matured was about a quarter of that of ma les homozygous for the virulence gene of the selected population. A co ntamination of the Ro 1 population used with 8.7% Ro 3 genes was deduc ed from the susceptibility of cv. Alcmaria to this population of 0.3% of that of cv. Irene. The relative susceptibilities <0.05% relative to cv. Irene at initially 5 eggs/g soil of cvs Cardinal, Saturna and Ehu d (resistance ex andigena) to the same Ro 1 population (relative susce ptibility to the selected population and to pathotype Ro 3 about 30% o f that of cv. Irene, cv. Alcmaria 50%) is, most probably, due to a neg ative effect of the presence of Ro 1 juveniles on the development of R o 3 juveniles in the latter cultivars which did not operate in cv. Alc maria. A range of potato cultivars resistant to pathotypes Ro 1 and Ro 3 did not select more virulent populations from one of pathotype Ro 3 in three or four generations. These cultivars are, therefore, only pa rtially resistant against pathotype Ro 3.