In 1943 the breeding work on immunity to blight (Phytophthora infestans)
on the Plant Breeding Station of the Central Bureau at Hoofddorp started by using the two races then known in the Netherlands; later 7 new races of Phytophthora
were found. The spectra of all known races were investigated and test series composed. The relationships and differences between the races were outlined in schemes.
Segregations in the crosses investigated by repeated inoculations with distinct races could be explained by the hypothesis of three genes for immunity. Segregation ratios indicated that these genes were dominant allelomorphs of three independent genes.
Other authors postulated genetical factors in explaining their observations. To coordinate the hypotheses test series were exchanged.
Although differentiation had been known already for about 25 years the 'new' races were not found on varieties of Solanum tuberosum
in practice. This supported the supposition that the 'common' race was superior to the other races on varieties of S. tuberosum.
Some specimens of S.demissum
and other Solanum
spp. were tested for susceptibility to 8 races. There were great differences between the varieties of S. demissum.
Some of the progenies of single plants were homozygous for immunity.