In an assay which involved 838 potato seedlings, the relative resistance to leaf-Phytophthora (KR(G)) was tested at the 4 - 6 leaf growth stage, and the relative tuber resistance to infection was determined on small pot-grown tubers (B(K)) and, in the following 2 years, in slices from field-grown potatoes (B(S)). The small tuber assay is described and its value as a measure of disease resistance, compared with the potato slice assay, discussed. Among 357 leaf-Phytophthora susceptible clones (KR(G) less than or equal to 5) were 10 clones with a good to very good tuber resistance. 18 of 204 leaf-Phytophthora highly resistant idiotypes were extremely susceptible to tuber rot. After discarding those seedlings which showed KR(G)-values less than or equal to 5 and B(K)-values less than or equal to 5, 208 of the remaining 241 (= 86%) tuber rot resistance carrying seedlings were selected. 14% of the clones which showed adequate resistance in the potato slice assay were falsely discarded according to the small tuber assay. 80 clones remained as ballast in the field trials (B(K) over or equal to 5, B(S) less than or equal to 6.4), 160 (33% of the idiotypes tested) had been correctly eliminated. The work-intensive small tuber assay presents a possibility by which breeding material can already be reduced in the seedling stage. In this respect there must be a pronounced genetic differentiation present in the potato population and the selection procedure carried out must not be too harsh

Protoplast fusion allows the transfer of both mono- and polygenic traits between species that are sexually incompatible. This approach has particular relevance for potato, and somatic hybridisation has been used to introduce a range of disease resistance genes from sexually incompatible wild species into the cultivated potato gene pool. In addition, protoplast fusion allows the resynthesis of tetraploid genotypes from pre-selected diploid or dihaploid donor parents. A limiting factor for the efficient exploitation of this technology in potato breeding is the difficulty of unequivocally identifying nuclear hybrids (heterokaryons). In order to facilitate the identification of hybrids at an early stage following fusion, Randomly Amplified Polymorphic DNA markers (RAPDs) have been used to characterise molecularly both inter- and intra-specific somatic hybrids of potato. RAPD markers detect naturally occurring polymorphism in the donor genotypes and utilise short oligonucleotide primers of arbitrary nucleotide sequence in combination with the polymerase chain reaction (PCR). The exploitation of RAPDs in the characterisation of both somatic and sexual hybrids is discussed.