PARTIAL RESISTANCE TO LATE BLIGHT (PHYTOPHTHORA-INFESTANS) IN HYBRID PROGENIES OF 4 SOUTH-AMERICAN SOLANUM SPECIES CROSSED WITH DIPLOID SOLANUM-TUBEROSUM

Resistant genotypes of the diploid tuber-bearing South American specie s Solanum arnezii x hondelmannii, S. berthaultii, S. leptophyes and S. microdontum were crossed with three diploid genotypes of S. tuberosum that varied in resistance and maturity type. The progenies were field tested for 2 years for resistance to a complex race of Phytophthora i nfestans. A wealth of genetic variation for resistance was found in mo st of the progenies. At least two susceptibility groups could be disti nguished in some progenies of S. microdontum. This could be explained by the presence of several major resistance genes in the wild parent a nd, unexpectedly, in the susceptible parent SH 82-44-111. In most of t he wild parents and in the susceptible parent SH 77-114-2988 there app eared to be minor resistance genes. General combining ability effects were predominant; small specific combining ability effects were detect ed in some crosses of S. microdontum. Gene action appeared dominant in some crosses.