A candidate-gene approach to analyse the resistance of plants to phytopathogenic fungi is presented. The resistance of sunflower (Helianthus annuus L.) to downy mildew (Plasmopara halstedii) shows a gene-for-gene interaction (monogenic resistance), whereas resistance to white rot (Sclerotinia sclerotiorum) is quantitative, with different levels of resistance for different plant parts. By homology cloning, probes were obtained homologous to some plant resistance genes (nucleotide binding site-like, NBS, genes and serine-threonine protein kinase-like, PK, genes). These clones were used as probes for linkage mapping of the corresponding genes. It was demonstrated that at least three NBS-like loci are located on linkage-group 1, in the region where downy mildew resistance loci have been described. Quantitative trait loci for S. sclerotiorum resistance to penetration or extension of the mycelium in different tissues were studied in three crosses. Major QTLs for resistance were found on linkage group 1, with up to 50% of the phenotypic variability explained by peaks at the map position of the PK locus, 25 cM from the downy mildew loci.