Euphytica (2005) 143: 189–199 DOI: 10.1007/s10681-005-3444-8 C  Springer 2005 Genetic linkage of QTLs for late blight resistance and foliage maturity type in six related potato progenies M.H.P.W. Visker 1,2,3,∗ , H.J.B. Heilersig 1,2,4 , L.P. Kodde 1 , W.E. Van de Weg 1 , R.E. Voorrips 1 , P.C. Struik 2 & L.T. Colon 1 1 Plant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands; 2 Crop and Weed Ecology, Wageningen University, P.O. Box 430, 6700 AK Wageningen, The Netherlands; 3 Present address: Human Nutrition, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands; 4 Present address: Laboratory of Plant Breeding, Wageningen University, P.O. Box 386, 6700 AJ Wageningen, The Netherlands; ( ∗ author for correspondence: e-mail: marleen.visker@wur.nl) Received 3 December 2004; accepted 9 March 2005 Key words: earliness, Phytophthora infestans, Solanum tuberosum Summary A set of test crosses of diploid potatoes was used to identify QTLs for foliage resistance against Phytophthora infestans and QTLs for foliage maturity type, and to assess their genetic relationship. The most important locus for both traits was found on chromosome 5 near marker GP21: the allele of marker GP21 that is associated with resistance to late blight is also associated with late foliage maturity. An additional QTL with a small effect on foliage maturity type was identified on chromosome 3, and additional QTLs for late blight resistance were found on chromosomes 3 and 10. Another QTL was detected on chromosome 7 when resistance was adjusted for the effect of foliage maturity type. The additional QTLs for resistance against P. infestans on chromosomes 3 and 10 seem to be independent of foliage maturity type and are not affected by epistatic effects of the locus on chromosome 5. The effects of the additional QTLs for resistance are small, but early maturing genotypes that necessarily have the allele for susceptibility for late blight on chromosome 5 may benefit from the resistance that is provided by these QTLs on chromosomes 3 and 10. Abbreviations: AUDPC: area under the disease progress curve; QTL: quantitative trait locus Introduction Potato (Solanum tuberosum) production throughout the world is threatened by late blight epidemics that are caused by Phytophthora infestans. Devastating out- breaks of the disease in North America and Europe in the 1840s initiated breeding for late blight resis- tance (Wastie, 1991), which at first resulted in intro- gression of race-specific resistance (R) genes from S. demissum (Ross, 1986). Unfortunately, this race- specific approach turned out not to be durable, because compatible races of P. infestans appeared rapidly and are now present for (combinations of) all 11 known R genes (Turkensteen, 1993). Race-non-specific foliage resistance to late blight in potato appears to be more durable (Thurston, 1971), though true race-non- specificity is impossible to prove (Johnson, 1979). This type of resistance is characterised by a continuous vari- ation in phenotypic appearance and a polygenic inheri- tance that complicate breeding considerably (Umaerus, 1970). The use of molecular markers has provided breed- ers with more knowledge of the genetic background of race-non-specific foliage resistance against P. infestans in potato. Quantitative trait loci (QTLs) for late blight resistance have been identified in several studies (e.g. Leonards-Schippers et al., 1994; Meyer et al., 1998), of which the joint results suggest that all 12 potato