Mapping Late Blight Resistance in Solanum microdontum Bitter

foliar late blight resistance in other populations (Collins et al., 1999; Sandbrink et al., 2000). Association between A diploid Solanum population was developed with the objective QTL conferring foliar late blight resistance, tuberizaof mapping quantitative trait loci (QTL) associated with resistance tion, and vine maturity was found in four out of five to Phytophthora infestans (Mont.) de Bary, the causal organism of chromosomes (Ewing et al., 2000). The knowledge about late blight. The mapping population was a cross between a late blight resistant selection of Solanum microdontum Bitter and a susceptible late blight resistance associations with undesirable traits diploid breeding clone. The progeny of 109 clones and the parents is also of value before introgressing a new source of rewere field tested for foliar late blight reaction in 1999 and 2000 and sistance. for vine maturity in 2000 and 2001. Parents and progeny were genoThe South American diploid species S. microdontum typed with isozymes and simple sequence repeats (SSR) markers. A has shown high levels of resistance to late blight (Colon total of 161 pairs of SSR primers were screened, from which 74 ampliand Budding, 1988; Colon et al., 1995a, 1995c; Douches fied polymorphic bands in Metaphor agarose or polyacrylamide gels et al., 2001). Among 618 clones representing 24 accesyielding a total of 109 SSR loci. The isozyme and SSR combined sions, 56 clones were selected as highly resistant to the analyses resulted in a total of 118 marker loci in the population, of US8 genotype of P. infestans, from which 27 clones which 54 were heterozygous (band present) in S. microdontum and represented three accessions of S. microdontum (Douches 64 in MSA133-57. High phenotypic correlation (r 0.89, P 0.0001) et al., 2001). Dominant gene action was identified in was found for late blight reaction between years and no correlation was some crosses between S. microdontum and susceptible found between late blight and vine maturity. Solanum microdontum clones (Colon et al., 1995c). Strong hypersensitive reactransmitted high levels of resistance to more than 50% of the offspring. tion or infection efficiency, lesion growth rate, and sporThere was a major QTL linked with the SSR marker STM0020b associated with foliar late blight resistance located at the same position ulation time were associated with high levels of resisin both years of field testing explaining over 60% of the phenotypic tance in S. microdontum (Colon et al., 1995a). variance. This major QTL is suitable for marker-assisted selection to The purposes of this research were to map QTL assointrogress a new source of resistance to P. infestans to the cultivated ciated with late blight resistance from a S. microdontumtetraploid germplasm of potato. derived population and to examine the association between late blight resistance and late maturity. I the mid-1990s, the USA and Canada experienced a late blight epidemic caused by new, more aggresMATERIALS AND METHODS sive, and metalaxyl-resistant races (Goodwin et al., 1995; Selection of Resistant Parent and Mapping Population Peters et al., 2001). The development of genetic resistance has become a major strategy for late blight control A total of 175 clones representing six accessions of S. microin cultivated (2n 4x 48) potatoes (Solanum tuberodontum were tested in the greenhouse with US8 genotype/ sum L.) and a priority in many breeding programs (CoA2 mating type of P. infestans in 1997. The most resistant clones were retested in 1998 and 27 highly resistant clones lon et al., 1995a), with horizontal (field, partial, or genwere selected (Douches et al., 2001), from which one clone eral) resistance being the only durable type of resistance (PI595511-5) was selected as the late blight resistant parent to late blight (Colon et al., 1995b; Umaerus et al., 1983; to develop the mapping population. The selected clone was Kamoun et al., 1999). very distinct from cultivated potato, from S. berthaultii Diploid (2n 2x 24) and tetraploid populations (Hawkes), and from other clones of S. microdontum with have been used for QTL associated with late blight reported late blight resistance (Bisognin and Douches 2002). resistance introgressed from a variety of wild species The diploid mapping population chosen was a cross between of potato. Genetic mapping at the diploid level avoids the Michigan State University (MSU) late blight susceptible interpretation problems associated with tetrasomic inbreeding clone MSA133-57 [(S. tuberosum S. chacoense) heritance (Meyer et al., 1998). The results of mapping S. phureja] with S. microdontum PI595511-5. A progeny of 109 clones that produced tubers at greenhouse conditions QTL associated with late blight resistance showed that (16-h day-length with supplemental lighting from high presthose QTL could also be associated with other traits. sure sodium lamps) was used for field phenotypic evaluations This is the case of chromosome V on which QTL were and molecular analysis. Greenhouse tubers were used to plant mapped for foliar and tuber late blight resistance, vine the late blight trial of 1999 at the MSU Muck Soils Research maturity and vigor (Oberhagemann et al., 1999), and Farm, Bath, MI, and the seed increase field at the MSU Montcalm Research Farm, Entrican, MI. Late blight trial of 2000 and maturity trials were planted with field-increased seed D.A. Bisognin, Dep. of Fitotecnia, Federal Univ. of Santa Maria, Santa Maria, RS, 97105-900, Brazil; D.S. Douches, L. Buszka, and D. tubers. Wang, Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI, 48824; G. Bryan, Scottish Crop Research Institute, InverLate Blight Reaction in Field Tests gowrie, Dundee, Scotland. Received 23 Apr. 2003. *Corresponding author ([email protected]). The P. infestans isolates collected in Michigan (MS94-1, MS94-4, MS95-7, and MS97-2) were characterized as US8 Published in Crop Sci. 45:340–345 (2005). genotype/A2 mating type as described in Bisognin et al. (2002). © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Those isolates were infectious on all R-gene differentials ex-