Mapping late blight resistance in Solanum microdontum Bitter

A diploid Solanum population was developed with the objective of mapping quantitative trait loci (QTL) associated with resistance to Phytophthora infestans (Mont.) de Bary, the causal organism of late blight. The mapping population was a cross between a late blight resistant selection of Solanum microdontum Bitter and a susceptible diploid breeding clone. The progeny of 109 clones and the parents were field tested for foliar late blight reaction in 1999 and 2000 and for vine maturity in 2000 and 2001. Parents and progeny were genotyped with isozymes and simple sequence repeats (SSR) markers. A total of 161 pairs of SSR primers were screened, from which 74 amplified polymorphic bands in Metaphor agarose or polyacrylamide gels yielding a total of 109 SSR loci. The isozyme and SSR combined analyses resulted in a total of 118 marker loci in the population, of which 54 were heterozygous (band present) in S. microdontum an 64 in MSA133-57. High phenotypic correlation (r = 0.89, P < 0.0001) was found for late blight reaction between years and no correlation was found between late blight and vine maturity. Solanum microdontum transmitted high levels of resistance to more than 50% of the offspring. There was a major QTL linked with the SSR marker STM0020b associated with foliar late blight resistance located at the same position in both years of field testing explaining over 60% of the phenotypic variance. This major QTL is suitable for marker-assisted selection to introgress a new source of resistance to P. infestans to the cultivated tetraploid germplasm of potato.