Introgression of late blight resistance from wild species and unadapted germplasm to cultivated potato

Since the mid-1990's, the United States and Canada have experienced late blight (Phytophthora infestans (Mont.) de Bary) epidemics caused by new, more aggressive and metalaxyl resistant races that impose new disease management strategies in potato (Solanum tuberosum L.). Breeding offers the opportunity to identify and release advanced germplasm with late blight resistance. The general objective of this research was to introgress late blight resistance from unadapted germplasm and wild species to cultivated potato. The first effort was to combine late blight resistance from eight (B0718-3, Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan and Zarevo) unadapted cultivars with tuber quality and marketable maturity found in cultivars/advanced breeding clones adapted to North America. A total of 408 field selected clones from 95 crosses were evaluated in single- and eight-hill plots for tuber quality (tuber appearance, specific gravity and chip color). The same clones were assessed for foliar late blight reaction using a mixture of complex races of US8/A2 mating type of P. infestans isolates in greenhouse and field studies in 1998 and 1999. The late blight resistant parents differ in their ability to transmit late blight resistance and tuber quality to the offspring. In addition, late blight resistance can be combined with marketable maturity and tuber quality. Moderate selection intensity for tuber quality traits can be initiated at the single-hill generation before testing for late blight resistance. The second effort was to select, within plant introduction accessions, clones with high levels of late blight resistance in greenhouse. A total of 60 selected clones representing South American species, hybrids between wild and cultivated species and cultivars/advanced breeding clones were than evaluated for their genetic diversity based on isozymes and simple sequence repeats (SSR). There is a high level of genetic diversity within and between accessions, species and ploidy levels of the late blight resistant germplasm from S. microdontum Bitter, S. berthaultii Hawkes and S. sucrense Hawkes that should be introgressed and combined in a breeding. The last effort was to map quantitative trait loci (QTL) conferring late blight resistance and other agronomic traits using isozymes and SSR markers in a S. microdontum derived population. Progeny of 110 clones and parents were field tested for foliar late blight reaction in 1999 and 2000, and for maturity, tuber number and size, yield and tuber quality in 2000. 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 with other evaluated trait. There was only one marker linked with late blight resistance and another trait (tuber size). Solanum microdontum has a QTL associated with foliar late blight resistance that is not associated with late maturity or any poor tuber quality traits that explains 70% of the phenotypic variance of two years of field testing. A SSR marker closely linked to the QTL that can be followed through polyploidization is suitable for using in a marker assisted selection strategy.