Assessment of solatium hougasii in Washington and Mexico as a source of resistance to late blight

A segregating introgression population, established by crossing an accession of Solanum hougasii from the central highlands of Mexico with two successive recurrent corky ringspot resistant parental lines, was screened against Phytophthora infestans . Foliage and tuber reactions were compared under natural epidemics of US-8 and US-11 P. infestans at Mount Vernon, Washington, natural epidemics at Toluca, Mexico, and by laboratory assays with US-8 and US-11 P. infestans at Pullman, Washington. Relative area under disease progress curve (RAUDPC) values in the field ranged from 3 to 80 and 2 to 42 for the 1998 and 1999 populations at Mount Vernon, respectively, and from 5 to 63, 2 to 79, and 4 to 76 in 1998, 1999 and 2000 for the populations at Toluca, respectively. Of the progeny lines tested during 1998 and 1999 at Mount Vernon, 7% were resistant, 60% were intermediate, and 33% were susceptible. Of those tested during 1998, 1999, and 2000 at Toluca, 33, 31, and 36% were resistant, intermediate and susceptible, respectively. RAUDPC values in the field at Mount Vernon and Toluca were significantly ( P = 0.0001) correlated. Late blight severity on detached leaflets inoculated with US-8 and US-11 P. infestans in the laboratory ranged from 0 to 64% or 65%, respectively. Severity of infection on inoculated tubers ranged from 0 to 68% for US-8 and 0 to 80% for US-11. Disease severity on leaflets in laboratory tests was significantly correlated with field RAUDPC values, but tuber severity in laboratory tests was not, although some lines exhibited resistance in both the foliage and tubers. Foliar resistance in the field was characterized by leaf chlorosis, as well as limited lesion expansion and sporulation. Estimate of broad-sense heritability was relatively high. Only 15% of this BC 1 population showed significant instability, giving little indication of specific interactions between genotypes and populations of the P. infestans pathogen that would be indicative of R-gene interactions. The introgression population (BC 1 ) appears to be expressing highly heritable durable resistance. The high heritability estimate suggests that utilization of highly resistant and stable BC 1 genotypes, such as 53.78, as a parental source of foliar late blight resistance will transmit substantial nonrace specific genetic resistance to future progenies, and that ultimately, after several cycles of backcrossing and selection, this genetic potential could be deployed in new potato cultivars.