Genetic Stability in Potato Germplasm for Resistance to Root Galling Caused by the Pathogen Spongospora subterranea

Spongospora subteranea, the causal agent of potato powdery scab is becoming increasingly important worldwide. Little is known about the genetic basis of resistance to this disease. The present study tested the hypothesis that potato genotypes with stable genetic resistance to Spongospora root galling were present in potato germplasm. Root galling index values of 24 genotypes screened for resistance in four field trials (environments) in 2004 and 2005 in Washington State and Idaho were analyzed. Genotypes tested included five resistant, four industry standards and advanced selections from the USDA-ARS, Prosser, WA program. Broad-sense heritability was calculated as 0.76 with a 95% confidence interval of 0.55-0.89, indicating a fairly high genetic component of the trait. Of the 24 genotypes that were tested, eight showed no genotype*environment interactions while six of the remainder had significant variance (i.e., they were unstable) after removal of genotype*environment variance. Among the five resistant genotypes, PA95B2-4 was stable, and PA98N5-2, PA98NM38-1, P094A009-7 and POROOHG5-1 were stable after the removal of environmental heterogeneity. Among the four industry standards, Shepody was unstable, whereas Ranger Russet, Russet Burbank and Umatilla Russet were stable after the removal of genotype*environment variance. Stable resistance to Spongospora root galling was identified. A large portion of the variation was genetic, which will enable breeders to use resistant and stable potato genotypes as parents in future breeding to develop superior commercial potato cultivars with resistance to Spongospora root galling.