Genetic variance models for the evaluation of resistance to powdery scab (Spongospora subterranea f. sp. subterranea) from long-term potato breeding trials

Breeding for resistance to soil-borne powdery scab in potato is an important component of the integrated management of this disease. Different genetic variance models within a mixed model framework were applied to data from long-term potato breeding trials, for the genetic evaluation of breeding lines. The multi-environment trial (MET) data came from 12 growing seasons (“years”, synonymous with environments) of New Zealand field trials screening for resistance to powdery scab on potato tubers. Pedigree information on a total of 1,031 genotypes was available. Additive components of the genetic effects were important with narrow-sense heritability estimates (and 95 % confidence intervals) from single-year analyses ranging from 0.26 (0.20, 0.44) to 0.57 (0.53, 0.85). Spatial components estimated from the residual plot effects were not important for most years and even when they were significant, estimates were small. In MET analyses, different variance structures for the genetic effects were tested; a homogeneous correlation model (CORH) gave a better fit to the data than a factor analytic FAk model of order (k), 1 and 2. The year-to-year genetic correlation estimate from CORH was 0.81 and compared with a range of 0.59–0.95 estimated from the FA1 model. There was no strong evidence of non-additive genetic effects with zero or boundary estimates for most years. Models which included the pedigree provided a better fit to the data than models that did not include this relationship information. There was no evidence for genetic improvement in resistance for powdery scab on tubers in the breeding population studied. This suggests that selection pressure for resistance in the past has been weak and greater consideration should be given to selecting parents on empirical breeding values to genetically improve breeding populations for resistance to powdery scab.