Effects of one cycle of recurrent selection for early blight resistance in a diploid hybrid solanum phureja-S. stenotomum population

Early blight, caused by Alternaria solani, is an important foliar disease in potatoes around the world. Heritable early blight resistance was previously identified in a population of Solanum phureja-S. stenotomum (phu-stn). Seventy-two clones, consisting of the most early-blight resistant clone from each of the 72 phu-stn families in this population, were randomly intermated and four horticulturally good clones from each family were selected to constitute the second cycle population. The objective of this study was to determine if this recurrent selection strategy would improve levels of early blight resistance in the second cycle population. Clones from the second cycle population were evaluated in a replicated field trial for two years in Pennsylania, USA. Area under the disease progress curve was calculated from estimates of the percentage of necrotic lesions and defoliation. Broad-sense heritability for resistance was 0.75, with a 95% confidence interval of 0.69-0.81. Narrow-sense heritability was 0.63 ± 0.29. However, the second cycle population was more susceptible and later in maturity than the first cycle population. Because of the negative association between earliness and resistance to early blight, it is likely that selecting horticulturally good clones to constitute the second cycle population favored susceptibility. Alternate breeding strategies to improve early blight resistance will have to be devised.