Screening lettuce germplasm for new sources of resistance to corky root

Corky root is a major disease of lettuce (Lactuca sativa L.) observed in many production areas of the world. The pathogen Sphingomonas suberifaciens (van Bruggen et al.) Yabuuchi et al. varies with regard to virulence, and several strains have been isolated that can cause disease symptoms even on cultivars that have the only known resistance gene, cor. It is desirable to find new sources of resistance to diversify the genetic basis of the resistance and to confer resistance against isolates that are not adequately controlled by cor. More than 1000 plant introduction lines and cultivars were screened in assays conducted in the greenhouse, growth chamber, and field. Three L. serriola L. lines (PI 491239, PI 491096, and PI 491110) and a L. virosa L. line (PI 273597c) were highly resistant to corky root in all tests. Disease severity ratings in the field were correlated with the ratings in the greenhouse (r = 0.722) and in the growth chamber (r = 0.650). Significant genotype x environment interactions were observed for corky root severity. None of the four resistant lines had the two molecular markers closely linked to the cor allele. The information on disease resistance for these genotypes will be useful in future breeding work.