Infection of plant material derived from Solanum acaule with Clavibacter michiganensis ssp. sepedonicus: temperature as a determining factor in immunity of S. acaule to bacterial ring rot

The development of potato ( Solanum tuberosum ) cultivars immune (completely free of infective agents) to bacterial ring rot disease, caused by Clavibacter michiganensis ssp. sepedonicus , would be a significant step toward eradication of the disease. Earlier results suggested that the wild potato species Solanum acaule , acc. PI472655 (genotype 7-8), was immune to C. michiganensis ssp. sepedonicus . The goal of the present study was to investigate the possibility of transferring this desirable trait to cultivated potato through interspecific somatic hybridization. Eight different somatic hybrids between S. acaule and S. tuberosum , with three different genome ratios, were tested for susceptibility to infection in the glasshouse using two C. michiganensis ssp. sepedonicus strains. All the somatic hybrids expressed symptoms of ring rot and were susceptible to infection as determined by IFAS (indirect immunofluorescent antibody staining) tests. The genome compositions of the hybrids influenced bacterial titres. Most of the hybrids with a higher proportion of the S. acaule genome contained lower numbers of C. michiganensis ssp. sepedonicus cells than hybrids with lower proportions of the S. acaule genome. In growth chamber tests, temperature was found to be a determining factor in the expression of immunity in the tetraploid S . acaule 7-8 line. At 21°C, S . acaule 7-8 was immune to infection by C. michiganensis ssp. sepedonicus , but at 15°C, plants supported large populations of the pathogen. However, none of the S. acaule plants expressed disease symptoms. Thus, S. acaule exhibits temperature-dependent immunity to infection by C. michiganensis ssp. sepedonicus and should be considered tolerant rather than immune.