High soil moisture reduces common scab caused by Streptomyces turgidiscabies and Streptomyces europaeiscabiei in potato

Common scab (Streptomyces spp.) is a worldwide economic important disease of potato (Solanum tuberosum L.) because it degrades the appearance of the tubers and thereby decreases market value. For the newly reported Streptomyces turgidiscabies and Streptomyces europaeiscabiei in Norway, little is known about their specific environmental requirements, particularly their pathogenicity under varying moisture levels. Scab development after soil inoculation of S. turgidiscabies and S. europaeiscabiei in dry, normal and wet soils was studied in three pot experiments (2009-2011) under controlled climate conditions at natural light in Tromso, Norway (69.7 N). With increased soil moisture, statistical significant reductions in scab severity were found in two out of three experiments for S. turgidiscabies (about 50% reduction). Similarly, scab caused by S. europaeiscabiei was reduced significantly in one experiment (65% reduction), and with a tendency of reduction in the other experiments. Scab development from seed-transmitted inoculum was investigated for both species. In one trial with four scab severity levels on mother tubers (0%, 10%, 30% and 50% of surface covered), we found that seed-transmitted disease development occurred for both species although few lesions were recorded. There were no significant differences in scab severity on progeny tubers, no matter the infection levels on seed tubers. These results indicate that seed tubers might play a minor role for transmitting common scab for S. turgidiscabies and S. europaeiscabiei, but further work is warranted. Related to growers practice, the results implicate that potato common scab caused by S. europaeiscabiei and S. turgidiscabies may be reduced by irrigation during the early stages of tuber formation, similar to currently recommended control measures for Streptomyces scabies.