Resistance toPseudomonas solanacearum in potato: Infectivity titrations in relation to multiplication and spread of the pathogen

When several wilt-resistant potato clones were tested against a highly virulent strain from Mexico (No. 276) by standard stem inoculation methods, only one clone ofSolanum phureja was resistant. When different inoculum concentrations were introduced quantitatively for infectivity titrations, however, different levels of resistance were clearly defined among clones previously rated as susceptible. Linear regression was used to estimate the ED₅₀ values (dosage required to wilt 50% of the population) for each of seven clones. These values ranged from 3 and 100 colony-forming units (CFU) for Katahdin and Russet Burbank, respectively, to 2.1 x 10⁶ CFU for S.phureja clone 1386.15. The distribution of bacteria in stems of Russet Burbank plants inoculated with the compatible strains 276 and K-60 and the incompatible strains B1 and S-210 was very different. In all cases, incompatible bacteria were not detected 10 cm above the inoculation point in the stem, even by 12 days after inoculation, whereas compatible strains multiplied rapidly at this site by 6 days. At the inoculation site and at sites immediately adjacent to it, however, incompatible bacteria attained populations that only differed by one order of magnitude from those of compatible bacteria. Similar results were obtained when a resistant clone (MS 118.24) was inoculated with the virulent strain K-60. Thus, resistance was characterized both by reduced acropetal spread and by tolerance to large numbers of the bacterium.