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Populations of Rhizomonas suberifaciens on Roots of Host and Nonhost Plants. R. Douglas O’Brien, Department of Plant Pathology, University of California, Davis 95616; Ariena H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 81:1034-1038. Accepted for publication 24 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1034.

The influence of plant species and cultivar on populations of Rhizomonas suberifaciens (causal agent of corky root of lettuce) on roots was investigated in field and growth chamber experiments. Populations on the roots of resistant and susceptible cultivars (440-8 and Salinas, respectively) of iceberg lettuce (Lactuca sativa L.) grown in microplots were measured over time by an enzyme-linked immunosorbent assay (ELISA). Populations recovered from roots decreased over time and ranged from 4 × 103 to 4 × 105 cfu/g of fresh root. Populations recovered from Salinas roots were equal to or higher than those recovered from 440-8. In growth chamber experiments, populations of a rifampicin-resistant strain of R. suberifaciens on the roots of these cultivars were compared with those of prickly lettuce (Lactuca serriola L.) and a nonhost (barley, Hordeum vulgare L.). Root populations were determined by dilution plating on rifampicin-amended medium 6, 12, 18, and 24 days after inoculation. At all harvest dates, the susceptible lettuce cultivar Salinas contained higher populations than the other plant species and cultivars. At three harvest dates, barley had lower densities than the other plant species and cultivars. Prickly lettuce and the resistant lettuce cultivar 440-8 had similar populations at all but one harvest date. Populations of R. suberifaciens on roots of potted plants fluctuated over time and did not fit significantly to any simple model. The results support previous observations that 440-8 and prickly lettuce are resistant to R. suberifaciens because they supported lower populations of the bacterium. R. suberifaciens did not increase on barley roots but survived for at least 24 days.