An assay to determine the relative virulence of wild-type and mutant strains of Agrobacterium tumefaciens on leaves of Bryophyllum daigremontiana has been developed. This assay is reproducible, is easy to learn, is not time consuming, and requires little space. The relative virulence of cellulose-minus mutants of A. tumefaciens was investigated with this assay. Some of these mutants were unaltered in virulence, while others showed a marked reduction in virulence. The wound sites were scored as positive or negative for tumor formation once a week for 6 weeks. Tumors began to form on the plants after 1 or 2 weeks; new tumors ceased to form after 4 to 6 weeks.We found that the best way to quantitate bacterial virulence in this assay was to determine the number of bacteria needed to form tumors at 50% of the inoculated sites. Other parameters, such as tumor weight, diameter, and length of time to form tumors, were explored but were not found to give a linear response as the number of bacteria inoculated increased. Figure 1 shows the effect of changes in the number of

No abstract