| Abstract: | Screening large collections of microorganisms for potential biological control activity on economically important weeds is often diY cult. Seeds required as indicator species are often not available in large supply and are highly variable in viability and germination, resulting in uneven seedling development. A bioassay system for rapidly assessing the phytotoxic eVects of rhizobacteria was developed based on leafy spurge ( Euphorbia esula L. ) callus tissue culture in multiple-well plates. Callus pieces (0.5 g) were placed in 24-well plates containing Gamborg's B5 medium, inoculated with rhizobacterial suspension and incubated at 27oC for 48 h. By rating inoculated callus for cellular damage, about 30% of rhizobacteria isolated from weedy Euphorbia spp. collected in Europe and North America were identified as being highly phytotoxic. Symptoms of phytotoxicity included growth reduction, discoloration and extensive cellular leakage. A high proportion of isolates found to be phytotoxic in tissue culture bioassays were similarly eVective in standard leafy spurge seedling bioassays. The method is rapid, host specific and more uniform compared with assays using seedlings, and should be adaptable to other weed species for screening microorganisms for potential biocontrol activity. |
