Submitted to: Microbial Ecology Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 5/8/2006
Publication Date: N/A
Interpretive Summary: The perennial invasive weed leafy spurge has an extensive root system and attains dense monocultures in extensive stands. Biological control occurs through feeding of larvae of a flea beetle on roots of the weed. It has been shown by one of the authors that the feeding and subsequent root leakage and root tissue damage stimulates fungal infection and ultimately higher soil levels of fungi that can extensively invade and kill the plants of leafy spurge. This would change the fungal community composition. In turn such changes may affect patterns of plant succession after biocontrol. As a companion to the results with fungi, the present study was done to examine patterns of bacterial community structure of the rhizosphere of leafy spurge. Rhizosphere soils of spurge stands with Aphthona present had large differences in community composition compared to sites without insect presence. For example, soft rotting Erwinia species were exclusive to sites with insects and such species as Pseudomonas syringae were found with much greater frequency in rhizospheres of spurge stands affected by the flea beetle. Thus, biocontrol also affects the bacterial community composition of soils of leafy spurge and such findings indicates that this community may also play a role in determining the plant succession patterns following weed biocontrol as well the success of any restoration efforts at reestablishing desirable native plant communities once substantial biocontrol of leafy spurge is attained.
Technical Abstract: The invasive perennial plant of Eurasian origin Euphorbia esula/virgata has been successfully controlled over large areas in North America with a synergism between larvae of Aphthona spp. and soilborne plant pathogens. However, a multitude of sites is not yet under control. Possible effects of rhizosphere bacteria on biocontrol success were investigated by comparing rhizosphere communities of bacteria associated with E. esula/virgata populations with a resident population of Aphthona with those without detectable insect presence. Hypotheses investigated were 1) that insect herbivory accelerates negative feedback or 1A) shifts in prokaryote community structure and that 2) E. esula/virgata rhizospheres at sites with insects but without biocontrol impact harbored distinct communities or constituent species. Sites representative of high biocontrol impact or a low level of impact were assayed by spiral plating root washes of E. esula/virgata, selecting colonies from the most dilute portion of the spiral (deemed as predominant), and with resulting pure cultures obtaining the results of GN or GP Biolog procedures or GC-FAME analysis to identify the isolates and further characterize community structures using principle component analysis. Bacterial species predominant at the unimpacted site included Stenotrophomonas maltophilia, Pseudomonas chlororaphis and P. putida. At biocontrol sites exhibiting rapid stand declines, high percentages were Bacillus spp. and several coryneform species. Some of the isolates from the impacted site tested in on E. esula/virgata caused ca 65% reduction in biomass in combination with Aphthona. Overall, at sites with insect presence, isolates identified as Pseudomonas syringae and Erwinia carotovora were exclusive constituents of rhizosphere communities.