Date of Award






Degree Type

Master of Science



First Advisor

John Davis

Second Advisor

Neha Potnis

Third Advisor

Lauren King


Recent studies have indicated the importance of resident microflora of plants in contributing towards overall plant health. Among difference components of the plant microbiome, Methylobacterium and Sphingomonas have been recognized as common residents of the phyllosphere for many host plants, however their role in disease control needs to be further investigated. The purpose of this study was to conduct experiments investigating the effectiveness of phyllosphere Methylobacterium and Sphingomonas isolated from red clover against common tomato phyllosphere bacterial pathogens, Pseudomonas syringae pv. tomato and Xanthomonas perforans. Additionally, this study uses X. perforans wild-type and X. perforans type VI secretion system (T6SS) mutant strains to observe the role of type VI secretion system in infection with a potential biocontrol agent. To explain interactions among the phyllosphere residents, nutritional similarity, motility and direct inhibition were observed in vitro. Based off the literature, it was hypothesized that Sphingomonas would prove to be a potential biocontrol agent against P. syringae pv. tomato, X. perforans wild-type and T6SS mutant. Methylobacteria would not prove to be a potential biocontrol agent against P. syrinage pv. tomato nor X. perforans wild-type and mutant. Lastly, it was hypothesized that X. perforans wild-type would be more virulent than the X. perforans T6SS when competing with a potential biocontrol agent (red clover commensal).

In planta experiments under growth chamber conditions indicated no significant change in disease with seeds that were soaked in a mix of red clover commensals when dipped in phyllosphere pathogens P. syringae pv. tomato and X. perforans wild-type and T6SS mutant. P. syringae pv. tomato was unable to infect control tomato plants in trial two under growth chamber conditions, and was not tested further in planta. In vitro testing indicated a red clover phyllosphere commensal, S. taxi 55669, inhibited X. perforans wild-type and mutant colonies on R2a plates. Therefore, S. taxi 55669 was studied for disease protection further in the greenhouse with seedling dip experiments. To assess the effectiveness of S. taxi 55669 in planta, foliar disease percent and bacterial population counts were recorded on bacterial dipped seedlings coinoculated with phyllopshere pathogens X. perforans wild-type and T6SS mutant. It is not recommended that S. taxi 55669 serve as a potential biocontrol for P. syringae pv. tomato 99B799 ba ed off the neutral effect S. taxi 55669 had with in vitro testing. Methylobacteria observed in this study, did not show any benefits against disease against P. syringae pv. tomato and X. perforans wild-type and T6SS mutant. However, the results from this study indicate S. taxi 55669 should be studied further for plant health, and has potential as a biocontrol against X. perforans. X. perforans T6SS mutant was found to be less virulent in the presence of S. taxi 55669, than X. perforans wild-type. Based off the high NOI and decrease in foliar disease, this study shows S. taxi 5669 has potential as a biocontrol for X. perforans. The decrease in motility and bacterial populations of X. perforans T6SS mutant when in the presence of S. taxi 55669 highlights the importance of icmF3 in motility and ability to attack resident phyllosphere bacteria. The lack of differentiation between direct inhibition on R2a plates of X. perforans wildtype and T6SS mutant in the presence of S. taxi 55669 indicates a part of the T6SS mutant system may still be functional.

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