Abstract/Summary

Aphids are a major pest to the agricultural and horticultural industry, causing significant yield losses by directly feeding on crop plants and acting as vectors for plant diseases. Insecticide resistance and concern for the environment and human health has placed growers under increasing pressure to find alternative methods of aphid control. Pseudomonas poae PpR24 is a novel potential biocontrol agent with plant-growth promoting properties, proven to be naturally virulent to aphids as well as act as a deterrent when sprayed on a plant. This project furthers our understanding of the impact of PpR24 on the green peach potato aphid, Myzus persicae, in a sweet pepper, Capsicum annumm L., model system. An experimental evolution approach was applied in a bid to evolve PpR24 isolates with improved aphid virulence and biofilm formation, traits that may be desirable in a biocontrol agent. After ten passage cycles, no improvement was seen in aphicidal properties but one isolate, PpR24b4, evolved significantly strong biofilms at a cost to aphid virulence, bacterial growth and motility. Whole genome sequencing and variant calling analysis of the wild-type and derived isolates found single nucleotide polymorphisms in the genes wspF and gacS of PpR24b4. Both genes have known associations with biofilm formation and secondary metabolite production, which may explain the phenotypic differences observed between the wild-type and biofilm-forming isolate. To elucidate PpR24s deterrent effect and investigate any changes to volatile emissions as a result of the evolutionary passages, Solid-Phase Microextraction and Dynamic Headspace Extraction with GC-MS were used to identify the volatile organic compounds (VOCs) produced by PpR24 in a spray suspension and when applied C. annuum plants. Anti-microbial and plant-growth promoting VOCs were identified from the bacterial suspensions, whereas green leaf volatiles used in plant defence and signalling aphid natural enemies were identified from plants sprayed with PpR24. Significant differences were observed in the compound emissions of the biofilm-forming isolate, PpR24b4, when compared to the wild-type PpR24. Non-target effects of PpR24 on three species of commercially available aphid natural enemies was investigated by simulating likely routes of exposure. PpR24 had no significant effect on the mirid bug, Macrolophus pygmaeus Rambur, although significant effects were observed on Aphidius colemani Viereck, and Orius laevigatus Fieber, depending on the route of exposure. The data presented in this study furthers our understanding of how PpR24 could be applied in an integrated pest management system to prevent and control aphid infestations.