Importance of microbial pest control agents and their metabolites In relation to the natural microbiota on strawberry.

Abstract

The main objectives of the this project were to examine the abundance of applied microbial pest control agents (MPCAs) and their metabolites compared to that of the natural microbiota and to examine the compatibility between MPCAs and conventional fungicides and their combination effects in disease control. A series of laboratory, growth chamber, semi-field and field experiments using strawberry as a model plant focusing on commercial microbial pest control products (MPCPs) or laboratory MPCAs expected to be on the market within 10 years served as our experimental platform. Initially the background level of indigenous microbial communities and their mycotoxins/metabolites on strawberries was examined in a field survey with 4 conventional and 4 organic growers with different production practise and geographic distribution. Culturable bacteria, yeasts and filamentous fungi were isolated and identified using both chemotaxonomy (fatty acids and metabolite profiling) and morphological characteristics. Microbial communities on strawberries were complex including potential plant pathogens, opportunistic human pathogens, plant disease biocontrol agents and mycotoxin producers. Bacteria were the most abundant and diverse group of strawberry microbiota followed by yeasts and filamentous fungi. Grower practice did not seem to correlate with the strawberry microbiota. Limited difference between microbial communities on strawberries from conventional and organic production systems was observed. Mycotoxins were not detected in strawberries from any of the 8 different growers covering both fresh and worst-case (semi-rotted) berries. However, filamentous fungi from the genera Penicillium and Aspergillus isolated from the field survey produced high amounts of carcinogenic mycotoxins when applied to strawberries in vitro. A broad range of microbial biocontrol agents (MPCAs) including the commercial Trichoderma based products TRI003, Binab-T and Supresivit and the laboratory MPCAs Clonostachys rosea and Ulocladium atrum were examined concerning their target and non-target effects, production of mycotoxins, fungicide sensitivity and performance (establishment, growth and survival). Among the MPCAs tested only the laboratory MPCAs C. rosea and U. atrum demonstrated biocontrol effects against the strawberry pathogen Botrytis cinerea and the background microbial community was unaffected by both C. rosea and U. atrum. None of the fungal MPCAs produced any mycotoxins when applied to flowers in semi-field and field experiments, but strawberries artificially inoculated with Trichoderma-based MPCPs in vitro contained biologically active fungal metabolites of the peptaibol family. 8 In general, fungicides employed in conventional strawberry production had no fungicidal effects on any of the MPCAs. Establishment of Trichoderma (TRI003) on berries following flower application was poor and applying fungicides prior to the MPCA application did not result in improved MPCA performance. In conclusion, the relative importance of deliberately released fungal MPCAs and their metabolites in relation to that of the natural strawberry microbiota seem to be limited when considering the potential risk from the natural microbiota including mycotoxin producers and opportunistic human pathogens. In addition, our results suggest that it is possible to combine MPCAs and fungicides in an integrated strategy, potentially reducing the levels of fungicide applications.