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

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.
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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.