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Dickeya fangzhongdai causing soft rot of Phalaenopsis orchids and bacteriophage biocontrol options

Špela Alič: National Institute of Biology

<div>Bacterial soft rots represent a serious limitation to the production of ornamental and others horticultural plants world-wide e.g. potatoes, tomatoes and orchids, with no effective chemical control. The aim of our study was to isolate and characterize bacteria causing soft rot of orchids (<em>Phalaenopsis</em> sp.) and to investigate bacteriophages as biocontrol agents for their management.<br /="/">Bacteria isolated from diseased tissues of Phalaenopsis orchids from a commercial production site were identified as <em>Dickeya</em> spp.. However, none of the performed tests was able to resolve classification to a species level. Therefore, genomes of two representative <em>Dickeya</em> sp. isolates were sequenced. As part of a comparative genomic study of genus Dickeya, average nucleotide analysis, DNA-DNA hybridisation and multilocus sequence analysis on six housekeeping genes were performed, placing the newly sequenced <em>Dickeya</em> to a recently described <em>D. fanhzhongdai</em>. To confirm the observed taxonomic position of our <em>Dickeya</em> isolates the phenotypic analysis was performed. Aggressiveness of the bacteria was tested phenotypically on the model plants, Belgian chicory and potato tubers, exhibiting extreme maceration. Beside, inoculation of the viable potato plants was performed, showing the bacteria ability to survive and spread through the plant tissue. On the other hand, possible approaches for the Dickeya biocontrol were studied. Bacteriophages effective against new <em>Dickeya</em> strains were isolated from the diseased orchids and sewage water. Three different groups of bacteriophages from <em>Caudovirales</em> family were identified. All bacteriophages were lytic and specific for the genus <em>Dickeya</em>, therefore represented suitable candidates for phage biocontrol.</div>