The first record of Dickeya zeae in the UK

The reclassification of Erwinia chrysanthemi into six new Dickeya species (Samson et al., 2005) has resulted in the need to confirm identities of the strains held in the National Collection of Plant Pathogenic Bacteria (NCPPB). This process is being done using fatty acid profiling, repetitive sequence PCR and phylogenetic identification based on several partial gene sequences, but primarily the recA gene. Prior to the proposal for Dickeya spp., E. chrysanthemi was classified both by a pathovar system based on host specificity (Bradbury, 1986) and biovar systems based on nutritional and physiological differences. The most widely used biovar system was that of Ngwira & Samson (1990) and Samson et al. (1990). Correlation between the systems was not good.

Two E. chrysanthemi strains isolated from wilting florist chrysanthemum plants (Chrysanthemum morifolium cv. Mayford’s Perfection) in different nurseries in Bedfordshire and Hertfordshire in 1970 were acceded to the NCPPB as E. chrysanthemi pv. chrysanthemi (NCPPB 2339 and NCPPB 2340). Fatty acid profiling using the MIDI system, showed that these strains did not cluster with other E. chrysanthemi pv. chrysanthemi strains including the pathotype reference strain NCPPB 402. Nutritional tests used to differentiate the biovars of E. chrysanthemi gave atypical results for biovar 5, regarded as being synonymous with E. chrysanthemi pv. chrysanthemi (Ngwira & Samson 1990, Samson et al., 1990), but had similarities to biovar 3.Biovar 3 strains have been reclassified largely into Dickeya zeae and Dickeya dadantii (Samson et al., 2005).

A phylogeny based on a 481 bp sequence of the recA gene, placed NCPPB 2339 and 2340 in the same phylogenetic clade as the type strain of D. zeae (NCPPB 2538) and all other maize strains (n=19.) tested from the NCPPB. This result is supported by fatty acid profiling and repetitive sequence PCR, using REP and ERIC primers, of all Dickeya species type strains. Both strains clustered exclusively with D. zeae strains in UPGMA dendrograms for all three assays. These results clearly show that these two strains do not belong to D. chrysanthemi bv. chrysanthemi (Samson et al., 2005) but are typical members of D. zeae.NCPPB 2339 was included in the study of Samson et al. (2005) and shown to be D. zeae but was wrongly quoted as being from the USA.Thus, examination of archival strains using current taxonomic methods has demonstrated that these are the first and currently the only records of D. zeae causing plant disease in the UK. This also highlights the importance of applying up to date identification methods to avoid overlooking significant new findings.