First report of Xanthomonas axonopodis pv. poinsettiicola, the bacterial leaf spot pathogen on Euphorbia pulcherrima in Austria

Abstract

In September 2007 symptoms typical of bacterial leaf spot were observed on pot-grown Euphorbia pulcherrima in a commercial plant nursery in Carinthia, Austria. Leaf symptoms were dark to rust-brown spots with yellow haloes that sometimes coalesced to larger areas. Isolation of bacteria was performed on yeast agar (YPGA) and King's B Agar and colonies were more or less yellow and glossy. A strain of X. axonopodis pv. poinsettiicola (LMG849) from the Belgian Coordinates Collection of Microorganisms and a strain of X. campestris pv. vesicatoria (DSMZ 50861) from the German Collection of Microorganisms and Cell Cultures were used as controls. Identification was performed using two PCR assays, which combined allowed precise identification of the pathogen. Three pairs of oligonucleotide primers specific for different hrp gene regions of several pathovars from the X. campestris/axonopodis cluster were used for identification to species level (Leite et al., 1994). The 1075 bp amplicon described by Leite et al. (1994) was digested with HaeIII and Sau3AI, producing restriction patterns similar to those of X. campestris pv. vesicatoria and X. axonopodis pv. poinsettiicola. Additionally the PCR amplicons (1075 bp) of the isolated strain and the reference strain (LMG849) were sequenced. Phylogenetic sequence comparison grouped the sequence from the isolated strain with that from strain LMG849, lending further support to identification as X. axonopodis pv. poinsettiicola. A second PCR assay specific for X. campestris pv. vesicatoria designed to amplify the rhs family gene (Park et al., 2007) was negative. We therefore concluded that the bacteria isolated from diseased E. pulcherrima plants was X. axonopodis pv. poinsettiicola. Pathogenicity was confirmed by inoculating young E. pulcherrima plants with bacterial suspensions. After two weeks bacteria were re-isolated from newly emerged leaves. Isolates were identified as X. axonopodis pv. poinsettiicola using the molecular methods described above. Combining these two PCR assays, X. axonopodis pv. poinsettiicola isolates could be identified from leaves by amplification of DNA extracted from diseased material. This is the first report of X. axonopodis pv. poinsettiicola in Austria. An outbreak of this disease in nurseries would have a major economical impact, since E. pulcherrima is produced in yearly quantities of approximately 3·5 million units. The authors would like to thank Mr. Kelvin Hughes for his helpful comments on this manuscript.