First report of latent infection of Cyperus rotundus caused by a biovar 3 Dickeya sp.(Syn. Erwinia chrysanthemi) in Israel

Recent outbreaks of potato blackleg in Israel, caused by Dickeya spp., on plants grown from seed tubers imported from Northern Europe, are of a great concern. The warm climatic conditions during the growing season favour disease expression, and may result in the establishment of the pathogen in the potato ecosystem and a spread to weeds and other crops (Tsror et al., 2009). Until recently, most of Dickeya spp. strains found in association with potato blackleg in Europe were characterised as D. dianthicola (biovars 1 or 7). These strains have a relatively lowgrowth temperature compared with other Dickeya spp. and seem to be more adapted to cool European climate conditions. However, during the last three years, Dickeya spp. strainsbelonging to a new biovar 3 clade, probably constituting a newspecies, have been isolated from potato tubers inseveral countries in Northern Europe (Sławiak et al., 2009 and references therein).

To study dissemination to weeds, surveys were conducted in potato fields where Dickeya-infected potato plants were detected during two consecutive spring seasons (2009 and 2010). Symptomless plants of 12 species of local weeds were randomly collected: Cyperus rotundus,Orobanche aegyptiaca, Amaranthus spinosus, Polygonum equisetiforme, Chenopodium sp., Heliotropium sp., Centaurea iberica, Sorghum haepense, Malva nicaeensis, Cynodon dactylon, Amaranthus blitum and Solanum elaeagnifolium. Roots or tubers (in the case of C. rotundus)of 15 plants of each weed were washed, surface sterilised and then sample homogenates were plated on crystal violet pectate medium (CVP). Cavity forming bacteria were transferred to nutrient agar and analysed. Dickeya spp. were isolated only from the perennial weed C. rotundus.Incidence of infected plants was 6.7 and 14.3% in 2009 and 2010, respectively). DNA extracted from the bacteria isolated from C. rotundus was positive in a PCR amplification procedure using pelADE specific primers which are specific for E. chrysanthemi (Nassar et al., 1996).Isolates were identified by biochemical assays as biovar 3 (Palacio-Bielsa et al., 2006). They were characterised as the new genetic clade, using dnaX sequence (Sławiak et al., 2009) and REP-PCR analyses (Tsror et al.,2009). These isolates caused maceration of potato tubers at 30°C (Laurila et al., 2008) and formed clear haloes on a polygalacturonic acid medium (Collmer et al.,1988). This is the first report of Dickeya spp. latent infection in one of the most prevalent weeds in potato fields in Israel. C. rotundus is difficult to control because of resistance to most herbicides. Therefore, it may serve as an alternative host for Dickeya spp. allowing the pathogen to survive in the absence of a host crop.