First Report of Fire Blight on Pear Trees Caused by Erwinia amylovora in Kurdistan Province, Iran
Authors: Mollaei S. N. , Harighi B. 
Published: December 2012

Pear (Pyrus L.) is one of the most widely grown crops in western Iran. Since 2010, an outbreak of a disease with symptoms similar to fire blight has been observed on pear trees in various locations of Kurdistan Province. Initial flower symptoms include water-soaking and rapidly shriveling, infected flowers that remained hanging on the trees. Immature fruits become water-soaked, turned brown, and shriveled. Infected flowers and immature fruits were collected from different locations in the province. Small pieces (about 1 mm2) were excised from infected tissues, surface sterilized with 0.5% sodium hypochlorite solution, followed by rinsing in sterile-distilled water (SDW). Each piece was macerated in 2 to 3 ml of SDW, streaked onto nutrient agar sucrose or eosin methylene blue agar media, and incubated at 27 to 29°C. After 48 to 72 h, single colonies were subcultured onto the same media and stored at 4°C. In total, 74 bacteria were isolated from infected tissues. All isolates were gram-negative and rod-shaped. Based on other phenotypic properties, strains were grouped into three clusters at a similarity level of 65% (data not shown). Forty-one and 23 strains showed properties as expected for Erwinia amylovora and Enterobacter sp., respectively. Other strains showed properties resembling Pantoea agglomerans. All strains identified as E. amylovora produced an expected DNA fragment of about 900 bp by PCR using primers PE29A and PE29B corresponding to plasmid pEA29 (1). The result was confirmed by using primers AMSbL and AMSbR derived from the ams region required for amylovoran synthesis of E. amylovora. E. amylovora strains produced an expected 1,600-bp fragment (2). For the pathogenicity test, a bacterial suspension was adjusted to approximately 1 × 107 CFU/ml from cell cultures grown in nutrient broth at 27°C for 48 h. Immature pear fruits sterilized with 70% ethanol and rinsed with SDW were injected with the bacterial suspension using a 25-gauge sterile needle. Fruits injected with sterile water were used as controls. Pear fruits were kept in a mist chamber at 27 to 29°C. Symptoms were assessed up to 2 weeks after inoculation. All E. amylovora strains produced typical symptoms on inoculated immature pear fruits. Necrosis and oozing of bacterial exudates were observed after 3 to 7 days. The phylogenetic position of two selected strains was analyzed by sequence comparison of recA gene among other species in the genus Erwinia and related bacteria. The recA sequence of bacterial strains identified as E. amylovora revealed high similarity (99%) to the E. amylovora type strain (CFBP 1430). Genetic diversity of selected strains was assessed and compared with E. amylovora reference strain CFBP 1430 using ERIC and REP primers in rep-PCR analysis. (3). UPGMA cluster analysis of the combined data obtained in the rep-PCR experiments using Dice's coefficient revealed that the majority of E. amylovora strains showed the same fingerprint patterns at a similarity level of 93%, indicating genetic homogeneity among strains but clearly separated from Enterobacter sp. and P. agglomerans strains. To our knowledge, this is the first report that characterizes the phenotypic and genetic properties of E. amylovora in western part of Iran.

References: (1) S. Bereswill et al. Appl. Environ. Microbiol. 58:3522, 1992. (2) S. Bereswill et al. Appl. Environ. Microbiol. 61:2636, 1995. (3) J. Versalovic et al. Mol. Cell Biol. 5:25, 1994.