Study of Virulence factor of Erwinia amylovora isolates obtained from Semnan province and resistance evaluation of different pear tissues, blossom and immature fruit, in different pear cultivars to Fire blight disease

Fire blight, caused by Erwinia amylovora, is one of the important bacterial diseases of pome fruit trees. It causes the blight of different organs of the tree (blossoms, shoots, leaves, fruits, and limbs) and exudates' production. The main symptoms of fire blight infection are the burnt-like appearance of infected tissues, bacterial ooze released from the infected tissues, shepherd’s crook, wilting, and water-soaked appearance. Recently, it has been included in the top 10 plant pathogenic bacteria. This study aimed to collect, identify and characterize E. amylovora isolates from provinces of Semnan in Iran and determine their pathogenicity factor and the current situation of fire blight disease in this province.

In the present study, 68 strains isolated from pear, quince, and apple hosts were identified in Semnan Provinces. Phenotypic, nutritional, and biochemical tests were performed on strains. The molecular identification of isolates was conducted using specific primers A and B of the plasmid pEA29. A pathogenicity test of the bacterial isolates on the immature pear fruit of the Spadona pear cultivar was performed, and the most pathogenic bacterium isolate was selected for further evaluation. Pathogenicity test was carried out on immature fruits and blooms of eight pear cultivars. Pathogenicity characteristics such as amylovoran, biofilm, and siderophore were studied in most virulence isolates.

The isolates were gram and oxidase negative, catalase-positive, and anaerobic. The isolates were able to produce a hypersensitive response in tobacco and levan production, but none of them could produce fluorescent pigment on King B medium and growth at 39 °C. The results suggested that of 68 strains collected, 52 isolates belong to Erwinia amylovora bacteria. The virulent isolate (D 43) was collected from Dibaj city. All 52 isolates under study proliferated 1000 base pairs. These tests, along with sequencing of PCR products, proved they were Erwinia amylovora species. Studying pathogenicity factors showed an almost direct relationship between all pathogenicity factors (other than biofilm production) and the severity of disease symptoms on tissues. The most resistant blooms tissues to D 43 isolate were shown to be related to Dargazi and Spadona pear cultivars; Duchess and Chini Hesar pear cultivars had the most resistant immature fruits tissues to D 43. It is therefore concluded that the susceptibility tissues to isolates of Erwinia amylovora were different in various cultivars. This point could help control fire blight disease and programs of breeding cultivars in the future.