Characterization of Pseudomonas syringae pv. syringae isolated from mango in Sicily and occurrence of copper-resistant strains

Mango (Mangifera indica) is grown throughout a wide range of frost-free climates and is one of the world’s most important fruit crops. During 2010-2014, severe symptoms of bacterial apical necrosis (BAN) caused by Pseudomonas syringae pv. syringae (Pss) were observed on mango plants throughout all mango-growing areas in Sicily (southern Italy). The causal agent was identified based on phenotypic, genotypic and pathogenicity tests. The genetic variability of strains obtained from different areas and cultivars were assessed by rep-PCR and allowed clear differentiation of the Pss strains isolated from mango from other representative strains of the pathovar. In addition, the respective production of syringomycin, syringopeptin and mangotoxin was checked by in vitro tests and PCR detection. All tested strains showed the presence of mgoA and mgoB of the mangotoxin operon. Some copper-resistant Pss strains showing cusCBA genes were also found in some orchards. This feature could explain the failure of the disease control using copper compounds. Among 71 strains tested using in vitro assay, no copper-sensitive strains were detected, whereas 44 strains (62%) had high resistance, 16 strains (22.5%) were resistant and 11 strains (15.5%) had low resistance to copper sulfate. To our knowledge, this is the first report of copper resistance among Pss strains causing BAN on mango in Sicily.