Characterization of Pseudomonas syringae strains from cherry orchards in Central Otago

Abstract

Bacterial canker of cherry, caused by Pseudomonas syringae pathovars, is a major constraint to cherry growing in New Zealand. The prevalence and pathogenicity of strains from cherry orchards in Central Otago were studied, with the aim of better understanding the epidemiology of the disease. Pseudomonas spp. isolates were collected from symptomatic and asymptomatic cherry tissue in 2015. Isolates were classified into strains belonging to three different taxonomic groups by determining the phylogeny of the gltA gene for all the strains and the Multi Locus Sequence Analysis (MLSA)¹ of four housekeeping genes for 35 strains. Pathogenicity tests in plant tissue were carried out to further characterise the strains. The two main taxonomic groups were P. syringae pv. syringae (Pss) and P. syringae pv. morsprunorum (Psm), in Phylogroup 2 and Phylogroup 3, respectively². The third group comprised non-pathogenic strains classified as Pseudomonas spp. It was concluded that Pss is the predominant pathovar in Central Otago. Lesions caused by Pss on inoculated-immature cherry fruit were observed as being black/necrotic, while Psm caused watersoaked lesions. Lesions on fruit inoculated with Pss strains increased rapidly in size, having a larger area under the disease progress curve (AUDPC) at 10 days post inoculation (dpi), compared with fruit inoculated with Psm strains. Symptoms on leaves infiltrated with Pss appeared within 1-dpi. At 7 dpi, the necrosis caused by most Pss strains, had extended considerably from the inoculation site and into the leaf veins, while necrosis caused by Psm strains remained restricted to the inoculation site. On 1-year-old cherry shoots, the smallest lesions were for strains classified as non-pathogenic Pseudomonas spp. Larger lesions were associated with Pss and Psm strains, which did not differ significantly in this assay. This is the first detailed study of the P. syringae species complex in cherry orchards in New Zealand and it provides the basis for future epidemiology studies.