Molecular characterization of genetically related isolates of Pseudomonas savastanoi pv. savastanoi recovered from olive and athel trees using specific PCR

Abstract

Olive knot is one of the most important bacterial diseases of olive orchards. Athel (Tamarix aphylla) has recently been reported as a new host of Pseudomonas savastanoi pv. savastanoi (Pss) in Libya, raising questions about the genetic relatedness and virulence of isolates from both hosts. This study characterized 23 Pss isolates recovered from olive (Olea europaea) and athel using primers designed from the 16S rRNA and ptz genes and related the molecular data to hypersensitive reaction (HR) and pathogenicity tests. Most isolates induced a necrotic HR on pepper leaves, whereas a few did not. Pathogenicity tests showed that several olive and athel isolates produced typical knots on olive seedlings, while no clear knots developed on athel seedlings, despite recovery of the bacterium from this host. All isolates yielded the expected amplicon of the 16S rRNA gene, confirming their affiliation to the genus Pseudomonas, whereas the plasmid-borne ptz gene was detected only in eight isolates from both hosts. Combining HR, pathogenicity and PCR results allowed the isolates to be classified into four groups reflecting differences in genetic load and virulence, and indicated a genetic relatedness between olive and athel isolates. These findings contribute to improving diagnosis and management of olive knot disease under local conditions.