Investigating the spread of Pseudomonas savastanoi pv. savastanoi the causal agent of Olive knot disease in Algeria

Olive knot disease is one of the most important diseases affecting olive cultivation, and Pseudomonas savastanoi pv. savastanoi is recognised as its causal agent. The aim of our research is to explore the reasons behind the alarming spread of olive knot disease in Algeria for the regions of Bouira, Bejaia and Tizi-ouzou and to study the factors implicated on its infestation. Field surveys were carried out during the period 2019-2020 in several olive orchards in the Kabylie area on the northern tip of Algeria. The isolation of P. savastanoi was carried out on King-B medium from the sampled knots, the identification of the pathogen was done by biochemical, biological and molecular methods. The field survey revealed a heterogeneous distribution of the disease depending on the site. However, all varieties of Olea europea grown in the study area were affected regardless of the age of the tree. A total of 14 bacterial strains were isolated from knots on semi-selective medium (King B) and identified as P. savastanoi based on biochemical characteristics (LOPAT test), and phyto pathogenicity test on olive plants.The amplification of the iaaL gene confirmed that the selected strains concernP. savastanoi. The biochemical characterisation of the bacteria associated with olive knots found alone or in association with P. savastanoi revealed the identification of mainly Pantoa, to which were added Stenotrophomonas, Rahnella, Seratia,Pseudomonasfluorescens, Enterobacter cloacae and Citrobacter freundi. These associated bacteria would appear to contribute to the virulence of the causal agent and/or have antagonistic activity that may depreciate the disease.