Bacterial Species as Causative Agents Involved in Pistachios Dieback in Iran

Abstract Die-back of pistachio (DBP) is a major disease affecting pistachio trees in Iran and has become a serious problem in Kerman province in recent years. In the current study, the role of bacteria as causal agents of DBP was investigated under laboratory and field conditions. Samples were collected from infected pistachio orchards in Kerman province from 2015 to 2016. The ability of bacterial isolates to produce disease and vascular colonization was studied through different inoculation methods. Identification of isolates was carried out using biochemical and physiological assays, amplification of 16SrDNA region combined with partial analyses of gyrA gene. A total of 281 bacterial isolates were obtained from infected trees from which 148 induced the hypersensitivity reaction on tobacco leaves. From those, 128 isolates were able to colonize vascular tissues in sub-bark inoculations of pistachio branches under laboratory conditions. In field experiments, 24 selected isolates were able to extend in vascular tissues of pistachio branches and twigs using sub-bark and apical inoculation methods, although disease severity varied. Staphylococcus pasteuri, Bacillus pumilus, Bacillus sp., Acinetobacter radioresistens, Xanthomonas sp., Curtobacterium flaccumfaciens, Arthrobacter oxydans and Pseudomonas koreensis were identified to be involved in die-back of pistachio trees. The work presented here shows that a range of bacterial genera and species may be involved with the DBP, which may be used to improve the urgently needed management of the disease.