Bacterial dieback of pistachios in Australia : preliminary epidemiological studies

Abstract

The onset of the production of nuts in commercial quantities in Australia (1996) coincided with the advent of symptoms of decline, dieback, internal staining, trunk and limb lesions and, in some instances, tree death. This disease has been termed “pistachio dieback”. Bacteria of the genus Xanthomonas have been identified as the most probable causal agents of pistachio dieback in Australia. Xanthomonas bacteria were consistently recovered from shoots (1-2 years old) with internal staining. No other plant pathogens have been isolated consistently from diseased tissue. Koch’s postulates have been only partially fulfilled, as the complete set of symptoms has not yet been observed on young trees inoculated in the glasshouse. Identification by GC-FAME and Biolog suggested that the species involved is X. translucens, although the similarity indices were low. Likewise, 16S rRNA sequencing of one isolate showed it to be X. translucens. Whole-genome DNA fingerprinting using AFLP will be applied to identify the bacteria to species and pathovar. Epidemiological studies are in progress to provide a basis for the development of disease management strategies. The pattern of spread of the disease in the orchard is being examined and trees at different stages of the disease have been dissected to trace possible entry point/s of the bacteria and determine the pattern of colouration of the tissue.