Identification of the causal agent of pistachio dieback in Australia

Abstract

Symptoms associated with pistachio dieback in Australia include decline (little or no current season growth), xylem staining in shoots two or more years old, trunk l and limb lesions (often covered by black, superficial fungal growth), excessive exudation of resin, dieback and death of the tree. Bacteria belonging to the genus Xanthomonas have been suggested as the causal agent. To confirm the constant association between these bacteria and the disease syndrome, the absence of other pathogens and the identity of the pathogen, we performed a series of isolations and pathogenicity tests. The only microorganism consistently isolated from diseased tissue was a bacterium that produced yellow, mucoid colonies and displayed morphological and cultural characteristics typical of the genus Xanthomonas. Database comparisons of the fatty acid and whole-cell protein profiles of five representative pistachio isolates indicated that they all belonged to X. translucens, but it was not possible to allocate the isolates to pathovar. Pathogenicity tests on cereals and grasses supported this identification. However, KochÕs postulates have been only partially fulfilled because not all symptoms associated with pistachio dieback were reproduced on inoculated twoyear-old pistachio trees. While discolouration was observed, dieback, excessive resinous exudate and trunk and limb lesions were not produced; expression of these symptoms may be delayed, and long-term monitoring of a small number of inoculated trees is in progress.

Genetic, phenotypic and pathogenic diversity among xanthomonads isolated from pistachio (Pistacia vera) in Australia

“…Using DNA‐independent and DNA‐based analyses, the pistachio dieback pathogen, previously identified as similar to X. translucens , was found to be phenotypically, biologically and genetically heterogeneous. Two distinct groups were identified, supporting preliminary findings based on five strains (Facelli et al ., 2005). Group A included strains from three areas (Kyalite, Renmark and Red Cliffs) and rep‐PCR revealed genetic variation within this group.…”

“…Strains in group A induced a typical HR on tobacco leaves, indicating that they were pathogenic, whereas strains in group B did not. Groups A and B could also be distinguished by pathogenicity to the Anacardiaceae and Poaceae, whereas a preliminary study with a limited range of strains and hosts failed to reveal differences between groups (Facelli et al ., 2005). Group A appeared to be more aggressive than group B in terms of the HR response, development of symptoms in inoculated plants and recovery of the pathogen from inoculated buds.…”

“…Previous attempts to confirm the pathogenicity of one Xanthomonas strain isolated from pistachio by injection of bacteria into the trunks of 2‐year‐old grafted trees in pots in autumn resulted in discoloration of wood and reisolation of the bacteria from various parts of the plants. Monitoring of inoculated trees for trunk and limb lesions continues on the basis that such symptoms are observed in trees 6–7 years old (Facelli et al ., 2002, 2005). In the present study, however, multiple inoculation points (especially close to the apex), inoculation in spring, and providing a moist environment for several days after inoculation resulted in necrosis and dieback.…”

Effect of burning and high temperature on survival of Xanthomonas translucens pv. pistaciae in infected pistachio branches and twigs

“…(E. Facelli, The University of Adelaide, Australia, personal communication) and in 2009 from infected trees in a commercial orchard in Robinvale, Victoria (34°34′46″S, 142°46′47″E, 61 m a.s.l.) (Facelli et al. , 2005).…”

The Use of ARMS PCR in Detection and Identification of Xanthomonads Associated with Pistachio Dieback in Australia

“…Discolouration of woody tissue in shoots one or more years old, trunk and limb lesions with excessive exudation of resin, and dieback and death of the tree characterise the disease. Xanthomonas strains have been identified as the causal agent of the disease and previous studies have shown a close relationship between the pathogen and X. translucens (Facelli et al, 2002(Facelli et al, , 2005Marefat et al, 2006). The main location of the pathogen is in the young sapwood of the trunk, primary branches and young branches of diseased trees (Facelli et al, 2003) and the pathogen can be isolated most reliably from woody tissues with internal staining.…”