Since 2015, Araucaria araucana, an ecologically and economically important conifer native to Argentina and Chile, has suffered an unusual partial death of the crown throughout almost all of the distribution range in Argentina. No primary pathogen or pest was evident, associated with the phenomenon. Isolates of Mortierella, a poorly studied fungal genus in Patagonia, were obtained from the margins of necrotic phloem tissue of symptomatic trees. Five species of Mortierella were isolated from affected tissues. In inoculation tests, Mortierella alpina and M. aff. basiparvispora were pathogenic to A. araucana. These species caused necrosis of phloem, leading to chlorosis, foliar desiccation and eventually death, demonstrating that Mortierellales may play a role in the decline of the tree.
Phytophthora austrocedri is a pathogen of Austrocedrus chilensis causing “Mal del Ciprés” (cypress sickness) in Patagonia and killing Juniperus communis in Britain, along with other species in the Cupressaceae. The evident association of the pathogen with members of the Cupressaceae makes it necessary to study the susceptibility of other species related to A. chilensis to prevent the emergence of new diseases. The present study tested the pathogenicity of P. austrocedri to three native and endemic conifers in South America: Fitzroya cupressoides, Pilgerodendron uviferum and Araucaria araucana. The results showed that F. cupressoides and P. uviferum were highly susceptible to P. austrocedri. Since these are endangered tree species, the outcome of this study is crucial for governmental agencies, policymakers and stakeholders.
Phytophthora austrocedri was identified as the primary pathogen causing the "Mal del ciprés" disease in Patagonia, which affects the endemic conifer Austrocedrus chilensis. Trees present root necrosis that may extend to the stem. This study aimed to describe the histological alterations occurring during P. austrocedri infection of roots of A. chilensis seedlings. Plants were inoculated at the roots and histological studies were performed four weeks post-inoculation. Safranin-fast green, phloroglucinol-HCl, toluidine blue, lugol and diaminobenzidine stains were used to describe and compare anatomo-histological features observed in roots of non-inoculated versus inoculated seedlings. In healthy tissues, the presence of Phi thickenings in cortical cells is reported for the first time for A. chilensis. In inoculated roots, it was observed necrosis of the epidermis and of the cortical parenchyma, and alterations in parenchymal cells (loss of turgor and content, without starch, presence of phenolic compounds). Lignin content remained unaffected by the presence of P. austrocedri. The area occupied by Phi thickenings was smaller in P. austrocedri-colonized tissues, and these structures showed an accumulation of polyphenols, absent in healthy tissue. Parenchymal cells, tracheids, and rays, showed production of hydrogen peroxide. Results evidenced that A. chilensis triggers mechanisms to restrict the infection, but P. austrocedri manages to evade them.
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