Mango (Mangifera indica L.) is an economically significant crop that has been adversely impacted by tree dieback associated with members of the Botryosphaeriaceae fungal family worldwide. The Northern Territory (NT) is a key contributor to Australia's mango production; however, the prevalence of mango dieback is of concern to the NT industry as it results in reduced productivity. Mango dieback is characterised by the progressive and downward drying out or dying of the stems and branches, from the tip backward towards the roots, and can lead to complete defoliation and tree death. The baseline study by Burgess et al. (2019) on Botryosphaeriaceae in Australia used published records and identified 62 species in the country out of a total of 222 species worldwide and found no published records for the NT, highlighting a significant knowledge gap. In Australia, mangoes have been associated with fungi in the genera of Botryosphaeria, Lasiodiplodia, Neoscytalidium, Neofusicoccum and Pseudofusicoccum (Burgess et al. 2019; Sakalidis et al. 2011; Slippers et al. 2005). However, the identity of the casual agents of mango dieback in the territory remained anecdotal and un-published. Therefore, the overall aim of this study is to identify the causal agents of mango dieback in the NT through a systematic study of mango producing regions in the NT. Disease samples were collected from Darwin and Katherine, fungal isolations were carried out and the subsequent isolates were identified using morphological and molecular identification techniques. Mango seedlings were inoculated using a selection of isolates to confirm disease causality (Koch’s postulates) and investigate isolate virulence. A total of 54 isolates were identified through culturing and phylogenetic analysis of the partial internal transcribed spacer (ITS) and partial translation elongation factor 1 alpha-like gene (tef-1α) sequences. The study identified species within the Botryosphaeriaceae family including Botryosphaeria dothidea, Lasiodiplodia crassispora, Lasiodiplodia iraniensis, Lasiodiplodia pseudotheobromae, Lasiodiplodia rubropurpurea, Neofusicoccum sinoeucalypti, Neoscytalidium dimidiatum, and Phyllostictaceae family including Pseudofusicoccum adansoniae.
The study showed significant difference in the fungal populations from each of the study areas, with Darwin being dominated by Lasiodiplodia species and Katherine being dominated by Neoscytalidium. All the inoculated isolates fulfilled Koch’s postulates indicating that they are pathogenic on mango, however they varied significantly in their virulence, L. rubropurpurea, L. pseudotheobromae, and N. dimidiatum were the most virulent isolates tested.