Mango malformation disease (MMD) caused by Fusarium mangiferae poses a significant threat to Australia’s mango industry valued at AUD $208 million through significantly reduced yields. MMD was eradicated from Queensland but is present in the Northern Territory and considered absent from other states/territories. A risk assessment of potential entry pathways was initiated to determine whether it is appropriate to establish domestic risk management measures to reduce biosecurity risks associated with MMD.
The IPPC requires phytosanitary measures to be “technically justified” based on the conclusions of a scientific pest risk analysis. The risk assessment encountered notable challenges. MMD is associated with multiple Fusarium species, with F. mangiferae the most prevalent. A comprehensive and reliable list of MMD pathogens of potential quarantine importance to Australia is needed to support the initiation of the PRA process. Fusarium mexicanum, F. pseudocircinatum, F. tupiense, F. sterilihyphosum, F. proliferatum, F. neocosmosporiellum and F. decemcellulare have been identified as causative agents of MMD but evidence of causation is lacking for F. nivale, F. avenaceum and F. chlamydosporum. Other species have been re-evaluated as endophytes, non-pathogenic or opportunistic pathogens.
Ongoing taxonomic revisions in the genus Fusarium complicates the accurate diagnosis of MMD pathogens. Former single species taxa like F. oxysporum, F. fujikuroi and F. solani are now recognised as species complexes with many of the constituent species yet to be fully elucidated. Recent taxonomic refinements have clarified new Australian Fusarium species such as F. dhileepanii and F. mariamitchelliae which were previously misidentified as F. proliferatum and F. sterilihyphosum. Their role in MMD remains uncertain.
Assessment of the likelihood of establishment is also challenging. Globally, MMD pathogens F. proliferatum, F. pseudocircinatum, F. neocosmosporiellum and F. decemcellulare infect multiple hosts, while F. mangiferae, F. sterilihyphosum, F. mexicanum and F. tupiense only infect mango. In Australia, F. proliferatum and F. pseudocircinatum have been isolated from mango and other hosts while F. neocosmosporiellum and F. decemcellulare have only been reported on hosts other than mango. This suggests that multi-species pathogenicity testing is essential to determine whether local strains pose a credible threat to the mango industry. Local knowledge would complement global historical host range when assessing establishment potential of these species.
Assessment of the likelihood of spread for MMD is hampered because most assessments have focussed on F. mangiferae. For instance, mango bud mite and mango inflorescence gall midge are reported to facilitate the spread of F. mangiferae but their role in transmitting other MMD pathogens remains uncertain. In addition, while the economic impact of F. mangiferae is significant, evidence of economic loss is limited for other Fusarium species such as F. proliferatum and F. pseudocircinatum.
Developing a scientifically robust approach to distinguish Fusarium species which cause MMD from those merely associated with it, and providing a framework for determining effective quarantine measures, market access and management strategies is important for managment of MMD in Australia.