Resin canal discolouration in mango (Mangifera indica L.) fruit

Resin canal discolouration (RCD) in mango fruit is a quality defect and a major concern to the Australian mango industry. RCD appears as red-brown resin canals that form networks through the flesh and irregular brown mottling across the peel. It occurs sporadically and has been observed at all stages along the supply chain from field harvest to retail display with symptoms becoming more obvious during fruit ripening. The propensity for RCD appears complex as genetic, agronomic and environmental factors have been posited as possible causes. This thesis aimed to elucidate factors associated with RCD expression including its possible causes in order to identify potential pre- and post-harvest interventions to manage RCD. The impact of RCD on consumer perception of mango quality and their specific decision to purchase and consume mangoes with RCD was investigated in a consumer survey of three supermarkets in Sydney. The survey of 135 mango purchasers revealed that the majority of consumers (89%) were regular buyers and preferred to buy Kensington Pride (KP) (36%) and Calypso\(^{TM}\) (32%) cultivars because of their taste (50%), aroma (19%) and fruit colour (18%). Moreover, the majority of the respondents (87%) were not willing to buy RCD-affected fruit. This study suggests that RCD can significantly affect consumers’ perception on fruit quality and influence their purchasing behaviour. To characterize the progression of RCD symptoms during fruit ripening, the incidence (%) and severity (rating score) of RCD on fruit peel, flesh and seed along with other physio-chemical attributes (peel colour, flesh colour, firmness, dry matter content and total soluble solids) were explored at eating ripe stage. Fruit with RCD had significantly lower total soluble solids (TSS) compared with non-affected fruit. There were no other associations between these fruit attributes and the presence of RCD. To date, progress on RCD-related research has been stymied by an inability to consistently induce RCD expression in mangoes. In this thesis, evaluation of two inoculation methods (needle and spray) found it was possible to reliably induce RCD at different stages of development (harvest green to eating-ripe) by inoculating with the extracted flesh from RCD -infected fruit as an inoculum source. In particular, while all inoculated fruit developed RCD, regardless of the stage of fruit development, fruit inoculated at ‘harvest green’ had the highest RCD severity by the eating-ripe stage. A second study monitored RCD expression and severity by sequential sampling at 0, 3, 6, 9, 12 and 15 days after inoculation (DAI) at harvest green stage. Symptoms of RCD was first observed at 6 DAI on the seed testa, then in the flesh at 9 DAI and finally in the peel at 15 DAI. The bacteria present in RCD fruit were identified using 16S rRNA sequencing technique and next generation sequencing (NGS) techniques were used to assess overall microbial diversity and their abundance in both RCD-infected and control fruit. Significant difference in microbial communities were observed between RCD-infected and control fruit, suggesting a bacterial association with RCD expression. The two dominant genera identified in RCD-infected mangoes were Pantoea and Tatumella. Novel approaches to manage RCD expression were explored. The efficacy of a commercial postharvest sanitiser, Nylate\(^®\), to manage RCD was investigated. This postharvest treatment applied in varying concentrations, durations and timing were ineffective at controlling RCD expression. The susceptibility of mango cultivars B74 (Calypso\(^{TM}\)) and National Mango Breeding Program cultivars (1243, 1201 and 4069) to RCD was evaluated using the two artificial inoculation tests. Symptoms of RCD were observed in all cultivars, however the level of RCD incidence and severity differed among cultivar demonstrating a genetic difference in susceptibility to RCD. Overall, this study highlights the role of bacteria associated with RCD expression. It also documents the increase in RCD incidence with maturity along the supply chain. Despite the lack of postharvest control of RCD using Nylate\(^®\), alternative sanitisers and application at earlier stages such as pre-harvest and at harvest should be explored. Results from the cultivar screening offers new research possibilities to the longer-term reduction of RCD through genetic selection of less susceptible cultivars. A better understanding of the pre- and post-harvest factors affecting RCD will assist the mango industry in the reduction of RCD incidence and the associated economic losses