A forward estimate of mango (Mangifera indica L.) harvest timing is required for farm management (e.g., for organization of harvest labour and marketing). This forward estimate can be based on accumulated growing degree days (GDD) from an early stage of flowering to fruit harvest maturity, with fruit maturity judged on a destructive assessment of flesh colour and dry matter content. The current study was undertaken to improve GDD targets for Australian mango cultivars, to improve estimation of harvest maturity, and to document a methodology recommended for future work characterizing fruit maturation GDD for other mango cultivars. An alternate algorithm on GDD calculation involving use of a function that penalizes high temperatures as well as low temperatures was demonstrated to better predict harvest maturity in warmer climates. Across multiple locations and seasons, the required heat units (GDD, Tb = 12 °C, TB = 32 °C; where TB is upper base temperature of 32 °C and Tb is lower base temperature of 12 °C) to achieve maturity from asparagus stage of flowering was documented as 2185, 1728, and 1740 for the cultivars Keitt, Calypso and Honey Gold, respectively. GDD difference between the asparagus and two-thirds floral opening stages of flowering was 188 ± 18 for Calypso, 184 ± 12 for Honey Gold, 238 ± 21 for Keitt and 175 ± 10 for KP. Colour specifications for a colour card set suitable for maturity assessment of all cultivars was also proposed. A flesh colour harvest maturity card specification of 9 was proposed for the cultivar Honey Gold and 13 for the cultivar Keitt.

Forecasting the mango harvest window from flowering aids cultivar selection and harvest planning. This study evaluated the cross-site and cross-season generalizability of accumulated growing degree days (GDD) calculated using either a lower base temperature (Tb) alone or both lower and upper temperature thresholds (Tb and TB), relative to calendar days, for Australian mango cultivars. Operational values of Tb = 12 °C and TB = 32 °C were recommended for all assessed cultivars. The coefficient of variation across site-season populations for calendar days, GDD calculated using Tb alone, and GDD calculated using both Tb and TB was 13.5%, 2.5%, and 1.3%, respectively, confirming the robustness of the bounded GDD estimate across growing environments. Cultivar-specific GDD values associated with minimum and maximum fruit maturity were established using two complementary approaches. The first was based on ripening time (GDDr), with time-to-ripen at 20 °C durations of 12 and 6 days used to define lower and upper maturity boundaries (minGDDr and maxGDDr). The second was based on internal attributes at harvest (GDDi), including flesh color, dry matter (DM), soluble solids concentration (SSC), titratable acidity (TA), and flesh firmness. Minimum maturity was based on cultivar-specific flesh-color and DM targets and an SSC:TA criterion of 7.0. The mean firmness of cohorts at maxGDDr ranged from 2.2 to 3.6 kgf; a 3 kgf individual-fruit benchmark was therefore used for maxGDDi assessment. There was close agreement between GDDr and GDDi estimates of minimum maturity. The average of minGDDr and minGDDi for ‘B74’, ‘Honey Gold’, and ‘Keitt’ differed from previously published preliminary values by only 9–13 °C d, equivalent to approximately one calendar day, while values for ‘Nam Dok Mai’, ‘1201’, and ‘4069’ had not previously been estimated. Overall, minGDD values were established for nine cultivars, while complete harvest windows were resolved within the current study for four. For the remaining five cultivars, upper-boundary estimates were based on a single assessment approach, legacy observations, or remained unresolved.