Influence of orchard conditions on fruit quality of ‘Scilate’ apple

The production of Class 1 fruit that maintains quality during long-term storage remains one of the top priorities for the Australian apple industry. The physiological disorders internal flesh browning (IFB) and fruit softening (FS), compromise post-harvest quality and limit fruit storage potential. The development of IFB related disorders involves membrane disruption which results in compromised structural and functional stability of the cell membranes or components. The cellular de-compartmentalisation leads to confluence of phenolic compounds and polyphenol oxidases resulting in enzymatic oxidation and browning of flesh tissue. Multiple studies in cultivars such as ‘Cripps Pink’, ‘Braeburn’, ‘Fuji’ and ‘Empire’ have attempted to identify both pre- and post-harvest causes of IFB, yet little is understood on predictors for these disorders and management strategies. A thorough review of the literature (Chapter 2) highlighted gaps in our current understanding of pre-harvest influences of IFB, FS and drivers of fruit quality in relation to new and popular cultivars that are typically large, juicy, and have dense flesh and high firmness. With increasing industry concern about the incidence of IFB related disorders and FS in these high value cultivars, a better understanding of pre-harvest orchard conditions and orchard management practices is needed in order to identify the best predictors of IFB related disorders and FS and to devise strategies to mitigate its risk. Using ‘Scilate’ apple as the model cultivar, we investigated the influence of crop load management, thinning methods, nutrition, and harvest management using pre-harvest application of 1-MCP and different harvest timings in relation to fruit quality outcomes and the incidence of IFB related disorders and FS.
Predictors of IFB related disorders and FS
A survey of multiple commercial orchards/blocks (Chapter 3) was undertaken over three consecutive seasons (2018-19, 2019-20 and 2020-21) to investigate the impact of crop load, fruit and leaf mineral nutrient concentrations, and quality parameters in both fresh and stored fruit to determine whether any predictors of IFB related disorders and FS could be identified. The results showed that two types of IFB related disorders, CO₂ injury and radial flesh browning (RFB), occur in ‘Scilate’. Crop load, fruit weight, flesh firmness, starch pattern index (SPI) and malic acid (MA) content were identified as predictors of both FS and CO₂ injury. A negative relationship was observed between FS and crop load and a positive relationship with flesh firmness. CO₂ injury had a positive correlation with fruit weight and dry matter content (DMC) and negative with SPI, but no such relationship was observed for RFB. Fruit mineral nutrients were found to be better predictors of IFB related disorders than leaf nutrients. Fruit phosphorus (P), potassium (K), calcium (Ca), zinc (Zn), K:Ca and Ca to magnesium (Mg) ratio were associated with FS or CO₂ injury, whereas RFB was only influenced by fruit manganese (Mn) and K:Mg ratio. This study emphasised the need for the manipulation and optimisation of crop load to address FS and IFB related disorders in ‘Scilate’, as optimising crop load management can aid in preventing biennial bearing and regulating fruit size, quality and balancing fruit mineral nutrients
.Crop load management
As crop load can have a major impact on fruit quality, yield, storability, nutritional balance and physiological disorders, a two-year trial (Chapter 4) was undertaken to compare two thinning methods, artificial bud extinction (ABE) and hand thinning (HT), at three different levels of crop load: 3, 6, or 12 fruit cm−2 limb cross-sectional area (LCSA). This study found that ABE consistently improved fruit set, return bloom, fruit weight and yield over HT, and also improved fruit quality attributes such as flesh firmness, total soluble solids (TSS), DMC and MA content. Crop load significantly influenced fruit quality with higher quality under the low and medium crop loads and poor-quality fruit under high crop loads, however, a marginally higher incidence of IFB related disorders (largely RFB) and FS was observed in fruit from low crop load trees. Fruit mineral nutrient content N, Ca, Mn, and Zn was increased while K declined with increased crop load. Further, severe biennial bearing was induced in high crop load trees regardless of the thinning method, and in HT trees with a medium crop load. Consistent yields and good fruit quality were observed over the two seasons in ABE managed trees with a medium crop load. Optimised crop load and fruit quality in ‘Scilate’ can be achieved with ABE management with around six fruit cm⁻² LCSA.
Pre-harvest 1-MCP and harvest timings
There are several harvest management technologies, including pre-harvest application of the ethylene inhibitor 1-methylcyclopropene (1-MCP), that are commonly used by apple growers in order to schedule harvest by expanding the harvest window and improve quality of fresh and stored fruit. We examined the influence of pre-harvest 1-MCP (Harvista^TM) applied one week prior to harvest on fresh and stored fruit quality, IFB, FS and respiration rate of ‘Scilate’ (Chapter 5). Pre-harvest application of 1-MCP slowed respiration rate, improved post-harvest flesh firmness and MA content and prevented the incidence of RFB, but increased the severity of CO₂ injury.
Finally, we investigated how maturity at harvest can influence fruit quality, FS and incidence of IFB related disorders in ‘Scilate’ by harvesting fruit at 187 days after full bloom (DAFB) (early harvest), 194 DAFB (mid-harvest) and 202 DAFB (late harvest) (Chapter 6). The different harvest timings influenced fruit maturity, fresh and stored fruit quality parameters, FS and RFB (only type of IFB related disorder detected). Later harvest increased fresh fruit SPI, weight, DMC, and improved colour but decreased MA content. Flesh firmness and MA content in stored fruit declined significantly with late harvesting while TSS and juice pH were increased. FS and incidence of RFB were also greatest with late harvest and lowest at mid harvest, but no significant difference was observed between early or mid-harvest timings. Therefore, delayed harvesting of ‘Scilate’, despite some benefits in fresh fruit quality attributes, could be detrimental for post-harvest quality and storability as it increases the risk of FS and RFB. These research outcomes improve our understanding of pre-harvest influences on fruit quality, IFB related disorders and FS and provide management options to the apple industry in order to improve fruit quality, storage potential and mitigate the risk of IFB related disorders and FS.