Crop load, fruit quality and mineral nutrition as predictors of fruit softening and internal flesh browning in modern firm fleshed apple cultivars

Internal flesh browning (IFB) is a physiological disorder of apples that significantly reduces market acceptability of the fruit. The incidence of IFB in high value apple cultivars that have dense firm flesh is of concern to the apple industry, yet there is little understanding of the conditions that may influence its presence in apple fruit. Using the dense, firm fleshed 'Scilate' cultivar as a model crop, this study examined crop load, fruit and leaf mineral nutrient status and fresh and stored fruit quality attributes over three seasons in commercially managed orchards in southern Tasmania, Australia to determine whether these variables could be used as predictors for fruit softening (FS) and the incidence of IFB. Results showed that 'Scilate' could be prone to biennial bearing, following on and off year patterns when subjected to heavy crop loads in any given season. Crop load, fruit weight, flesh firmness, starch pattern index (SPI) and malic acid content can be used as predictors of FS and IFB expressed as CO2 injury and radial flesh browning (RFB). Fruit softening had a negative correlation with crop load and positive with flesh firmness. CO2 injury showed a strong positive correlation with fruit weight and dry matter content (DMC) and negative with SPI. No correlations were found for RFB with these quality parameters. Fruit mineral nutrition and/or their ratios were better predictors of IFB than leaf nutrition. CO2 injury was positively correlated with fruit phosphorus (P) and potassium (K) to calcium (Ca) ratio and negatively with calcium, zinc (Zn) and calcium to magnesium (Mg) ratio. RFB was positively correlated with FS, but showed a negative relationship with fruit manganese (Mn) and positive with K:Mg. A positive relationship was found between FS and fruit K, K:Ca, leaf K and K:Mg, while there was a negative relationship with fruit Mn, leaf Mg and S. This study concludes that pre-harvest factors, orchard conditions and imbalance in fruit mineral nutrients critically influence FS and IFB in dense-fleshed cultivars such as 'Scilate'. Crop load management, in order to avoid biennial bearing and regulate fruit size, quality and balance mineral nutrient levels, should be central to the strategies to mitigate the risk of FS and IFB.