Do your apples look good on the outside but brown and ugly on the inside?
Research & ExtensionResearchers have identified predictors of flesh browning by examining the factors that may lead to flesh browning, including crop load, fruit size and quality, and fruit and leaf mineral nutrition.
Flesh browning is a devastating and highly erratic physiological disorder, found in many apples, but particularly in modern firm-fleshed cultivars. Some cultivars are more susceptible than others, depending upon their parentage, growing region and management practices. There can be considerable variation in both the incidence and severity of flesh browning; up to 100 per cent of fruit can be affected, with differing degrees of severity from no brown flesh to browning of the entire flesh.
Flesh browning occurs due to compromised structural integrity and functional stability of the cells in the fruit flesh, resulting in deterioration of fruit quality and reduction in storage potential. This can cause substantial economic loss to the apple industry and poses a serious threat to brand reputation of various established firm-fleshed apple cultivars.
Types of flesh browning
Despite variation in the expression and symptoms, flesh browning can be broadly categorised into chilling injury, CO2 injury or radial browning (Figure 1), depending on the underlying cause of occurrence.
- Chilling injury: browning of the flesh, generally showing defined margins between damaged and healthy flesh.
- CO2 injury: characterised by brown portions throughout the fruit flesh, with small to large lens-shaped cavities in the cortex tissue.
- Radial browning: shows a typical radial pattern of browning extending outwards from the vascular bundles, but there is no clearly defined margin between affected and unaffected tissue.
Figure 1: Images of three different broad categories of flesh browning found in apple fruit: a) chilling injury, b) CO2 injury and c) radial browning.
Studies conducted in Tasmania
In a three-year study, undertaken as a part of a PhD project, over 10,000 individual apples were sampled across 11 different orchard blocks from five commercial orchards in the Huon Valley region of Southern Tasmania. Apples were assessed for fruit quality attributes, mineral nutrient concentrations, and the incidence of flesh browning and data was analysed to identify potential predictors of flesh browning. The incidence of flesh browning was calculated in terms of percentage of total symptomatic fruit per block or site, irrespective of the type of flesh browning.
Predictors of flesh browning
Considerable variation was found from season to season among the orchard sites, orchard blocks and even within each orchard site or block. The incidence of flesh browning across three seasons ranged from 0 to 51 per cent (Figure 2). The major seasonal variation was seen in blocks with greater variation in crop load.
Figure 2: Incidence of flesh browning, expressed as a percentage of total symptomatic fruit per block or site. Error bars represent 95 per cent confidence interval.
Crop load and fruit quality parameters as predictors of flesh browning
Flesh browning showed a negative correlation with crop load and a positive correlation with fruit weight, flesh firmness and dry matter content (Figure 3). This means that higher crop loads reduce the risk of flesh browning, while greater fruit weight, flesh firmness and dry matter content increase the risk. Fruit from lighter crop load (off-year) trees are larger, firmer and denser and are more prone to flesh browning because these fruit tend to be low in calcium (Ca) and experience poor diffusion of gases due to the dense flesh.
Figure 3: Crop load and fruit quality parameters as predictors of flesh browning (FB) in apples. Spearman’s correlation coefficients (* p < 0.05, ** p < 0.01, *** p < 0.001).
Mineral nutrients as predictors of flesh browning
Fruit nutrition was found to be a better predictor of flesh browning than leaf nutrient content. We found that flesh browning was positively correlated with fruit potassium (K) and K:Ca and negatively correlated with fruit Ca, Zinc (Zn) and Ca:Magnesium (Mg) (Figure 4). This means that higher fruit K and K:Ca can increase the risk of flesh browning, while higher fruit Ca, Zn and Ca:Mg can reduce the risk, and vice versa. Calcium in fruit plays an essential role in imparting structural and functional stability to fruit cells, but excessive K and Mg can induce Ca deficiency due to their antagonistic relationship with Ca. Zinc is known to complement fruit Ca uptake (to a certain limit), hence it can play a role in reducing flesh browning.
Figure 4: Fruit mineral nutrients as predictors of flesh browning (FB) in apples. Spearman’s correlation coefficients (* p < 0.05, ** p < 0.01, *** p < 0.001).
Key findings
- Fruit from lighter crop load (off-year) trees tend to be larger, firmer and denser and were prone to flesh browning.
- Optimising crop load to avoid biennial bearing can mitigate the risk of flesh browning.
- Managing crop load to ~6 fruit per cm2 trunk cross-sectional area can prevent biennial bearing and reduce flesh browning.
- Incidence of flesh browning was lower in fruit weighing <200 grams.
- Higher fruit K and Mg and lower Ca and Zn can increase risk of flesh browning.
- Balancing mineral nutrition, particularly fruit Ca, is critical.
Ramandeep Singh Sidhu, Post-Doctoral Research Fellow, Tasmanian Institute of Agriculture, University of Tasmania.
This article was first published in the Spring 2023 edition of AFG.
