Fruit Splitting (Cracking) – What causes it and can we do anything about it?
Like much of biology, it’s a multifactorial phenomenon which has no simple one-step answer. Research is on-going, but detailed mechanistic understanding and appropriate management strategies are still incomplete. Almost all fruits and nuts can be variously affected, and for some may represent a significant economic loss for growers. As well as damaged fruit being unmarketable, they also serve as entry points for fungal infections. Frequency and extent of splitting varies widely across different species, and even within species, different cultivars can show great variation. Outcomes are driven by plant genetics and environmental and management conditions and how these factors interact.
Causes - general.
Unlike soft and flexible animal cells, plant tissues including fruit cells are encased within cell walls and other stiff materials to maintain form and function. For fruit to grow and expand, there has to be a controlled loosening of these firm structures in concert with cellular expansion through increased turgor pressure. If this coordination is out of phase, then cells may be ruptured where loosening gets ahead of expansion, or there may be no increased space for cells to expand into if behind; proper cell function may then be compromised. This synchronisation is controlled at the genetic level where families of proteins are synthesised and released, and environmental factors such as temperature and management conditions such as soil moisture may impact. The desired outcome is obviously for an orderly unfolding of these events such that fruit mature with sugars, acids and other substances developing properly, astringency is removed, fruits soften, etc.
Causes - specific.
For a given plant, probably the most important factor is the state of fruit hydration throughout development. This is influenced by foliar absorption such as when it rains or through the roots. Most fruits increase in size in the early stages by cellular division and in later stages by cell expansion. If moisture status is too low in the early phase, there may be insufficient cells to accommodate cellular expansion later. Usually, rainfall or over-watering around harvest time increases the percentage of split fruit. The relative importance of root and aerial delivery of moisture seems to vary for different fruits. However, the observation that splitting may be just as prevalent in greenhouse-grown plants where moisture levels can be controlled precisely illustrates that moisture is not the sole answer to the problem. As with grapes, reducing water in the approach to harvest has one other plus, namely of increasing fruit solute concentrations and flavour.
- Those fruits where there is a depression at the stem end can have a natural welling of any rainwater and this increases the exposure time for absorption. Rain droplets also tend to remain on the apical end for an extended time, again representing increased opportunity for absorption.
- Splitting tendency can develop from the interior flesh outwards or can develop from cracks in the skin or cuticular layer and progress inwards. Cuticles are extra-cellular waxy substances designed to help reduce desiccation risk In plants. If deposition of these coatings does not keep up with cellular expansion which then goes beyond their ability to stretch, then cracks form. If continued, these cracks (initially less than 5 square microns) join up to form bigger and bigger splits. If rain occurs when these are present, then uptake of water increases dramatically and compounds the problem.
- As fruit ripen and become sweeter, polysaccharides are converted into smaller molecule sugars, resulting in higher osmotic potential. This draws in increased water which then increases pressure on the skin. So, the sweeter the fruit the stronger the skin has to be, eg grapes.
- The elasticity and firmness of the cuticle, skin or all of the pericarp influences the degree of splitting. In many instances firm cultivars have been developed to improve commercial handling and transport requirements, but these properties may be at odds with degree of splitting.
- The epidermal arrangement of cells is a factor determining the ability to withstand turgor-induced stress.
- The shape of the fruit is significant, with non-symmetrical or flattened sections being more prone. For example, navel oranges usually split first at the navel end where the skin is thinner and weaker. Figs and pomegranates behave similarly. The stem end is also a relative weak point.
- Calcium and boron are important in maintaining proper cell wall structure, so inadequate supply increases fruit splitting.
- Crop load is another important factor in two ways. With few fruits, size usually increases and this may be beyond the point where fruit integrity can be maintained. Alternatively, too heavy a crop load may result in undue competition for nutrients so that cell walls, cuticles etc do not have sufficient supplies for normal synthesis.
- Warmer temperatures usually increase splitting, as do increased day/night temperature differentials.
- Light intensity, apart from producing higher temperatures, has its own direct effect on increasing fruit soluble solids and growth rate, leading to increased stress on the skin. Apples provide an illustration, as fruit on the outside of the tree suffer increased splitting.
- Leaf to fruit ratio can be another variable – presumably the opportunity for water absorption increases as leaf area increases. Alternatively, increased foliar shading of fruit can reduce splitting by reducing incident light and temperature of the fruit.
- Lenticel and stomatal density seem to be positively correlated with water absorption.
What can we do?
Obviously if you are only at the planning stage of growing a particular crop of 10,000 trees and your chosen species has a propensity for significant splitting, then you need to select the most resistant cultivar available in balance with other desirable properties. Climate is the next concern, and usually drier conditions around harvest time are preferable. For example, rain near harvest with grapes, blueberries and cherries can cause massive splitting. So, the further out of range your site is from the plant's naturally adapted climate, the more problems may ensue.
If your plant is already in the ground, then genes and climate are decided and you have to rely on management. To repeat the general statement above, foolproof and universal remedial strategies have not yet been devised because we still don't have a complete understanding of mechanisms.
- Avoid dramatic swings in fruit moisture status from fairly early fruit set till maturity. The ripening period for many fruits in WA is in the hotter summer months, so reducing irrigation towards maturity to minimise fruit split has to be traded off against the very real need for plants to transpire – consequently a balancing act is required. The key message is not to overdo irrigation approaching harvest, as the rainfall source of water is not such a big problem for us compared to other areas where their wet season is in the hot months. In these other areas, growers have even resorted to using water-proof plastic sheeting to cover the whole crop in efforts to address the problem. So try and maintain constant soil moisture levels. If your plant is mixed in with others that require heavy summer irrigation, root pruning or placing an impermeable ground sheet around sections of the tree may reduce soil sources of moisture.
- Fruit thinning to produce average yearly crop loads rather than extremes will assist, particularly where the species has a tendency to be an alternate bearer.
- Nutrition has to be optimised to ensure adequate nutrients are available for the plant to synthesise requisite structural and functional proteins and waxes. Calcium sprays throughout development have often been found effective.
- Plant hormone sprays have been shown to help with certain crops under very particular circumstances, but general rules have so far not eventuated and they should only be used with great care.