Peaches, like all stone fruit, grow in three stages. Understanding this can help you to understand the factors contributing to stone split and gum to exude from peaches, Bas van den Ende* writes.

Stages of growth

The first stage is rapid cell division and growth, when the number of cells in the fruit flesh is determined.

The fruit grows slowly in the second stage, when the stone starts to form. During this stage, the entire stone hardens (lignifies) and the seed inside the stone also completes its development.

After this second stage, the cells of the flesh enlarge and fill with water and sugars. This is the third stage and ends in rapid growth called final swell.

Understanding stone splitting

Stones may split when they change from soft plastic to rigid lignified tissue during the onset of the second stage. The stone begins to harden from the tip and then hardens down the suture side of the fruit towards the base. The stone keeps on accumulating lignin until the end of the second stage.

If you examine a section of a peach fruit next to the stone under a microscope, you will see thousands of small cells compacted on the surface of the stone. Since cell division has stopped by the middle of the first stage, the number of cells per fruit is fixed. Subsequent growth of the fruit takes place because each cell enlarges.

While the stone stops growing and begins to harden, the cells of the flesh continue to enlarge. The cells with room to grow naturally tend to become roundish. The cells on the surface of the stone have no room to grow laterally and become elongated. This layer of elongated cells puts pressure on the stone.

The strength of this pressure depends on the number of fruit on the tree, leaf area and water content of the soil. When sugars (produced by the leaves), and water (taken up by the roots) both enter the fruit, it is like air being pumped into a tyre. Before the tree is hand thinned, this solution of sugar and water is distributed over hundreds, sometimes thousands of fruit, depending on the size of the tree.

When the tree is thinned, the remaining fruit suddenly has seven or more times more solution entering from the stem and the pressure on the stone increases. This pressure increases even more if you irrigate heavily after hand thinning, fertigate with nitrogen, or cincture the trunk or limbs to get better fruit size.

If the stone is sufficiently lignified, it can withstand the pressure. But if the stone is still changing from its plastic stage to the lignified stage, cracks may appear in the upper part of the stone which is rigid, while the lower (stem-end) part is still soft, causing the stone to split. This explains simply why stones split.

Susceptibility to split

Varieties differ in their susceptibility to split stone. Fruit of early varieties often enter their final swell before the stone is completely hardened. The stress exerted by the rapidly expanding cells of the flesh mainly causes the stone to fracture. Young trees with a large leaf-area-to-fruit ratio can suffer more from split stone than do mature trees.

Differences in the shape of the stone also influence split stone. Some varieties have round stones which can cope better with the pulling force that the cells create, than elongated or flat stones of other varieties do.

Weather also plays a part and perhaps can be overlaid across the other factors. Low temperatures before stone hardening may cause more split stone than do moderate temperatures before stone hardening. Also, when low temperatures are followed by moderate temperatures, split stone may increase. From this, a cool spring probably leads to more split stones, especially if temperatures suddenly rise in late spring.

To avoid stones from splitting:

1. Plant varieties which are less prone to split stone

2. Prune trees correctly in winter. This will cause less fruit to be removed (target pruning and shortening of laterals)

3. With varieties that split easily, do not hand thin until the stones have hardened. However, this could adversely affect fruit size

4. Time your irrigations carefully during hand thinning and as the stone hardens. Do not irrigate heavily immediately after hand thinning

5. Do not apply nitrogen fertiliser before or during hand thinning.

What causes fruit to gum?

Peach and nectarine fruit have two large ducts or vascular bundles on the suture side of the stone and another one on the side opposite the suture. These ducts go around the stone and meet at the tip of the stone. The ducts have many lateral branches, which transport sugars and water to the flesh of the fruit. Gum, which consist of complex carbohydrates made up of different sugars and related compounds, is found in these ducts.

When the crop is thinned and a surge of sugars and water enters these ducts, the ends and some weak points along these ducts give way. The gum in the ducts oozes out and forms pockets, usually at the tip of the peach or nectarine. If there are any weak places near the skin, the pressure of the gum causes the skin to rupture and, as it dries, it forms beads or blobs on the surface.

Susceptibility to gum

As with split stone, varieties differ in their susceptibility to gum. Cultural practices and environmental factors, which tend to induce splitting of the stone also encourage the fruit to gum.

Gum is not all bad

Gum and pectin make the juice in a peach or nectarine viscous and add sparkle and glaze. However, the peaches and nectarines may appear to be sweet, but gum and pectin do not add any sweetness.

*Bas van den Ende was a researcher at the Tatura Research Institute, author, and former consultant in fruit production. Contact: advhort@bigpond.net.au