Article by Bas van den Ende
Consultant in fruit production (ret.).
Spring and summer rains may be a joy to some, but orchardists often consider them a curse, because of the damage rains can do to their ripening fruit.
When it rains, fruit of many stone fruit varieties may suffer from cracking or splitting; defects that make them unsaleable. In cherry, plum and apricot, fruit cracking can be a severe limiting factor in the production of these fruits. Some apple varieties are susceptible to stem-end and/or calyx-end splitting.
Rain-induced cracking is a complex combination of factors that are constantly interacting with each other and changing within the orchard. Scientists around the world have studied how water enters the fruit that causes the splitting, with inconclusive results.
With cracking, the problem is usually one of superficial fractures, which occur in the peripheral tissues of the fruit, that is in the epidermis and the hypodermis layers (the skin), whereas with splitting the fractures extend more deeply into the cortex (fruit flesh).
Cracking and splitting can occur on the sides (cheeks) of the fruit or on their stem or calyx ends. Although in theory a clear distinction is being made between cracking and splitting, both words are often used indiscriminately. Hereafter, the term cracking will be mostly used.
Fruit cracking occurs at various development stages. In most fruits, cracking arises later in fruit development. In cherry, plum and apricot, this is usually during stage III (final swell) of fruit growth when the fruits grow very fast. Cracks and splits do not heal.
Varieties differ greatly in their susceptibility to cracking, pointing to internal, thus genetical, factors controlling the phenomenon.
Cracks arise in the most susceptible varieties at the calyx and/or in the stem cavity, although occasionally very sensitive varieties develop deep cracks or splits on the lateral cheeks of the fruit.
Gala and Fuji are susceptible to stem-end splitting, Pink Lady to calyx-end splitting, whilst other varieties do not or hardly suffer from these defects.
More than one type of cracking may occur on one fruit simultaneously, and cracking can lead to the occurrence of splitting. A stem-end crack may be the origin of a deep split extending to the sides of the fruit.
The occurrence of cracking is extremely variable both between years and locations, pointing to the importance of external factors. In cherry, plum and apricot, rainy weather during the fruit-maturation phase can lead to enormous losses due to cracking, whereas in dry weather during that same period, few problems arise. Regions with little rainfall during stage III of stone-fruit development have an advantage over regions where the chances of rain during that period are high.
Prolonged periods of drought followed by heavy rains or heavy irrigations during the later stages of fruit development, can lead to severe cracking in many stone-fruit species.
There is general agreement that fruit cracking is the result of tensions within the fruit caused by fast increases in volume, such as influx of water that cannot be properly accommodated by the outer tissues. We shall therefore concentrate on water influx.
It has been shown that the rate of water import is related more closely to cracking than the total amount of water accumulated. Fruits from varieties that have a slow rate of water uptake, take much more time to accumulate damaging quantities of water than cracking sensitive varieties with a fast rate of water uptake. This could be the reason that the former varieties are less susceptible to cracking.
There is a close negative relationship between osmotic potential and susceptibility to cracking, which appears to be an important factor associated with differences in cracking between varieties. There are no indications that differences in fruit size or firmness are involved in variety differences.
No correlation was found between the calcium content of the fruit and cracking.
In summary, a range of factors is involved in fruit cracking and differences in one or more of these factors among and within varieties might explain the variation in occurrence of cracking, even between fruits on a single tree.
From the above it is clear that water is involved in cracking. Although it is common knowledge that rain can induce splitting in nearly mature or mature stone fruits, it is impossible to conclude whether it is the water import into the fruit through the skin, the stem or both, that are the cause of splitting.
It has been reported that stem-end splits occurred immediately after irrigation under moisture stress. On trees that were adequately irrigated throughout the season, very little splitting was observed. This indicates, that water entering the fruit solely through the stem is sufficient to cause splitting.
We know that fruit cracking is related to fruit turgor, which typically peaks in the early morning hours and is influenced by irrigation amounts and frequency. When previously water-stressed trees are irrigated, the overall recovery in water potential and the subsequent movement of solutes to the calyx end of the fruit, resulted in excessive turgor pressures in this region, leading to stem-end splitting. Fruits from well-watered trees or from continuously droughted trees, did not show such changes and defects.
Irregular water supply, where fruit growth is mainly based upon water import, will lead to irregular growth of the fruit. Apparently, the wax-like structure of the cuticle is not sufficiently flexible and the surface tensions that arise lead to fractures. If cuticular fractures, however superficial, could be avoided by regular water supply, the chances of damage by rain-induced cracking would be lowered.
Cherries can split in foggy weather without rain. It has been suggested that such conditions of very low rates of transpiration lead to a sudden increase in the rate of water supply to the cherries. Cherry trees covered with polyethylene to prevent rain-induced fruit splitting can show as much as 30% fruit splitting, and lack of air movement and sufficient transpiration under the covers coupled with abundant supply of water via the roots, is thought to be the causal factors in such situations.
Temperature combined with conditions that cause high turgidity in the fruits are believed to promote cracking. Possibly heat may affect the permeability of the cell walls. Indeed, water influx through the fruit surface, especially cherries, increased with increasing temperatures in the range 20˚C to 40˚C.
These observations show that whilst cracking is much worse when fruit surfaces are exposed to water for extended periods of time in adequate temperatures, cracking also occurs, albeit at lower levels, when dry fruits and leaves fail to transpire adequately in warm conditions with abundant root moisture.
Many efforts have been made to control fruit cracking. Here are several suggestions, observations and methods that may help you reduce, and hopefully eliminate, fruit cracking and splitting:
- An indirect control method is to choose varieties that are not highly susceptible to rain-induced cracking. However, it is important to bear in mind that a variety, which shows only low cracking sensitivity in one area, may prove sensitive in another area, with no clear association with the amount of rainfall.
- Provide a regular water supply through drip or micro-jet irrigation, to avoid large fluctuations in stress to ensure that fruit growth avoids patterns of excessive expansion and shrinkage and weakening of the skin. Uniform growth will promote a well-structured cuticular covering. If soil moisture is low, the fruit will adjust accordingly by creating a stronger demand for internally supplied water. Under these conditions, a sudden addition of water from rainfall or irrigation will be quickly delivered to fruit. The difference in tensions that this creates can cause the skin to crack and/or split. Therefore, you must keep topping up the water in the soil with irrigation and keep it at almost Field Capacity, especially towards final fruit swell. Make sure you provide good drainage as well. Keeping the soil wet is generally recognised as the most effective and cheapest method to reduce fruit cracking. In addition, keeping the soil wet also has a positive effect on fruit size at harvest.
- Blow off droplets that adhere to the fruits with your orchard air blast sprayer (without water). This fruit drying method only helps when an occasional shower occurs.
- Raingard™ and SureSeal, two anti-splitting hydrophobic (water repelling) materials, have consistently reduced cracking. The coating stretches as the fruit grows, but good coverage to the point of dripping, is essential. It is also important that you use the correct surfactant. It has been reported, anti-splitting materials have reduced cracking by up to 50% compared with untreated control. So you need to think of the susceptibility to cracking of the orchard when considering if it is cost effective to apply these materials, and others. There is also anecdotal evidence that cracking can be prevented with spraying the trees with white summer oil before rain. The pretence is, that a thin oil coating around the fruit stops rain water adhering to the fruit.
- In general, low crop loads experience high levels of cracking.
- Rain covers are very expensive to install and maintain, and there still may be a certain significant percentage of the fruits that still crack.
- Calcium sprays, which may or may not help. Moreover, when due to much rain, many applications of 0.5 to 1.0% calcium chloride are needed, which can cause fruits to become salty and leaf damage may occur.