Ministry of Agriculture & Lands
Tree Fruit Leader, Vol. 3(2) July 1994
All About Cherry Cracking
The following are three articles originally printed in British Grower which review research on rain cracking of cherries. Authors are Tony Webster, research scientist at East Malling Research Station and John Cline, previously a research student at East Malling and now working as a researcher in Ontario.
There is considerable interest in cherries at the present time and it was thought an update on cherry cracking would be useful. Thanks to Grower for permission to reprint.
Cherries - Cracking the Problem
The area of land devoted to sweet cherry production in Britain was once significantly larger than it is at present.
Since the early 1950s, the acreage of cherries has declined from more than 18,000 acres to only 2,000. This decline was in response to several severe cultural problems and in no way attributable to a drop in demand for cherries by consumers. Indeed, consumer demand for cherries has increased and they are still perceived as a high value luxury product by most people.
Many of the cultural problems, which brought about the decline of English cherry production, have now been overcome. Self-fertile Scion varieties crop more consistently, and varieties from the John Innes breeding programme offer greater resistance to the damaging bacterial canker organism. Furthermore, dwarfing rootstocks are finally on the horizon and chemical methods of tree size control have also shown great promise in research trials.
Severe problem
One severe problem remains-the cracking of fruits in response to rain at, or close to, harvest. This is not a problem unique to the UK; producers in Switzerland, France, Belgium, the U.S., New Zealand and many other countries face similar worries when picking time approaches.
Rain in California caused losses of several million dollars a few seasons ago and New Zealand's cherry exports to Japan (to target the lucrative Christmas market) are frequently disrupted by rain close to harvest.
Why do fruit crack? Until a few years ago, conventional wisdom was that cherries only cracked in response to rain deposited on and taken up by the fruits; soil moisture and the water relations of the rest of the tree were thought to be of only minor importance.
More recent evidence from trials in New Zealand, and at East Malling, have resulted in this theory being amended slightly. Research suggests some cracking may be caused, and all cracking made worse, by high water levels in the rest of the tree, and high humidity surrounding the tree. Loss of fruit water by movement through the fruit stalk back into the tree is not thought to be possible once they approach maturity. The only way ripening fruit can lose excess water is by transpiration through the skin.
Consequently, conditions which reduce the fruit's ability to transpire, such as high humidity and minimal air movement within the tree canopy, are likely to increase the incidence of cracking following rain.
Microcracks
Fruits sensitive to cracking often show many small imperfections on the skin surface (cuticle microcracks). These are usually invisible to the naked eye and are only observable through the microscope.
Whether these microcracks have a part to play in facilitating water uptake or weakening cell bonding in the fruit epidermis, both of which would promote cracking, is not yet known. If they do influence cracking then it will be important to determine the orchard environmental conditions early in fruit development which may stimulate their formation.
Three possible control strategies can be pursued:
- Control by choice of resistant varieties or rootstocks;
- Control by sprays of minerals, hormones or other substances;
- Control by covering the crop and manipulation of the microclimate around the tree.
Sweet cherry varieties showing resistance to rain-induced cracking in Danish trials:
Adriana; Allers Spate; Annabella; Beta; Bianca; Black Oliver; Ceran Edra; Early Rivers; Ermstaler; Grosse Gemersdorfer; Hudson; Kristin; Lapins; Merton Heart; Merton Marvel; Ohio Beauty; Oktavia; Regina; Sam; Schauenburger; Schmidt; Seneca; Ulster; Vittoria
A cracking index test, which involves immersing ripe fruits in distilled water and then monitoring the time taken to crack, was used to determine the sensitivity of more than 200 varieties in trials in Denmark.
Not viable
Many of the varieties which show more resistance than others, listed in the panel, cannot be considered commercially viable on account of their poor fruit size, quality or yield potential.
Nevertheless some, such as Kristin, Ulster and Regina, may warrant testing in Britain.
Other varieties not included in these tests, but shown to have some resistance to cracking in tests conducted in other countries, are Castor, Kordia and Star. Amongst the newer Canadian varieties bred at Summerland in British Columbia, Sunburst, Lapins and Sweetheart have the lowest sensitivity to cracking.
One major problem with interpreting these cracking index tests is the variability in response from season to season and country to country. For instance, the variety Viva, which is considered quite resistant to cracking in its native Canada, was one of the most sensitive in Danish tests. Obviously, there are site and seasonal factors which greatly influence the susceptibility of some varieties to rain-induced cracking. So, from the British grower's viewpoint there is no substitute for testing promising varieties on their own farms and over a number of seasons.
Variability
Several theories have been put forward by researchers as to why varieties often differ greatly in their susceptibility to cracking. One is that varieties which take up more water, either in greater total amounts or at more rapid rates, from their fruit surface are likely to be more sensitive to cracking. An alternative theory is varieties which have skins with epidermal cells which are more strongly bound together or have more elastic cell walls are likely to tolerate light rain and resist cracking to some extent.
Several factors govern the rate and amount of water taken up by the fruit from its surface. More water is taken up when temperatures are quite warm following rain. The driving force for this uptake is the osmotic concentration of the fruit juices, i.e., their soluble solids (sugars) concentration. The higher the concentration of soluble solids in the fruit juice, the more water is expected to move into the fruit.
However, although differences in fruit juice soluble solids have been shown to influence water uptake when comparing fruits of a single variety, research in Denmark indicates these differences play only a small role in determining the cracking sensitivity of different varieties.
An alternative suggestion is that the fruit skin, its cuticle and epidermis, may be more of a barrier to water uptake into resistant varieties than into fruits of varieties with lower resistance.
Observations by researchers in Belgium in the late 1980s showed some cherry varieties had thicker cuticles and epidermal walls which might explain a higher resistance to cracking.
Varieties
More recent trials, also in Belgium, have shown varieties with thick cuticles and only a small number of fruit stomata are less susceptible to cracking. These varieties absorbed water through their skin more slowly than varieties more prone to cracking. Varieties with thick cuticles were also able to absorb more water in total before splitting than thinner cuticled varieties.
Research conducted by John Cline in collaboration with researchers in Norway showed much reduced cracking on the varieties Ulster and Sam, when compared with the sensitive Van.
The trials showed that juice from the fruits of these two tolerant varieties had lower concentrations of soluble solids and lower osmotic potential. Also, fruits of Ulster and Sam were less firm and took up water in immersion tests at much slower rates than Van. These varieties, however, were more sensitive to cracking when grown on F12/1 in comparison with Colt rootstock. This rootstock effect on cracking sensitivity warrants further investigation.
Although it has often been stated that varieties with large fruits are more sensitive to cracking than those with smaller fruits, this is not entirely supported by the evidence.
While it is true that for any single variety, sensitivity increases with increase in fruit size, the relationship is a poor one when comparing varieties with genetically determined differences in fruit size.
Possibilities
Other possibilities, as yet little explored, are that the resistance to cracking of some varieties may be partly attributable to the greater elasticity of their epidermal cells or to differences in the development of fruit cuticular microcracks early in the season.
Unfortunately, resistance to rain-induced cracking has rarely featured as an important selection criterion in fruit breeding programmes and it would appear many breeders have often inadvertently made poor choices of parents for their new varieties in this respect.
Recent work in the Ukraine and the Republic of Georgia indicates that varieties selected for drought tolerance during hot windy conditions at the time of blooming are quite resistant to cracking when grown in more humid, rainy climates. This fortuitous finding should be explored in future breeding programmes.
While there are no varieties which are fully immune to rain-induced cracking, some clearly offer greater tolerance than others.
Some of the new Canadian varieties now under test in Britain, such as Lapins, Sunburst and Sweetheart, should offer slightly greater resistance to cracking than many of the older varieties. Also growers may wish to test on a limited scale, varieties such as Regina, Castor, Kordia and Ulster under UK growing conditions.
Cherry Split - Will Chemicals Crack the Problem?
An answer to the problem of rain-induced cracking of sweet cherries has been sought since the 1930s. Although no chemical spray will completely stop fruits cracking, most effort has been put into finding one which will significantly reduce the proportion of fruits damaged.
Research has mainly focused on testing the effects of sprays of minerals (calcium, copper or boron compounds), plant hormones (gibberellins or auxins), and of antitranspirants or surfactants on the sensitivity of cherries to cracking.
Minerals
Some reports have indicated that sprays of calcium compounds, applied up to four times before harvest starting three or four weeks before the expected harvest date, have reduced the percentage of fruits splitting. Several different calcium compounds have been tested but the most success has been achieved with the chloride, the acetate or the hydroxide.
Unfortunately, worldwide results have been inconsistent, ranging from significant reductions to increases in cracking following application of calcium sprays.
One of the problems in interpreting these results and identifying reasons for the variation is that the trials have been conducted on different varieties, bearing different crop loads and in very different environmental conditions.
However, trials with sprays of calcium chloride and calcium metalosate (a chelated form of calcium), plus surfactants, were conducted on the variety Bradbourne Black at East Malling in 1990 and 1991 and on Van in 1991. Three sprays were applied, each of 3% calcium active ingredient, at ten-day intervals starting four weeks before harvest.
The results were most disappointing. None of the calcium sprays had any effect on the cracking of either variety. In 1990, the calcium-treated Bradbourne Black fruits actually cracked slightly more in immersion tests (cracking index tests) than untreated fruits. Further studies indicated the treatments had little effect on the amounts of calcium in the fruit skin or flesh and had no significant effect on the speed of water uptake by the fruits in immersion tests.
When calcium sprays have been successful in tests abroad, it has been argued (usually without much supporting evidence), that the calcium was getting into the fruits and strengthening the bonds between epidermal and other fruit cells, so improving their 'strength' and reducing cracking.
Another theory sometimes put forward is that calcium deposited on the fruit surface may reduce its capacity for water uptake. However, this is not supported by the East Malling results.
Danish experience
Research in Denmark some years ago showed that spraying cherries with calcium during rain showers did alleviate cracking. Daily fruits misting with calcium-rich water also produced similar effects. These treatments may have worked better than the more usual two or three pre-harvest sprays by constantly replacing the calcium washed off by the rain.
A major drawback of calcium sprays, however, is the unsightly residues left on the fruits at harvest which may need to be washed off or removed in some other way before marketing.
Trials many years ago indicated that sprays of Bordeaux mixture could have beneficial effects in reducing cherry splitting. Bordeaux is a mixture of copper sulphate and lime and the results may have been attributable to one or both of these components.
Copper sulphate applied alone is damaging at the concentrations needed to have any beneficial effect on cracking, so the lime is included primarily to reduce the scorching effect of the copper. In recent trials in Tasmania, Bordeaux mixtures have successfully reduced cherry cracking and increased both fruit firmness and epidermal thickness.
Trials in the U.S. examined the effects of sprays of aluminium or boron compounds on cracking. As with the calcium and copper sprays. the results were inconsistent.
Sprays of copper or aluminium also leave unsightly deposits on the fruit at harvest. The fruits usually need washing after picking.
Plant hormones
Several reports suggest one or two sprays of gibberelic acid, (GA3, at 15 to 30ppm) applied three to four weeks before harvest can reduce cracking of cherries. These sprays also delay maturity and may increase fruit size and firmness.
In trials in Belgium, GA3 appeared to thicken the cuticle and the radial epidermal walls of treated fruits while work in Oregon in the U.S. has shown that GA3-treated Lambert sweet cherries exhibited more cracking following rain than unsprayed controls.
Further studies in Oregon, using simulated rain, showed that the GA3 treatments increased cracking of immature fruits decreased it for a short period at about two weeks before maturity and had no effect at fruit maturity. Also, GA3 appeared to cause more side cracks and fewer tip cracks as maturity progressed.
Work at Wye College some years ago also showed that although cracking around the neck of cherry fruits was reduced by GA3 sprays, the more damaging lateral cracks were not. Research in Washington State on Rainier cherries showed increased cracking of GA3-treated fruits. The studies also showed that cracking was worse on gibberellin-treated cherries if the fruit remained wet for more than four hours.
Studies at East Malling last year showed no reduction in cracking following GA3 treatments to fruits, spur leaves or both spur leaves and fruits. However, fruit firmness and size were increased and maturity delayed by the GA3 treatments (table 1).
Table 1. Influence of GA3 (5ppm + surfactant)* on Van fruit quality
| Treatment | Fruit size (g) | Optimum harvest date N/cm | Fruit firmness (%) | Soluble solids |
| Control | 6.9 | July 10 | 8.31 | 17.9 |
| GA3 | 9.3 | July 19 | 11.20 | 17.9 |
So although sprays of gibberellin consistently increase fruit size, firmness and storage potential, their effects on cracking are likely to be poor, or even negative, if rain is prolonged.
Trials in Belgium at Wye College and at East Malling have looked at whether cherries from trees sprayed with paclobutrazol show more or less fruit cracking following rain. Although the Belgian trials found positive benefits, those conducted at Wye College and East Malling showed no consistent reductions in cracking.
However, paclobutrazol-sprayed trees do yield fruits with higher calcium content which is beneficial for shelf life. Also, the short internodes and tightly clustered leaves on sprayed trees may act as umbrellas to protect fruits from direct rain and so in some cases reduce cracking.
Research in the 1950s in the U.S., and more recently in Spain, showed that sprays of the sodium salt of naphthalene acetic acid (NAA, applied at 1mg/litre at approximately 30 to 35 days before harvest), reduced sweet cherry cracking by up to 50%. However, applications closer to harvest (four to 18 days) increased cracking.
The Spanish work also found that varieties differed considerably in their response to NAA. NAA had no effect on fruit mineral status but seemed to inhibit swelling of fruits placed in water. One suggestion is, therefore, that where NAA reduces cracking it acts by restricting water uptake by the fruit. In trials in Oregon the beneficial effects of NAA sprays were additional to those of pre-harvest sprays of calcium hydroxide.
Elsewhere, NAA has been found to have no benefits in reducing cracking. In France, auxin sprays applied to the variety Burlat had no effect although the year of testing was one of low natural cracking.
Wye College research showed that sprays of NAA, applied four weeks before harvest reduced both the field cracking and the cracking index, and increased the firmness of two sweet cherry varieties. However, the NAA treatment did reduce the size of one of the varieties, Stella. So auxin sprays can reduce the severity of rain-induced cracking on some cherry varieties but may also reduce fruit size slightly.
Antitranspirants
Cherries are known to have a less efficient covering of waxes on the fruit surface than apples and pears. Antitranspirant sprays, if effective, should add to this wax covering and limit both water uptake and water loss through the cuticle and stomata. Unfortunately, covering the skin in this way also reduces gas exchange by the fruits which may also limit photosynthesis and the build up of soluble solids (sugars).
As with all other chemical sprays used to reduce cracking, there are conflicting reports. Research conducted in California, Britain and Italy has shown pre-harvest sprays of antitranspirants to sweet cherries can reduce the incidence of rain-induced cracking and may also inhibit post harvest water loss and shrivel of fruits.
The British research showed that waxy antitranspirants, such as Clarital, cut down water uptake and fruit cracking. These treatments also increased fruit size and firmness, but reduced soluble solids and titratable acidity.
Other U.S. work found antitranspirant sprays actually increased cracking, while trials in Canada showed no effect. All effective antitranspirants leave an unsightly deposit on the fruits at harvest.
Surfactants
Sprays of wetting agents/surfactants like Pril and Citowett just before ripening (one week before harvest) have been shown to reduce cracking by as much as 50% in German trials. Austrian research showed similar beneficial results on a range of cherry varieties.
Belgian trials found that two sprays, each of 100ml Citowett/100 litres water, 18 and eight days before harvest reduced cracking on six cherry varieties to very low levels compared with unsprayed controls. Similar trials in Holland also showed promising results using Citowett.
Surfactant treatments, like those of antitranspirants, are thought by some researchers to work by reducing the rate or amount of water taken up by fruits. They are easily washed off, however, and are unlikely to be effective after very heavy rainfall or if applied too soon before harvest.
Sprays of calcium and copper compounds, gibberellins, auxins, antitranspirants and surfactants have all reduced sweet cherry cracking in some trials. However, the effects are very variable and in some cases the sprays may make cracking worse.
The reasons behind the inconsistent results need to be established.
Cherries - Under Wraps
If we accept that most cherry cracking is the result of rain water deposited on and taken up by the fruits, and not from water translocated to the fruits from the roots, then covering the crop with some form of rain shelter should offer the best solution to the problem.
Rain shelters were first tested in Switzerland many years ago and since then growers and researchers across the world have experimented with covers.
By far, the biggest investment has taken place in New Zealand where many orchards of large standard trees have been enclosed by expensive and highly sophisticated polythene structures. The expense of these structures is to some extent justified by the very high returns on cherries exported from New Zealand to Japan to satisfy the demand during the Christmas period.
European covering systems for sweet cherries have been more modest in design and cheaper to erect than the New Zealand canopies. Moreover, they have relied on the grower being able to control, at least partially, tree size. Canopies erected in Norway, where many trees are covered commercially, are extremely simple. Although they do occasionally break free in high winds they are, for the most part, reported to be reliable.
In Switzerland and Belgium the covering systems have been similar in design to the one we have tested at East Malling over the past three years.
Metal hoops attached to stout poles, with either lightweight metal rods or wires between each hoop, support polythene umbrellas over the trees. Attaching the polythene firmly to the structure is the biggest problem and the metal rods on the system tested at East Malling (manufactured by the Rovero, Rolloos Sorensen bv in Holland) make it much simpler than wires.
In the East Malling experiment the polythene covers (150 micron Visqueen) were put in place over hedge-trained trees of Merchant on Colt rootstock in the middle of May each year from 1991 to 1993. Covers which simply provided an umbrella over the tops of the trees were compared with longer covers, which draped to within 1m of the orchard floor on both sides of the hedge of trees. Similar, uncovered, trees were monitored as controls.
All trees were protected from birds by enclosing within netting. Two of the trees in each of the five-tree experimental plots were irrigated by trickle, providing eight litres/tree/day starting once the shelters were erected.
The covers reduced the percentage of fruits which cracked in all three years (table 2).
Table 2. Fruit Cracking (%) Over Three-Year Trial
| Covers | 1991 | 1992 | 1993 |
| None | 24 | 20 | 41 |
| Short | 13 | 16 | 20 |
| Long | 15 | 14 | 16 |
| Irrigation | |||
| + | 21 | 27 | 29 |
| - | 20 | 20 | 22 |
The differences between the effects of long or short covers on fruit cracking were small and not significant. Long covers gave excellent protection from rain and yet despite this, approximately 15% of the fruits split in each year. As condensation on fruits beneath the covers was minimal, the indication is that even when fully protected from uptake of rain through the fruit skin, an appreciable proportion of fruits may still crack.
Irrigation increased the proportion of fruits cracking, which adds further evidence to the theory that the water status of the whole tree has a bearing on the severity of the problem.
It is thought water entering the fruits via the fruit stalk cannot escape by the same route. In order to relieve the high turgor pressure which may build up when water enters from the rest of the tree, the fruit must lose some of its excess water by transpiration through the skin.
Records at East Malling show that humidity builds up beneath the covers to levels that can severely limit water loss from the fruits via transpiration. This, coupled with the high water status of the trees following rain, is almost certainly the cause of cracking recorded beneath the covers.
What is needed in the future is a cover which can breathe or which can be drawn off immediately after rain to allow better air circulation around the trees. Close woven plastic materials may be one answer and some have been tested in grower trials. Unfortunately, driving rain tends to result in fine mist sprays penetrating the materials and wetting the fruits beneath.
In the U.S., one grower has developed a system of covering trees with thick, transparent tarpaulin-type materials which are hung from wires above the trees and are drawn on and off the crop as necessary. This system has proved to be very effective.
Other Benefits
Apart from reducing the percentage of fruits cracking, covering cherries grown in Britain may have other advantages.
In 1993, trees beneath the long covers yielded 43% more fruit than the uncovered controls and short and long covers increased the yield of marketable fruits by over 40% and over 50% respectively.
Fruit size and colour were increased in two of the three years of trial at East Malling. Fruit firmness, which was recorded in only one year (1993), was also improved. Fruit soluble solids (% sugars) were, however, reduced in fruit from beneath the long covers in all three years; the short covers had no significant effect on soluble solids.
Whether covering sweet cherries with polythene or other rain-proof materials is economically viable is not yet known. More trials will be necessary over several seasons before the benefits in terms of increased yields of marketable fruits can be quantified. Such economic appraisals are already under way in Michigan where experimental shelters are under test.
Cracking of many fruits, such as tomatoes and grapes, is often attributed to erratic supplies of water. A regular supply of water applied throughout the season may reduce this problem. However, results in our trials at East Malling indicate irrigation of sweet cherries, even when applied daily from mid May until harvest, may increase rather than decrease the risk of cherries cracking at harvest (table).
Indeed, some slight water stress in trees as they approach harvest may be beneficial; fruits with a reduced water content should be able to take in more water after rain before splitting is induced than fruits with a higher water content.
Choice of rootstock or root manipulation treatments may help in this respect and new trials are planned to examine these possibilities.
