Reducing rain cracking in sweet cherry

Sweet cherry fruit split open by rain — Figure 1. Cracking in sweet cherry fruit after rainfall during the ripening period.

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

One of the most serious problems in sweet cherry growing is the high sensitivity of certain cultivars to rain-induced fruit cracking. Heavy rain during ripening causes extensive splitting in the fruit of most cultivars. There is at present no way of excluding cracking altogether; certain applications can nevertheless raise the resistance of the fruit and so reduce the final loss. These applications, of course, only work up to a certain depth of rainfall.

Contents

Three different kinds of cracking
The mechanism — what actually happens
The second route for water: the pedicel
Cultivar susceptibility
Calcium
Silicon
Glycine betaine and L-proline
Gibberellin
Rain covers and nets — the only measure that changes the order of magnitude
Irrigation and nutrition
How susceptibility is measured
The overall strategy
Sources

1. Three different kinds of cracking

The first step in any serious approach is to stop speaking of «cracking» as though it were a single phenomenon. There are three, with different causes, a different genetic basis and a different response to control measures.

Type	Where it appears and what causes it	Heritability
Apical cracking (pistillar-end)	At the tip of the fruit, where the stigma once was. This is where the cuticle is thinnest and where the floral scar forms a structural weakness. It is associated with a droplet sitting directly on the apex.	0.608 – 0.905 very high
Stem-end cracking (stem-end)	In the cavity around the pedicel. Water stands there for hours after rain, and this is also where the xylem route of the pedicel ends.	0.575 – 0.742 high
Side cracking (side)	On the cheeks of the fruit. It is the most purely mechanical of the three: it arises from strain in the cuticle over an expanding surface.	0.354 – 0.557 moderate

What these numbers mean in practice Broad-sense heritability indicates how much of the difference observed between trees is due to genes and how much to environment and management. A value of 0.9 for apical cracking means the phenomenon is almost entirely a matter of cultivar — no spray programme is going to solve it. The value of 0.35–0.56 for side cracking, by contrast, means that here management has real room to work. A specific genetic locus (QTL) has in fact been identified on linkage group 2 of the cultivar Regina, relating specifically to tolerance of side cracking.

2. The mechanism — what actually happens

The traditional explanation is familiar to everyone: rainwater enters the flesh through the skin, the fruit swells, internal pressure rises and the fruit finally «bursts like a balloon». That explanation is intuitive, has been taught for decades — and is not confirmed by the experimental data.

Why the «critical turgor» hypothesis collapsed

Measurement showed something that overturned the model: the ripe cherry fruit, at stage III, has remarkably low turgor. And, most critically of all — turgor did not rise when the fruit took up water, nor fall when it transpired.

If pressure does not rise when water enters, then it cannot be that pressure which splits the fruit. The paper that stated this explicitly carries a title to match: «Rain cracking in sweet cherries is not due to excess water uptake but to localized skin phenomena».

The practical consequence is considerable: any measure aimed simply at «getting less water into the fruit overall» — cutting irrigation a few days before rain, for instance — is addressing the wrong problem. What matters is not total volume but what happens locally, over a few square millimetres of surface.

The «zipper» model

The explanation that now prevails is a different one, and it begins long before the rain.

Step 1 — The cuticle stops thickening, the fruit keeps growing

During stage III, when the fruit is expanding rapidly, cuticular mass stays essentially constant. The same quantity of waxes and cutin has to cover a surface that keeps growing. The result is increasing strain in the cuticle — like a membrane stretched over a balloon that is being inflated without any material being added.

Step 2 — Microcracks appear

Once strain exceeds its strength, the cuticle fails locally and microcracks form. They are not visible to the naked eye and are not damage in themselves. They are, however, open doors: water passes through them without meeting the cuticular barrier. The critical point is that the microcracks are already present before it rains. The fruit arrives at the rain either primed to crack or not.

Step 3 — Local failure and propagation

Water entering through a microcrack reaches the cells immediately beneath it. Those cells burst, and their vacuolar contents — rich in sugars and organic acids — spill into the apoplast, the space between cells. There they act osmotically and draw in still more water, which bursts the next cells in turn. The crack «unzips», running from a microscopic starting point to ever greater length.

Two further mechanisms that reinforce the picture

Swelling of the cell walls. In ripe fruit the cell walls swell on contact with water. That swelling weakens the middle lamella, the «glue» holding cells together, and makes them easier to separate.
The flesh is not homogeneous. The osmotic potential of flesh cells differs from point to point within the same fruit. This spatial heterogeneity determines where absorbed water is directed — and explains why the same fruit cracks at one particular place and not another.

3. The second route for water: the pedicel

Rainwater does not enter only through the fruit surface. There is a second route, and it is systematically underrated.

The pedicel is five times more permeable than the fruit

The permeability of the pedicel surface to water is five times that of the fruit surface. Nor is that all: the apoplast of the fruit and the xylem of the pedicel remain hydraulically connected even after the abscission layer has formed.

What this means in the field: water standing in the stem cavity is not merely a cosmetic problem. It is the fastest route by which water enters the fruit. For that reason every practice that dries the fruit quickly after rain — air blowers, helicopters where they are used, an open canopy that lets air through — pays back out of all proportion to its cost. It does not reduce the rain; it reduces the number of hours the water stays on the fruit and in the stem cavity.

4. Cultivar susceptibility

The most susceptible cultivars are all the early ones, together with Giant Red, Tsolakeika and others. By contrast, Sabrina, Samba, Canada Giant, Summit, Grace Star and Regina, among others, are considerably more resistant.

What the international literature says about the same cultivars

The ranking from international trials agrees with our own experience on the main points. Regina is among the most resistant cultivars worldwide, while Kordia, Lapins and Hedelfingen also show some tolerance.

One observation deserves attention: Sweetheart, regarded in some regions as a safe choice because it is late, showed the highest cracking susceptibility in trials. Lateness on its own is no guarantee — it merely reduces the chance that ripening will coincide with the spring rains.

Why early cultivars suffer more It is not simply that they ripen when it rains. Early cultivars complete stage III over a shorter period, which means the fruit surface expands faster. Because cuticle deposition cannot keep pace, the strain developing in the membrane is greater, and microcracks appear earlier and at higher density. The susceptibility of early cultivars is therefore a structural characteristic, not bad luck with the weather.

5. Calcium

Foliar calcium applications substantially improve the postharvest storage life of sweet cherry fruit and flesh firmness. More specifically, two to three calcium applications are enough for a good result.

What the trials show — and why the results differ so much

Calcium is the most studied treatment for cracking and at the same time the most misunderstood, because published results range from striking to nil. In one series of trials calcium chloride reduced cracking by 62%, while in others the effect was negligible.

The difference is not random — it is a matter of timing. The applications that worked were those placed early: in trials, CaCl₂ applied at 39 and 62 days after full bloom reduced the incidence of cracking in thinned trees. Applications made only close to harvest did not achieve the same. The reason connects with section 2: calcium has to be already incorporated into the cell walls and middle lamella before the critical expansion begins, so that it can reinforce cell-to-cell cohesion. A spray on the eve of the rain has no time to play that part.

On firmness and storage the data are distinctly more consistent: early calcium chloride sprays improved firmness at harvest, firmness after cold storage and the modulus of elasticity of the fruit.

The limit that applies before any increase in rate The main drawback of direct calcium applications is the visible residue left on the fruit, which downgrades its market appearance. This is the real reason rates and repetitions are not increased indefinitely. In practice it means that the calcium has to be «spent» on the early applications, where it also has the greatest physiological effect and the lowest risk of residue at harvest.

6. Silicon

Silicon increases the elasticity of the cherry skin, reducing the proportion of cracked fruit. Applications are made from petal fall onwards; some products, however, may leave white spotting on the fruit. In recent years calcium silicate has also become available, covering the requirement for both silicon and calcium. Application can also be made through the soil.

The size of the benefit, in figures

The figures in wider circulation — a 15% reduction in cracking and a 9% increase in firmness against the control — come from trials by a manufacturer of a foliar silicon product, not from independent published work. They are recorded here with that reservation stated openly. The documentation that stands up to scrutiny is the soil-applied work that follows.

As regards soil application, referred to above: trials with 1% potassium silicate applied to the soil every 3 weeks from bloom gave consistently firmer fruit in the cultivar Bing (409 against 386 g·mm⁻¹ for the control). The size of the difference is moderate but real and consistent — and the soil route bypasses the problem of white spotting on the fruit entirely.

7. Glycine betaine and L-proline

This combination is well suited to reducing the osmotic stress caused to plants by environmental conditions. A glycine betaine product with L-proline at a rate of 1.5 kg per tonne of spray solution, applied at the change of colour to yellow and repeated 10 days before harvest, substantially reduced cracking in several of the cultivars on which it was tested. The same application also markedly increases frost tolerance during the flowering period.

How it works and what the trials show

Glycine betaine is a quaternary ammonium compound that accumulates inside cells and stabilises cellular structures, maintaining membrane integrity under abiotic stress through osmoregulation. That mechanism fits the model in section 2 exactly: if the cells beneath a microcrack hold without bursting, the «zipper» never starts.

The available data indicate that increasing the rate significantly reduces the proportion of cracked fruit, and that efficacy rises as ripening advances — meaning the timing described above, with the second application close to harvest, agrees with the literature. In a trial on the cultivar Skeena comparing calcium, gibberellic acid, abscisic acid, salicylic acid, glycine betaine and Ascophyllum nodosum extract, the two most effective treatments for raising yield and reducing cracking were the seaweed extract and abscisic acid.

8. Gibberellin

Gibberellin applied at the change of colour from green to yellow markedly increases fruit size (by roughly 1 mm), flesh firmness and resistance to rain-induced cracking. One study reports that in the cultivar 0900 Ziraat cracking increased rather than decreased. It does, however, delay ripening by 3 to 6 days.

Gibberellin is a plant growth regulator — a prescription is required In Greece, gibberellin products must be prescribed before they can be applied, and must hold an authorisation from the Ministry of Rural Development and Food. This is not a fertiliser or a biostimulant but a growth regulator, with everything that implies for record-keeping and pre-harvest intervals. Requirements differ from country to country and the national rules always apply.

Rate and timing — what is documented

Stage: the application is placed at the transition from light green to straw yellow. This is exactly the stage described above.
Rate: the established recommendation in British Columbia, where the practice is applied systematically, is a single spray at 20 ppm at the straw-yellow stage.
Repetitions: there is no benefit from multiple applications over a single one. In a three-year trial, yield was affected neither by one application at 20 or 30 ppm, nor by two or three weekly applications at 10 ppm. Raising the rate or the number of sprays adds cost and risk without a corresponding return.

What is gained and what is paid: the fruit is consistently firmer and larger, can be harvested later, and titratable acidity is higher. Colouring is delayed — which has to be taken into account when the market wants deep colour on a particular date.

Why the contradiction in 0900 Ziraat is not a measurement error The observation that in one cultivar cracking increased with gibberellin has a physiological basis and should not be dismissed as an anomaly. Gibberellin makes the fruit bigger. A bigger fruit means a bigger surface — and, on the model in section 2, a bigger surface over roughly the same cuticular mass means greater strain and more microcracks. Gibberellin therefore acts in two opposite directions at once: it raises flesh firmness (favourable) and also fruit size (unfavourable). Which of the two prevails depends on the cultivar. The conclusion for practice: on a new cultivar, a trial on a limited area comes first.

9. Rain covers and nets

All the applications above reduce the damage by some percentage. One measure alone changes the order of magnitude of the problem: keeping the water off the fruit entirely.

What the trials showed, Greek ones included

Plastic rain covers are the most effective method of controlling cracking. Covering the trees from three weeks before harvest and throughout it significantly reduced cracking in 4 trials out of 5.

And a second, equally important benefit: the same covers also significantly reduced fruit rots. In Greek orchards specifically, a study by Thomidis and Exadaktylou recorded a significant reduction in rots on covered fruit in 6 out of 8 trials, the principal fungi being Monilinia laxa and Botrytis cinerea. That reduction translates into fewer fungicide sprays — meaning the cover recovers part of its cost from plant protection as well.

The drawbacks are real and have to be weighed: high installation cost, higher temperature and humidity under the cover, which can damage leaves and fruit and raise disease risk, and poorer fruit colouring from reduced light. Even so, the assessment in the literature remains that a covering system is the profitable option for reducing rain-induced cracking.

Nets as an intermediate solution Beyond full plastic covers, it is documented that cover nets too improve resistance to cracking. They do not match the effectiveness of plastic — they do not stop the rain — but they reduce the force of impact, modify the microclimate, and cost distinctly less with a smaller penalty on colouring. For holdings where a full cover is not economically feasible, they are the logical intermediate option.

10. Irrigation and nutrition

What makes sense in irrigation

Stability, not deprivation. The aim is not to make the tree thirsty before the rain. It is to avoid abrupt swings in soil moisture, which cause waves of rapid fruit expansion and therefore sharp increases in cuticular strain.
Monitoring by measurement. Irrigation has to be set from soil moisture readings during the critical periods and according to the susceptibility of the cultivar — not by the calendar.
Late preharvest deficit irrigation. This has been studied as a tool in combinations such as Regina on Gisela 6. It is a technique that requires measurement and experience; applied blind, the cost in fruit size can exceed the benefit.

The nutrition that matters Deficiencies in calcium, potassium, boron and phosphorus make cultivars more prone to cracking. Before any special application is added, the base has to be right — and that is checked by leaf analysis, not by estimation. An orchard with a deficiency is not going to be rescued by two silicon sprays.

11. How susceptibility is measured

The cracking index and its limits

The established laboratory method is the Christensen method: fruit is immersed in water and the percentage that cracks is recorded at set intervals. The resulting cracking index is the basic tool for comparing cultivars in the international literature.

Care is needed in interpretation, however: the method does not adequately simulate real field conditions — immersed fruit receives water from every side and continuously, which is not what rain does. A Greek research group (Michailidis and co-workers) proposed, for precisely this reason, a complementary method using rainfall simulation. In practice: the statement that a cultivar «has a cracking index of X» is a relative ranking, not a prediction of what will happen in a particular orchard.

12. The overall strategy

Ranked by effectiveness

Cultivar choice at planting. With heritability up to 0.9 for apical cracking, no application corrects a wrong cultivar choice in a wet region.
Cover or net, where the economic scale of the holding allows it. It is the only measure that changes the order of magnitude of the loss, and it reduces rots and sprays as well.
Rapid drying of the fruit after rain. Low cost, high return — because of the fivefold permeability of the pedicel.
Early calcium applications, with timing more important than rate.
Silicon, preferably through the soil so as to avoid white spotting.
Osmoprotectants at colour change and close to harvest.
Gibberellin, on prescription, tested first on a limited area for each cultivar.
Steady irrigation and correct nutrition as the foundation for all of the above.

13. Sources

Sweet cherry fruit cracking mechanisms and prevention strategies: A review. Scientia Horticulturae. General review of mechanisms and measures.
Rain cracking in sweet cherries is not due to excess water uptake but to localized skin phenomena. Journal of the American Society for Horticultural Science 141(6):653. The paper that overturned the classical osmotic model.
Sweet cherry fruit: ideal osmometers? Frontiers in Plant Science. Source for the low turgor of ripe fruit and the absence of any turgor response to water uptake.
Factors influencing fruit cracking: an environmental and agronomic perspective. Frontiers in Plant Science (2024). Description of the «zipper» model and the role of microcracks.
Swelling of cell walls in mature sweet cherry fruit: factors and mechanisms. Planta. Source for cell-wall swelling and the weakening of the middle lamella.
Spatial heterogeneity of flesh-cell osmotic potential in sweet cherry affects partitioning of absorbed water. Horticulture Research. Source for the heterogeneity of the flesh.
Xylem, phloem, and transpiration flows in developing sweet cherry fruit. Trees. Water flows in the developing fruit.
Water uptake through sweet cherry fruit pedicels: influence of fruit surface water status and intact fruit skin. Acta Agriculturae Scandinavica B. Source for the fivefold permeability of the pedicel and for the hydraulic connection between apoplast and xylem.
Multi-year analyses on three populations reveal the first stable QTLs for tolerance to rain-induced fruit cracking in sweet cherry. Horticulture Research. Source for the three cracking types, the heritability values and the QTL on linkage group 2 of Regina.
Detection of breeding-relevant fruit cracking and fruit firmness QTL in sweet cherry. Frontiers in Plant Science. Source for the ranking of cultivar susceptibility, Regina and Sweetheart among them.
Cracking susceptibility of full-sibs of a cross of a cracking-tolerant and cracking-susceptible sweet cherry: relation to cuticle characteristics, microcracking and calcium. The link between cuticle, microcracks and calcium.
Effect of preharvest calcium treatments on sweet cherry fruit quality. Comparison of calcium caseinate, chloride, hydroxide and nitrate on the cultivar 0900 Ziraat. Source for the 38–66% reduction in the cracking index and specifically for the 62% figure for calcium chloride.
Calcium sprays and crop load reduction increase fruit quality and postharvest storage in sweet cherry. Agronomy 12(4):829.
Enhancing cracking resistance and post-harvest quality of sweet cherries through calcium and potassium-based foliar treatments. Horticulturae 11(1):30.
Effects of soluble potassium silicate soil drenching on sweet cherry fruit quality. Acta Horticulturae 1020. Source for the 1% potassium silicate soil application and the firmness values in the cultivar Bing.
Prevention of cherry fruit cracking using soluble potassium silicate — Washington Tree Fruit Research Commission.
Gibberellic acid increases fruit firmness, fruit size, and delays maturity of «Sweetheart» sweet cherry. Journal of the American Pomological Society. Source for the 20 ppm at the straw-yellow stage and for the absence of benefit from multiple applications.
The effect of gibberellic acid applications on the cracking rate and fruit quality in the «0900 Ziraat» sweet cherry cultivar. The study in which cracking increased.
Foliar application of calcium and growth regulators modulate sweet cherry tree performance. Plants 9(4):410. Comparison of calcium, GA₃, ABA, salicylic acid, glycine betaine and Ascophyllum nodosum extract on the cultivar Skeena.
Effect of a plastic rain shield on fruit cracking and cherry diseases in Greek orchards. Crop Protection (2013). Greek data on rain covers, the reduction in rots and the reduction in fungicide use.
Use of a plastic rain shield reduces fruit decay and need for fungicides in sweet cherry. Plant Disease 87(5):523.
Orchard net covers improve resistance to cherry cracking disorder. Foods 12(3):543.
Understanding and preventing sweet cherry fruit cracking — Oregon State University Extension, EM 9227.
Sweet cherry fruit cracking: follow-up testing methods and cultivar-metabolic screening. Plant Methods 16:51. Greek work (Michailidis and co-workers) on the limits of the Christensen method and the complementary rainfall-simulation method.
Effect of late preharvest deficit irrigation on physiological and agronomical responses in «Regina»/Gisela 6 sweet cherry.

Disclaimer This text is informative and educational in character and combines the literature with the author's empirical conclusions from orchards in the region. It does not replace on-site agronomic assessment and does not constitute a prescription for application. Authorisations, rates and pre-harvest intervals change and differ by country and by crop; the label instructions and the national authorisations always apply. Foliar sprays applied over fruit can cause fruit spotting and leaf scorch: a trial on a limited number of trees, with the same product at the same rate, and a wait of 5 to 7 days, is required before any general application, and spraying is done in the evening or early morning, never in a heatwave and never on foliage under water stress. Pastopoulos Agronomics accepts no liability for the use of the information above.