Side cracking is generally caused by a rapid water uptake by the root system and is more frequent when it rains during the harvest period. At a histological level, side cracking begins with the formation of micro-wounds on the cuticle. From these discontinuities there is a leak of malic acid that promotes the loss of Ca2+ from the cell walls with consequent epidermal cell death and weakening of the middle lamella. This cascade effect propagates on the surface until the skin splits. This propagation model proposed by Schumann et al. (2019) is known as the “zipper model”. Cultivars with thicker fruit skin were found to be more resistant to cracking. Fruit size is another important predisposing factor to fruit cracking. Cultivars with big fruits are generally more susceptible to cracking. Similarly, agricultural practices aiming to increase cherry size, like the application of growth regulators (gibberellic acid and calcium cyanamide), can increase fruit susceptibility to cracking.

An elevated soluble solids content is another fruit trait that can induce an increase in cracking incidence (Simon et al., 2004). This effect is mainly due to the decrease in the osmotic potential that stimulates water uptake through the fruit skin and this induces a sudden increase in cherry volume. This is particularly the case when there is free water on the fruit surface. Skin elasticity plays a pivotal role in the reduction of the insurgence of cherry cracking because it makes the fruit more resistant to the tension exerted by the increased turgor. Skin elasticity mainly depends on the cultivar and the developmental stage of the fruit. This characteristic makes cultivars like ‘Regina’ naturally more resistant to fruit cracking. The incidence of cherry cracking is also expected to increase in the future as a consequence of climate change, because of the forecasted increase in the frequency in extreme meteorological events. In fact, most climatic models point out that, together with the increase in air temperature, an increase in the evaporation demand is expected to occur with a consequent increase in the precipitation intensity. Altogether these conditions can make some areas no longer suitable for cherry cultivation.

Considering the great economic importance of cherry cultivation and the relevant negative economic impact that cracking can cause, many strategies were proposed to reduce cracking incidence for this crop. The use of plastic anti-rain covers was proposed as a suitable strategy to reduce cracking incidence, but there are many disadvantages related to their adoption, like high installation costs, and the increase in temperature and humidity under the cover that could promote the attack of harmful fungi. Another strategy proposed in the literature is the application of antitranspirant sprays, but these treatments may exert negative effects on fruit composition. In addition, their suitability in reducing fruit cracking is still controversial mainly because of the difficulty in applying these products uniformly on the whole fruit surface.

Growth regulators were widely used in cherry cultivation mainly for increasing fruit size. Probably for this reason, in most of the studies, the use of growth regulators is associated with an increase in cracking incidence. Gibberellic acid, on the other hand, resulted to be effective in containing cracking by increasing skin elasticity. Another possible treatment is the foliar application of calcium, which plays a pivotal role in the strengthening of the middle lamella and pectins. Indeed, the application of this element in many different forms was found to reduce cracking incidence (Simon, 2006).

However, the growing concern for crop sustainability has increased the interest of both growers and researchers in plant biostimulants, a complex and heterogeneous group of products that is resulting very useful for inducing interesting responses in plants even under abiotic stress conditions. Some of these products have also been evaluated to reduce the susceptibility of cherries to fruit cracking. Even though the scientific literature on this subject is still scarse, there are several indications that some biostimulants may be useful for this purpose. Vercammen et al. (2008), in some experiments carried out in Belgium on the “Sweetheart” cultivar, found that the application of protein hydrolysates or algae extracts can induce a reduction of the incidence of cherry cracking even when compared to calcium chloride applications. The authors compared the effects of two protein hydrolysates and a product based on Ascophyllum nodosum, and they highlighted that the foliar application of these biostimulants every two weeks (starting from veraison) at concentrations of 5 L/ha of product (diluted in 700 L of water) allowed to reduce the onset of cracking. These products were found to be very effective in reducing the incidence of cracking caused by water absorption through the skin, while among the three products tested – the one based on A. nodosum also prevented the occurrence of side cracking which often occurs when there is a sudden increase in the availability of water in the soil. The authors also highlighted that protein hydrolysates of plant origin are effective in reducing the incidence of cracking if applied at least two hours before the expected rainfall. Correia et al. (2020) in an experiment carried out in Portugal on ‘Sweetheearth’ cultivar evaluated a biostimulant based on A. nodosum applied three times during the growing season (at the beginning of fruit development, at skin color veraison from green to yellow, at skin color veraison from yellow to red) by foliar application in combination with calcium chloride. In this experiment, biostimulant-based treatments involved the preparation of a solution of 2.5 L per plant containing 1.25 mL of A. nodosum and 12.5 mL of CaCl2 (1.08 L/ha of A. nodosum and 10.8 of CaCl2). The authors showed the effectiveness of this product in preventing cracking. It was observed that the application of the biostimulant induced a cuticle thickening compared to the untreated control and to the plants treated only with calcium.

Gonçalves et al. (2020) in a recent study showed how the application of A. nodosum extracts stimulated the production of vitamin C, increasing its content by 74%. Furthermore, the authors observed an increase in the antioxidant activity and an increase in the sugar content of the fruits of the ‘Staccato’ cultivar. The experiment involved a total of three foliar applications of the biostimulant (four, seven, and eight weeks after flowering) at a concentration of 200 mL per 100 L of water.

Basile et al. (2021) in an open field experiment on ‘Kordia’ and ‘Regina’ cultivars studied the effects on fruit quality and yield of the foliar application of a tropical plant based biostimulant in combination with calcium. The treatments included a first application at the open sepals phase with 3.75 L/ha of A. nodosum extract plus 3 L/ha of organic fertilizer (5% of calcium and 3% of organic nitrogen from a plant-derived protein hydrolysate). Then 1.5 L/ha of an extract from tropical plants was applied in combination with the same organic fertilizer used in the first treatment. This was done at full bloom, petals fallen phase and a week after petal fall. The results indicated that, in particular in the ‘Kordia’ variety, this strategy improved the calcium absorption into the fruit by 26% which, as mentioned, is one of the aspects that can prevent the onset of cracking. In addition, the application of the biostimulants also induced an interesting increase in fruit yield in both varieties (of 7% in ‘Kordia’; 13% in ‘Regina’), an improvement in fruit size and color, and a soluble solids content increase of 2.16 °Brix in ‘Kordia’ fruit.

In general, the scientific literature seems to agree that biostimulants are an important resource for the control of fruit physiological disorders. It is important to underline the several studies have showed the significant improvement of plant nutrient use efficiency induced by biostimulants. This effect is very important considering the massive effort being made to make agricultural production more sustainable. Greater efficiency in the use of nutrients can allow a reduction in the use of mineral fertilizers, with positive implications on the sustainability of agricultural systems. Furthermore, the use of biostimulants could represent a valid alternative to synthetic plant growth regulators for organic crops. Despite this, further researches are required to better elucidate the suitability of biostimulants for the prevention of cherry cracking and their mode of action.