Grecia Hurtado
Moritz Knoche- 1Centro de Investigación de Alimentos (CIAL), Facultad de Ciencias de la Ingeniería e Industrias, Universidad UTE, Quito, Ecuador
- 2Institute for Horticultural Production Systems, Leibniz-University Hannover, Hannover, Germany
Strawberry (Fragaria × ananassa Duch.) is a high-value crop grown for the fresh fruit market. An attractive appearance is a key quality factor but is often compromised by fruit-surface disorders. Common ones are water soaking, cracking, bronzing, albinism and dried calyx. This review summarizes our current understanding of the mechanistic bases of these disorders, their triggers and their effective mitigation strategies. Water soaking and cracking both require the formation of cuticular microcracks, which develop when the fruit cuticle fails under tensile strain during expansion growth. Formation of microcracks is aggravated by surface wetness and exposure to high relative humidity. Cracking manifests as visible splits that penetrate the underlying tissues, whereas water soaking originates from excessive water uptake through the pre-existing microcracks, resulting in translucent, water-logged areas on the fruit surface. Bronzing results from a range of causes, most commonly high temperature and ultraviolet stress, which damage epidermal tissues and induce repair responses that produce brownish or yellowish, desiccated surface patches. Albinism and dried calyx, by contrast, are associated with physiological and nutritional imbalances, including excessive vegetative vigor and impaired calcium transport under saline conditions, leading to poorly colored, soft fruit and necrotic sepals, respectively. Although these disorders differ in symptoms, they share common drivers related to environmental stress, tissue integrity, and mineral nutrition. Management therefore targets multiple leverage points, including reduced moisture exposure, moderation of thermal stress, optimized mineral nutrition, and the selection of genotypes with more resilient surface tissues. Better management of surface disorders in strawberry requires a more thorough understanding of their mechanistic bases. It is likely the ultimate solution will be an integrated approach that combines cultural measures and selective breeding.
1 Introduction
Strawberry is a soft, fleshy fruit of very high significance globally. It is consumed both fresh and processed (mostly as jam) and valued for its desirable organoleptic and nutritional qualities (Hummer and Hancock, 2009). Strictly, strawberry is a ‘false fruit’ because the edible portion is the swollen fleshy receptacle, while the achenes (tiny pips) on the receptacle surface are the true fruit. Strawberry is one of the most widely commercialized fruit crops worldwide (Hancock, 2020). In recent years, production has averaged 10.5 million tons annually from about 435 thousand hectares (FAO, 2024). The rising demand for strawberries in the fresh market requires high-quality fruit. However, strawberries deteriorate rapidly and so have an extremely short shelf-life. There are no practical options for long-term storage, even under optimal temperature/atmosphere conditions. This poses a challenge to harvesting, transport and marketing. Fruit appearance is the quality trait most valued by consumers. Fruit appearance is primarily a property of the skin, with aspects such as color, texture, shape, surface integrity and the absence of defects being the main ones. Appearance plays a crucial role in determining market value and consumer acceptance (Steenkamp, 1990). Poor surface quality increases production costs due to additional labor required for harvesting, sorting, rejection and (re)packing. Strawberries are particularly susceptible to a range of pre- and postharvest surface disorders, especially when grown under open-field conditions.
Fruit surface disorders are mostly confined to the skin, with the internal tissues often being unaffected. Common surface disorders include water soaking, cracking, bronzing, albinism and dried calyx – the fruit is harvested with the calyx attached. Given their high importance to fruit quality, to shelf life and to market value, the surface disorders of strawberries have received inadequate research attention. As far as we are aware, a systematic review of the causes of the more important pre-harvest surface disorders of strawberry and their associated avoidance strategies is not available. This review summarizes the current understanding of the five most common surface disorders of strawberry. We focus on the symptoms, the physiological bases, the environmental factors triggering the disorders and some potential countermeasures. In this way, gaps in our knowledge are identified and directions for new research are outlined.
2 Search strategy
We explored the databases of Scopus and Web of Science for publications between 1960 and 2025 and limited the results to research articles, reviews and books. For each disorder, Boolean search queries were constructed using relevant synonyms and phrase matching, as detailed in Table 1, including the number of hits retrieved. After de-duplication, titles, abstracts, and keywords were screened to identify studies meeting predefined relevance criteria: studies focused on strawberry that (i) address surface-related physiological disorders, (ii) disorders with preharvest or developmental origin, (iii) report mechanistic or causal evidence linking symptoms to environmental, structural, or nutritional drivers; and (iv) evaluate management or mitigation strategies, including cultural practices, environmental control, nutritional interventions, or breeding-related approaches. These studies were used for critical evaluation and synthesis. For each disorder, the corresponding number (n) includes both strawberry-focused studies and closely related supporting literature that underpins the mechanistic interpretation, resulting in water soaking (n = 20), fruit cracking (n = 17), albinism (n = 18), bronzing (n = 12), and dried calyx disorder (n = 8). Additional cross-cutting references were used to provide broader context on fruit development, cuticle properties, calcium nutrition, vascular transport, and stress physiology.
Table 1. Search keywords and number of scientific papers (1960–2025) related to major surface disorders in strawberry.
3 Strawberry fruit: morphology and growth
Botanically, the edible portion of the strawberry derives from the enlarged floral receptacle. In this sense, the strawberry is a false fruit comprised largely of accessory tissue. The true fruit are the ‘pips’ – the achenes embedded in the receptacle surface (Darrow, 1966). Achenes are distributed on the receptacle in spirally-arranged rows. The achenes’ position on the surface varies among cultivars, ranging from being in deep depressions to being superficial. The area of the surface lacking achenes is known as the neck (Figure 1A) (Mathey et al., 2013; Hurtado and Knoche, 2023c). Fruit may have an elongated neck or a normal neck. The reason for the elongation of the neck is unknown. The strawberry fruit is composed of a succulent receptacle and a cup-shaped calyculus formed by inner sepals and outer bracts (epicalyx or epiphyllous sepals). The calyculus is commonly called a ‘calyx’ (Darrow, 1966; Hollender et al., 2012). The receptacle has four tissues: epidermis, cortex, bundle zone and pith (Figures 1A, B) (Darrow, 1966; Antoszewski, 1973).
Figure 1. Morphology of strawberry fruit: (A) whole fruit, (B) longitudinal cross-section through the receptacle illustrating inner architecture with vascular bundles, pith and cortex (C) close-up of the epidermis with a single achene depression, and (D) close-up of the calyx, stamina, junction, and proximal receptacle. Microphotographs of microcracks in the cuticle of the strawberry fruit skin. (E) image taken in incident bright light, (F) image shows the same specimen viewed in fluorescent light after 5 min incubation in the fluorescence tracer acridine orange. Acridine orange does not penetrate an intact cuticle, but penetrates through microcracks that bypass the cuticle as a penetration barrier. Fluorescence color indicates local concentration of the fluorescent tracer: (orange>yellow>green for high>intermediate>low concentrations). The achene depression has numerous transverse (white-traced) and radial microcracks (dashed-white-traced), Scale in A and B = 5 mm, C = 500 µm, D = 2 mm and E, F = 100 µm.
The strawberry skin acts as the physiological and mechanical interface between the fruit and its aerial environment. The primary skin of the strawberry consists of a thin cuticular membrane that overlays a single-layer epidermis and a subepidermal parenchyma (Polito et al., 2002). Trichomes occur on the surface (Figure 1C) (Mauseth, 2016). Epidermal cells are polygonal, elongated, and thick-walled (Szczesniak and Smith, 1969; Polito et al., 2002). The cuticle serves critical barrier functions. It limits water loss and gas exchange, restricts osmotic water uptake and prevents infections by pathogens (Knoche and Lang, 2017). The cuticle of strawberries is exceptionally thin (0.62 g m-2~ 0.52 μm) and among the thinnest reported for fleshy fruit (Petracek and Bukovac, 1995; see Table 1 in Hurtado et al., 2021 for a compilation). It is composed primarily of cutin (mainly C16 and C18 hydroxy fatty acids), epicuticular and embedded wax and phenolic compounds (Järvinen et al., 2010; Straube et al., 2024).
Unlike many fleshy fruit, there is no distinct hypodermis beneath the epidermis. Instead, a subepidermal layer of cortical cells is present (Polito et al., 2002). The cortex is the fleshy part that accounts for most of the volume of the receptacle. It is composed of rounded cells with large intercellular spaces. As these cells enlarge, the tissue becomes parenchymatous, characterized by thin-walled, isodiametric, vacuolate cells (Polito et al., 2002; Hancock, 2020). The bundle zone consists of xylem and phloem. Vascular bundles form a central cylinder with radially orientated branch vessels connecting to the achenes. These offer both nutrient supply to the achenes and mechanical support to the tissue in which they are embedded (Figure 1B) (Maas and Line, 1995). The pith is located in the center and composed of thin-walled cells, which frequently pull apart during fruit growth. Because the cortex often grows more quickly than the pith, large air-filled cavities can form in the center of the fruit (Szczesniak and Smith, 1969).
Strawberry fruit develop rapidly over 20 to 40 days after anthesis. The growth pattern is sigmoidal or double sigmoidal, depending upon conditions, cultivar and sampling frequency (Nitsch, 1950; Perkins-Veazie and Huber, 1987). Cell division ceases between 10 and 15 days after anthesis (Cheng and Breen, 1992). Approximately 90% of the subsequent fruit volume growth is accounted for by cell enlargement (Havis, 1943; Hancock, 2020). In the epidermis, cell diameter increases occur to accommodate the volume growth of the underlying tissues (Szczesniak and Smith, 1969). The expansion of the parenchymatous cortex is restrained in the immediate vicinity of the achenes by limited growth of the radially orientated vascular bundles resulting in their depression, whereas the areas between achenes lack the radial vascular bundles and so grow slightly more. This results in a ‘corrugated’ fruit surface with the achenes being pulled more or less deeply into the skin. Depression depth varies between genotypes.
Cuticle deposition in strawberries decreases during fruit development and eventually ceases completely. The cessation of cuticle deposition is a result of downregulation of the genes involved in cuticle synthesis and deposition (Straube et al., 2024). Thus, cuticle deposition no longer keeps pace with continued fruit expansion. Hence, continued fruit growth results in a thinning of the cuticle and significant increases in tangential strain. This strain causes microscopic damage to the cuticle, the so-called cuticular microcracking. Microcracks are minute fractures of the cuticle layer that are not visible to the naked eye (Hurtado and Knoche, 2023b). The strawberry’s ‘corrugated’ surface exacerbates this effect by focusing the tangential stresses around the achenes (Hurtado and Knoche, 2023b). Consequently, microcracks radiate from the achenes like the spokes in a bicycle wheel, with the achene at the hub. These microcracks result from the widening of the funnel-shaped achene depressions, and are consistent with the increasing tangential strain as fruit surface area increases (Figures 1E, F). Microcracks are also found on the raised ‘rims’ between achenes, at the calyx-receptacle junction, and at the corky (mechanically stiff) scars left as the stamens and petals abscise (Figure 1D) (Hurtado and Knoche, 2023a). Cuticular microcracks impair the barrier properties of the skin (Peschel and Knoche, 2005) and so play critical roles in the surface disorders of many fruit crop species, including of strawberry (Bangerth, 1979). At this point, the Ca relations of strawberry fruit deserve mention. Calcium is a key mineral for fruit quality (Cieniawska et al., 2023). During development of strawberry fruit, the Ca/dry-mass ratio increases with time only in the calyx, but it decreases in the skin, flesh, pith and achenes. Spatially, the Ca/dry-mass ratio is highest at the proximal end of the fruit (nearest the calyx) and declines distally (toward the tip) (Hurtado et al., 2025a). The reason for the decrease in Ca concentration during fruit development is the consequence of the interplay between several factors. First, Ca moves only in the xylem sap, not in the phloem sap (Marschner, 1995). Next, fruit volume increases hugely between fruit set and fruit maturity so, for the fruit’s Ca concentration to be maintained, requires a steady influx of Ca throughout development to match the volume growth. But there is a progressive decrease in the proportion of the xylem sap water inflow to phloem sap water inflow to the fruit (from 80% down to 36% of the total xylem+phloem water inflow). Meanwhile, the proportion of phloem sap water inflow increases towards maturity (from 20% to ~64% of total xylem+phloem water inflow) (Winkler et al., 2021). The decline in xylem sap flow is due to a decline in xylem functionality due to the stretching and breakage of xylem vessels as a result of growth strain (Düring et al., 1987; Lang and Ryan, 1994; Grimm et al., 2017). Consequently, Ca inflow (via the xylem) decreases, whereas K inflow (via the phloem) increases (Hurtado et al., 2025a). Hence, fruit Ca concentration decreases during development and fruit K concentration increases.
4 Water soaking
Water soaking (WS) is a common surface disorder of field-grown strawberries. It occurs more in regions and in season with higher incidences of rainfall during the ripening period. The disorder typically develops when ripe fruit is exposed to extended periods of surface wetness or to high humidities (Herrington et al., 2011). Field studies under subtropical conditions report that rainfall can damage approximately 55–80% of fruit, with water soaking symptoms accounting for >80% of rain-related damage (Herrington et al., 2013). Water-soaking also increases the incidence of gray mold (Botrytis cinerea) and other fruit rots (Herrington et al., 2009). Water soaking causes severe economic loss due to severe yield loss and also the additional labor required for grading out the affected fruit (Menzel et al., 2017).
4.1 Symptoms and mechanism
The skins of ripe, water-soaked strawberries show irregular deliquescent patches – watery, pale, translucent and slightly pinkish and dull (Hurtado and Knoche, 2021) (Figure 2). These symptoms are largely limited to the thin skin itself and rarely extend into the flesh. Occasionally water-soaking when dried is also referred to as ‘surface etching’ (Herrington et al., 2011). Water soaking often begins on the rims between the achene depressions (Figure 2B). Fluorescence microscopy reveals numerous cuticular microcracks in the water-soaked patches (Figure 2C) but none in the adjacent sound (healthy) areas (Hurtado and Knoche, 2021). The extent of microcracking can be quantified using the water soluble fluorescent tracer acridine orange (AO) (Peschel and Knoche, 2005). The tracer AO does not penetrate an intact cuticle but penetrates a microcracked cuticle. Following incubation in AO solution, the fruit is rinsed in water, blotted and then viewed by fluorescence microscopy. The microcracked areas appear as bright orange, yellow or green fluorescing zones for high, medium or low tracer concentrations (Peschel and Knoche, 2005). The fluorescent areas may be quantified using image analysis.
Figure 2. (A) Macroscopic view of fruit with water soaking symptoms; (B) Micrographs of developing symptoms of water soaking viewed under incident and (C) fluorescent light. Scale in A = 5 mm, B,C = 500 µm.
The mechanism of water soaking in strawberry resembles that of rain-cracking in sweet cherry (Winkler et al., 2016). First, surface moisture and – to a lesser extent – high humidity trigger or exacerbate microcracking which compromises the cuticle’s barrier functions (Hurtado and Knoche, 2023b). The high concentration of soluble carbohydrates and organic acids in the cells of the skin and outer flesh layers lowers their (osmotic) water potential and so creates a steep water potential gradient driving water uptake by the fruit. Microcracks in the cuticle now provide a pathway for rapid, localized water uptake by viscous flow, a much faster influx than the slow diffusion through an intact cuticle. Not surprisingly, the permeability of the strawberry skin to water is among the highest reported for any soft, fleshy fruit (Hurtado et al., 2021). Second, the rapid water uptake causes the underlying cells to burst. Their cell contents are released into the apoplast (Winkler et al., 2015). Strawberries are rich in citric and malic acids (Herrmann, 2001), particularly the epidermis that has a two-fold higher concentration of acids than the flesh (Kim et al., 2010). Third, the leakage of these acids dramatically increases the permeability of the plasma membranes of neighboring cells, causing further leakage. The organic acids extract Ca from the cell wall, resulting in decreased cross-linking of pectins and decreased cell-to-cell adhesion (Winkler et al., 2015). This chain reaction causes water soaking to spread across the skin, and in some cases deeper into the flesh.
4.2 Factors and triggers
Water soaking in strawberry fruit is closely linked to structural and genetic traits. Microcracks are a prerequisite to WS. Developmental stage and fruit firmness are also important factors influencing susceptibility to microcracking and WS. During ripening, cell walls loosen and the flesh softens. This decreases the structural support for the thin cuticle and increases the likelihood of microcracking and cortical cell bursting (Hurtado and Knoche, 2021). Additionally, during ripening, the fruit’s water potential becomes more negative due to the accumulation of carbohydrates and organic acids. The turgor of the parenchyma cells is negligibly low and hence the osmotic potential essentially equals the fruit water potential. The decrease in osmotic potential at constant low turgor increases the driving force for water uptake through the skin (Hurtado et al., 2021). The bursting of cells promotes localized swelling during wetness, particularly near the fruit tip (distal end) where the osmotic potential is more negative than in the calyx region (proximal end). Furthermore, the degradation of cell walls associated with softening also decreases cell-to-cell adhesion. These ripening-associated changes increase the susceptibility of mature strawberries to water soaking. At maturity, larger fruit are more susceptible than smaller fruit – probably as a result of greater straining of the skin (Herrington et al., 2011; Hurtado and Knoche, 2021).
Surface wetness and high humidity increase microcracking and thus are critical factors in WS (Hurtado and Knoche, 2023b). Water soaking does not occur unless liquid water contacts the skin (Hurtado and Knoche, 2021). In the orchard, this usually means rainfall, heavy dew or poorly-scheduled overhead irrigation that causes extended periods of surface wetness or high humidity. Also, the microclimate (particularly air flow and vapor pressure deficit) affects the duration of surface wetness, and, hence, the likelihood of WS. Dense stands are likely to be more prone to WS than stands having an open canopy structure.
Strawberry genotypes differ in their susceptibility to WS (Herrington et al., 2009; Chang and Brecht, 2020; Hurtado et al., 2024). A recent study of a total of 172 strawberry genotypes from three different populations reported a significant albeit moderate genetic component of the disorder. Water soaking was significantly correlated across different seasons with a coefficient of correlation of 0.46 (n= 31; p<0.01). Thus, approximately 21% of the phenotypic variation is accounted for by the genotype, indicating a substantial contribution of the environment. Furthermore, water soaking was consistently correlated with the skin’s permeance for water uptake. Genotypes that had higher skin permeances developed more water soaking (Hurtado et al., 2024). A high skin permeance is indicative of extensive microcracking (Hurtado et al., 2021). Averaged across the three populations of cultivars in that study, nearly half (~46%) of the variation in water soaking of the genotypes was explained by the variation in the skin’s permeance (Hurtado et al., 2024). In contrast, other traits such as fruit size (surface area) or the osmotic potential of the fruit’s expressed juice, were not or were only weakly correlated with water soaking. The Ca content of the fruit was not investigated in that study.
4.3 Control measures
Mitigation strategies to manage WS comprise cultural measures in the short term and breeding strategies in the long term.
Cultural measures include increasing fruit Ca content by sprays. Earlier studies demonstrate that fruit Ca content and susceptibility to WS are negatively related (Hurtado and Knoche, 2022; Hurtado et al., 2025b). Fruit high in Ca is less susceptible to WS. Fruit Ca concentration affects susceptibility to WS in several ways: (1) Ca decreases microcracking of the cuticle, (2) it stabilizes cell walls by cross-linking cell-wall constituents and (3) it maintains membrane integrity, thereby helping reduce the likelihood of cell bursting (Hocking et al., 2016; Khanal et al., 2021; Hurtado and Knoche, 2022; Winkler et al., 2024). Preharvest Ca sprays can increase fruit Ca content and reduce the susceptibility to water soaking (Hurtado et al., 2025b). Inorganic Ca salts such as CaCl2, that have a low point of deliquescence and, hence, favorable penetration characteristics, are more effective in increasing fruit Ca than organic Ca salts that typically have higher points of deliquescence (Hurtado et al., 2025c).
Lastly, all factors that reduce surface wetness duration are effective in reducing WS. These include rain shelters, greenhouses, drip irrigation instead of overhead sprinklers in open field cultivation, short irrigation cycles, open canopy structures, and timely harvesting ahead of wet weather.
In the long term, breeding of genotypes with reduced water soaking susceptibility will be effective. Important traits are the permeance of the skin to water, the mechanical strength of the dermal tissues, and the structural integrity and susceptibility of the cuticle to microcracking (Hurtado et al., 2024). These factors were all shown to affect WS susceptibility. Up to now, the genetic and molecular basis of WS in strawberries is unknown. Understanding their basis will lead to the identification of molecular markers that will greatly facilitate the development of tolerant genotypes through marker-assisted breeding.
5 Cracking
Macrocracking – also referred to as rain cracking or splitting – is a physiological disorder that impairs fruit quality in many soft-fruit species, including sweet cherry (Opara et al., 1997), grape (Considine and Kriedemann, 1972), blueberry (Marshall et al., 2009) and tomato (Domínguez et al., 2012). In strawberry, losses due to cracking can exceed 50% of yield after a single rainfall event in open-field production (Herrington et al., 2013). Cracked fruit must be discarded since it is readily infected by fruit rot pathogens. Despite its importance, macrocracking has been less studied in strawberry than in many tree-fruit species. A recent study concluded that in many respects, macrocracking in strawberry is similar to that in sweet cherry (Hurtado and Knoche, 2023c). Fruit with a short neck (Figure 3A) are less susceptible to cracking than fruit with an elongated neck (Figure 3B), particularly in the late stages of ripening (Hurtado and Knoche, 2023c).
Figure 3. (A) Normal shape strawberry fruit (short neck), (B) necked shape strawberry (elongated neck), (C) longitudinal macrocrack in the mid region of the neck (D), and a latitudinal macrocrack in the proximal region of the neck. Scale = 5 mm.
5.1 Symptoms and mechanism
Macrocracking (to distinguish it from cuticular microcracking) refers to visible fractures in the fruit skin that extend from the cuticle deep into the underlying flesh. In strawberry fruit macrocracking usually occurs as longitudinal (Figure 3C) or latitudinal cracks (Figure 3D) in the neck area, just distal to the calyx. Cracks may extend a few millimeters into the flesh (Hurtado and Knoche, 2023c). Microscopic observations show that macrocracks are preceded by microcracks. Microcracks result from excessive cuticular strain caused by the early cessation of cuticle deposition (Straube et al., 2024). In the field, water-soaking and cracking may appear together in rain-damaged strawberry. Sometimes water-soaked areas break open into cracks as the damage proceeds. Therefore, macrocracking in strawberry is similar to macrocracking in sweet cherry, where macrocracking follows the microcracking of the strained cuticle, as a microcrack propagates into the underlying flesh.
Macrocracking in the neck region occurs in two dominant orientations: latitudinal cracks in the proximal neck (Figure 3D), and longitudinal cracks in the mid and distal neck regions (Figure 3C) (Herrington et al., 2011; Hurtado and Knoche, 2023c). In each case, these orientations are normal to the directions of the underlying growth strain and in line with the orientations of cuticular microcracking (Hurtado and Knoche, 2023c). In the proximal neck, hoop stress around the calyx causes strain. In addition, stress concentrations occur around ‘stiff’ regions, such as petal and stamen abscission scars (Figure 1D). Furthermore, achenes and, hence, radial vascular bundles (that supply them with assimilates) are lacking in the neck region, and so the structural support they offer. When the diameter of the median and distal regions of the neck increase, the skin more readily ruptures in the longitudinal direction, where the mechanical support offered by vascular bundles is lacking. Latitudinal and longitudinal cracking is exacerbated by localized water uptake through cracks (Hurtado and Knoche, 2023a). More distally, in the main body of the fruit, the vascular bundles that supply the numerous achenes reinforce this part of the fruit (Figure 1B). In this way, the spatial pattern of macrocracking (both the location and the orientation) reflects the interplay between the mechanical architecture of the fruit and the stresses induced by expansion growth, with severity being driven by the durations of surface wetness (Hurtado and Knoche, 2023c).
5.2 Factors and triggers
Since macrocracks result from microcracks, all factors causing microcracking will increase the likelihood of macrocracking. For strawberry, these include growth strain and stress as the primary drivers of cracking (Darrow, 1966). Consequently, for a given genotype, larger fruit are more susceptible to cracking than smaller ones. Fruit with a neck are more susceptible than those without a neck – due to a lack of structural reinforcement by radial vascular bundles (Hurtado and Knoche, 2023c). Fully ripe fruit are more susceptible than immature ones. Ripe fruit have the most strained cuticles, the weakest support of the cuticle by the underlying epidermal cells and the most negative water potentials – so, an increased water uptake rate (Herrington et al., 2013; Hurtado et al., 2021).
Surface wetness and high humidity further increase microcracking of the strained cuticles (Hurtado and Knoche, 2023c). Even a brief shower during the harvest season can initiate significant cracking in ripe strawberry (Herrington et al., 2009). Water uptake exacerbates cracking in strawberry and many other soft fruit, such as sweet cherry and grape (Becker and Knoche, 2012; Knoche and Winkler, 2017). The macrocracking induced by water uptake in laboratory immersion assays is microscopically indistinguishable from that occurring naturally in the field. This observation indicates that water uptake through the fruit surface and tangential skin strain in the neck region are causal (Hurtado and Knoche, 2023a, 2023c).
5.3 Control measures
Preventing fruit surface wetness by protected cultivation in plastic tunnels or in greenhouses is effective in avoid cracking in strawberry. Keeping the fruit consistently dry reduces rain-induced damage by 20–40% compared with open-field production (Menzel et al., 2014). Good ventilation and proper design of the irrigation system are essential to avoid high relative humidities that can trigger microcracking – even in a protected environment (Hurtado and Knoche, 2023b).
To our knowledge, direct studies on the effect of spray applications of Ca salts on cracking susceptibility in strawberry are lacking. Spray application of CaCl2 reduced water soaking by approximately 40% (Hurtado et al., 2025b). Since the underlying physiological mechanisms are the same for both disorders, we expect similar reductions in cracking. Evidence from Ca sprays that lower rain-cracking in sweet cherry fruit supports this view (Demirsoy and Bilgener, 1998; Wójcik et al., 2013; Erogul, 2014; Winkler et al., 2024). In sweet cherry, Ca reduced rain-cracking by cross-linking homogalacturonans. As a result, cell-to-cell adhesion was increased, and cell swelling was reduced (Schumann et al., 2022). Winkler et al. (2024) demonstrated that Ca salts reduced macrocracking of sweet cherry when applied to wet fruit. Here, the Ca penetrated through pre-existing microcracks, bound to pectins, and prevented microcrack development into macrocracks. A similar mechanism may be expected in strawberry. This aspect merits experimental validation. However, Ca is expected to be less effective in preventing cracking in necked fruit.
Susceptibility to cracking varies markedly among strawberry cultivars. In subtropical Queensland, ‘Rubygem’ strawberry suffers about 25–20% more rain damage than ‘Festival’ and ‘Camarosa’ (Herrington et al., 2013). Meanwhile, the molecular basis of cracking susceptibility is not yet clear. It is thought likely that candidate genes in less susceptible genotypes will be those that result in thicker and more extensible cuticles, firmer fruit, increased cell-to-cell adhesion and reduced neck formation (Herrington et al., 2011; Hurtado and Knoche, 2023c). Hence, cracking susceptibility is likely a multi-gene property (Herrington et al., 2011; Wang et al., 2021).
6 Bronzing
Bronzing refers to fruit with discolored skin (Figure 4A). The damage is cosmetic and market quality is reduced. Three different types of bronzing occur. Type 1 bronzing (T1B) is associated with injury from arthropod feeding, most commonly by western flower thrips. Type 2 bronzing (T2B) results from phytotoxicity caused by agrochemicals. Type 3 bronzing (T3B) is linked to environmental stresses and is the most frequent bronzing damage (Polito et al., 2002; Larson et al., 2004; Koike et al., 2009). T3B has been reported to have an incidence of up to 70% in the crop, and causes severe economic loss (Larson et al., 2006; Koike et al., 2009).
Figure 4. Strawberry fruit with symptoms of (A) bronzing, (B) albinism and subsequent decay, and dried calyx disorder on (C) immature fruit (lateral view) and (D) ripe fruit (top view). Scale = 5 mm.
6.1 Symptoms and mechanism
Bronzed strawberries exhibit discolored and/or desiccated patches of skin with a brownish or yellowish tinge. Cracks often occur in the damaged tissue (Polito et al., 2002). The pattern and extent of bronzing depend on the type of bronzing. T1B produces a characteristic net-like russeting between achenes in patches in the neck and the body of the receptacle, where these pests commonly feed (Maas, 1998). T1B is the least frequent bronzing type, with less than 1% of bronzing damage being assessed as T1B and, when present, primarily attributed to thrips (Matos and Obrycki, 2004). T2B results from the phytotoxicity of agrochemicals. Symptoms are confined to areas having direct contact with the spray solution. Examples include damage from overhead spraying to run-off, forming hanging drops of agrochemical spray. It is most common with sprays containing sulfur or other caustic ingredients or surfactants (Larson et al., 2004). T3B typically affects the entire fruit surface uniformly, giving the whole berry a dull, brownish cast that resembles sun scald or heat injury (Polito et al., 2002). Damage is already apparent on unripe fruit that later ripens to leave the fruit with abnormal coloration and without gloss. While phytotoxicity and pests are the known causes for T1B and T2B, the mechanistic basis for T3B is not known. Circumstantial evidence suggests heat and/or UV damage are involved. Microscopic observation indicates that, in damaged areas, the cuticle is thin and disrupted. The subsequent defense response of the fruit involves thickening of the cell walls, with deposition of lignin and likely suberin, and cell death. Phenol oxidation and polymerization are also involved (Polito et al., 2002; Amil-Ruiz et al., 2011). These processes are likely causal in the color change typical for T3B in strawberry (Polito et al., 2002) and for sunburn in other fruit crop species (Renquist et al., 1989; Racsko and Schrader, 2012; Gambetta et al., 2020).
6.2 Factors and triggers
The incidence of all types of bronzing appears to be principally linked to environmental conditions. A combination of high temperature, low humidity and intense solar radiation seems associated with bronzing (Larson et al., 2004; Koike et al., 2009), Surprisingly, low temperatures are also reported to increase T3B bronzing although the effect is indirect (Koike et al., 2009). When strawberry crowns were exposed to prolonged freezing temperatures in winter, the meristematic tissues were injured. Consequently, vegetative growth was delayed, and the plants developed a very open canopy. The lack of a denser canopy not only depressed yield but also reduced shade, leaving early-summer fruit unprotected and highly susceptible to bronzing when temperatures and irradiation later rose. This explains why the frequency of T3B bronzing peaks in spring and early summer, and is lower in late summer when canopy development is complete and natural fruit shading is sufficient (Larson et al., 2004, Larson et al., 2006).
Susceptibility to bronzing differs between cultivars, suggesting genetic factors also play a role. Field reports and experiments have noted that some strawberry cultivars are more prone to bronzing than others under the same conditions. For example, the cultivar ‘Commander’ has been reported to be bronzing-sensitive (Larson et al., 2004). The mechanistic basis of these differences is unknown. Grower observations suggest that cultivars with lighter-colored fruit (i.e., berries that are paler when ripe) are more susceptible to T3B than those with dark red fruit (Koike et al., 2009). Darker fruit may have more UV-absorbing pigments (e.g., anthocyanins) per unit surface area than lighter fruit (Steyn et al., 2002; Pfündel et al., 2008; Duan et al., 2025).
6.3 Control measures
Integrated pest management mitigates T1B bronzing by a combination of monitoring, biological control, various cultural practices and effective pesticide use (Lahiri et al., 2022). T2B bronzing is reduced by restricting sprays to label-approved products and avoiding applications during hot, dry periods that increase phytotoxicity (Onofre et al., 2021). Field observations show that environmental bronzing (T3B) is the bronzing form responsible for the majority of symptomatic fruit (Koike et al., 2009). Cultural practices to minimize T3B are summarized in Table 2. The most effective methods are cooling via overhead sprinklers, the application of lignin sulfonates and derivatives and commercial pesticides such as carbamate, organophosphates and others for pest control (Table 2) (Koike et al., 2009). Shading and bed mulching either have no effect at all or inconsistent effects.
Table 2. Control strategies for bronzing (T3B) incidence in strawberry fruit. Results are compiled from literature sources.
Water evaporation cools the canopy and increases microclimate humidity. Californian strawberry growers use this strategy; on very hot days they operate sprinklers in brief cycles in mid-afternoon. However, this must be done with care as extended periods of wetness can induce water soaking and cracking and increase the risk of infection by pathogens. This cooling practice is similar to that used to reduce sunburn on apples (Lal and Sahu, 2017). An alternative is to spray lignin-based compounds whose dry residue absorbs UV and may be protective (Suota et al., 2021). However, since this has a contact mode of action, coverage of the fruit surface must be complete. Moreover, the use of these sprays must comply with local legal regulations, and the environmental conditions at the time of application and achieving the right dose are all critical, particularly with sulfonated lignin (Evdokimov et al., 2018).
Other approaches include the application of products against sunburn. Examples are suspensions of clay minerals or CaCO3 or emulsions containing carnauba wax can all be effective in reducing sunburn in apple, citrus and pineapple (Lal and Sahu, 2017; Rodriguez et al., 2019; Reig et al., 2020). Cultural measures like varying planting dates, installation of shade nets, covering beds with plastic mulch, or use of low tunnels can all reduce the risk of late-spring frost to improve vegetative growth and the provision of natural shade to the fruit (Koike et al., 2009; Hernandez et al., 2016; Hernández-Martínez et al., 2023).
7 Albinism
Albinism, also referred to as white fruit disease, is a physiological skin disorder that affects the color of the fruit skin (Figure 4B). Fruit suffering from albinism is pale pink or white at commercial ripeness (Ulrich, 1971). Albino fruit is highly susceptible to mechanical damage and decay. This disorder is particularly problematic in subtropical regions and under protected cultivation (Sharma and Sharma, 2003).
7.1 Symptoms and mechanism
Fruit suffering from albinism is bloated and has irregular patches of discoloration on the surface that progress from white, to yellow and eventually to brown (Figure 4B). The entire receptacle may become affected. The flesh often remains pale and glassy. The texture is soft. The achenes are sunken and surrounded by puffy pale tissue (Ulrich et al., 1992). Albino fruit are normal in size and shape but fail to ripen normally and uniformly. Flavor is poor and more acidic than in heathy fruit. Albino fruit is especially susceptible to mechanical damage during harvest and to fruit rots during storage (Lieten and Marcelle, 1993; Sharma et al., 2019). The dry matter content is lower than that of healthy fruit (Sharma et al., 2019). Albinism frequently occurs during the production peak, when vegetative growth is vigorous due to excessive nitrogen (N), periods of warm temperature, and cloudy skies. A shortage of carbohydrates during ripening exacerbates albinism (Ulrich et al., 1992).
The physiological mechanisms underlying albinism are unclear. Several hypotheses were suggested. Some evidence points to a disruption of anthocyanin synthesis, particularly of pelargonidin-3-glucoside (Lin et al., 2018). This glucoside accounts for over 70% of total anthocyanins in strawberry. Consistent with this is a decrease in total polyphenols and lipoxygenase (LOX) activity in albino, as compared to normal fruit (Sharma and Singh, 2010). Potential causes for the downregulation of anthocyanin synthesis may be elevated levels of auxins and gibberellins associated with more vigorous vegetative growth. In addition, imbalances in Ca nutrition and in LOX activity may further suppress the expression of genes involved in anthocyanin synthesis. Ultimately this could lead to a deficiency in fruit coloration (Xu et al., 2014; Zhao et al., 2021). Lipoxygenase is involved in the synthesis of jasmonic acid (Garrido-Bigotes et al., 2018). However, these mechanisms are hypothetical and must be experimentally proven.
7.2 Factors and triggers
Mineral nutrition plays a critical role in the development of albinism. High N supply is considered the main cause of the disorder (Sharma et al., 2019). Elevated N fertilization promotes excessive vegetative growth. Vigorously growing plants exhibit a higher incidence of albinism (Lieten and Marcelle, 1993). This effect is attributed to competition for carbohydrates between leaves and fruit, as well as hormonal imbalance, particularly due to increased activity of auxins and gibberellins. However, high N alone does not trigger albinism. Studies indicate that albinism results from an imbalance between K and Ca, with the ratios N:Ca and K:Ca being 37% and 49% higher in albino fruit than in normal fruit, respectively (Lieten and Marcelle, 1993; Sharma and Sharma, 2003; Sharma et al., 2006b). However, this conclusion is based on a small number of studies, and updated replications under diverse growing conditions are scarce. The loss of xylem functionality in developing fruit is likely another factor contributing to albinism.
Environmental factors, including light intensity and temperature, influence the occurrence of albinism. Hence, shading and/or overcast skies delay normal ripening and increase the incidence of albinism. In a study by Sharma et al. (2006b), shaded plants produced on average 49% more albino fruit than unshaded plants, though this effect varied significantly across cultivars. Similarly, strawberry production in greenhouse or high-tunnel during wintertime increases the likelihood of albinism. Also, shade resulting from high planting density exacerbates albinism. Extreme temperatures also affect the incidence of albinism. Thus, low temperatures during fruit development can inhibit key enzymes in the anthocyanin biosynthesis pathway, reducing both pigment accumulation and overall yield (Mao et al., 2022). On the other hand, high temperatures accelerate anthocyanin degradation (Ikeda et al., 2011). This poses a challenge for strawberry production in subtropical regions (>21 °C) (Menzel, 2021). Interestingly, elevated temperatures before flowering reduced the incidence of albinism (Dale et al., 2017).
Susceptibility to albinism varies significantly among strawberry cultivars (Ulrich et al., 1992; Lieten and Marcelle, 1993; Sharma and Sharma, 2003; Sharma et al., 2006a; Wani et al., 2017). There is a relationship between cultivar vigor and albinism (Sharma and Sharma, 2003). This genetic predisposition contributes to albinism and could be used in breeding programs to develop more vigorous cultivars with reduced susceptibility (Plante and Rousseau, 2001). It is important to distinguish the albinism disorder from genetically white-fruited strawberry. Cultivars with white strawberries are often derived from F. chiloensis or from specific breeding lines. Such genotypes have mutations in key genes for pigment synthesis, such as a non-functional FaMYB10 or a blocked anthocyanin pathway (Castillejo et al., 2020; Zhao et al., 2021).
7.3 Control measures
Managing the incidence of albinism in strawberry fruit requires an integrated approach of cultural practices. Optimizing N fertilization is crucial for avoiding albino fruit. Reducing or leaching excess soil N after symptom detection can mitigate the symptoms by restoring the equilibrium in carbohydrate partitioning (Ulrich et al., 1992). While N supply is critical, maintaining a proper K:Ca ratio is equally important. During fruiting, fertilizers with elevated K:N ratios are recommended to support carbohydrate translocation and anthocyanin synthesis (Sharma et al., 2006b). Calcium should be applied repeatedly via sprays because xylem functionality and hence, Ca import through the pedicel declines during development (Winkler et al., 2021; Hurtado et al., 2025a). Among the micronutrients, boron (B) affects the incidence of albinism. Foliar sprays of B alone or of B plus Ca can significantly reduce the incidence of albino fruit (Singh et al., 2009; Arunkumar et al., 2022). Singh et al. (2008) observed that supplementing soil with vermicompost (7.5 t ha-1) reduced the incidence of albinism by 71% compared to plants grown with only inorganic fertilization. In all treatments, total N–P–K was adjusted to the recommended 120–170–150 kg ha⁻¹; the vermicompost plots were supplemented with 51–79–41 kg N–P–K ha⁻¹ to reach this target. Clearly, the database on nutritional effects on the incidence of albinism is limited.
Mulching is reported to decrease albinism by regulating vegetative growth, temperature, soil moisture, and light intensity (Wang et al., 1998). The choice of the mulch material is important, particularly in warm climates. Natural mulches reduced the incidence of albino strawberries compared with standard plastic film mulches (Sharma et al., 2008). Sharma and Sharma (2003) and Wani et al. (2017) found plants mulched with black polyethylene were, on average, 72% and 70% more susceptible to albinism than those mulched with paddy straw, and 17% and 5% more susceptible than with no-mulch (the control).
8 Dried calyx disorder
Although the calyx is not consumed, its healthy appearance is an integral part in the fruit’s visual appeal so is a key aspect of quality (Mathey et al., 2013). The strawberry dried calyx disorder (SDCD) is a surface disorder that affects the marketability of strawberry fruit.
8.1 Symptoms and mechanism
The SDCD is characterized by a progressive drying and browning of the calyx. Symptoms typically start as a slight darkening of the calyx of newly-opened flowers and young fruit (Figure 4C). The initial discoloration of the calyx gradually intensifies, progressing to browning, with the sepals becoming desiccated and necrotic (Figure 4D). In severe cases, the disorder extends to the receptacle, eventually causing discoloration and deformation of the mature fruit (Santos et al., 2008). This disorder is non-pathogenic and may arise from incorrect fertigation that increases the salinity of the soil or growing medium (e.g., perlite or coconut fiber mix). Stress from low temperatures or high irradiance that increases transpiration and accelerates water loss from the calyx can exacerbate SDCD (Santos et al., 2008; Martínez et al., 2014).
Strawberry is particularly susceptible to salinity for two reasons: (1) Disturbed water relations and (2) ion toxicity from Na+ and/or Cl-. High levels of these ions in the soil solution are associated with high values of electrical conductivity (EC) and lowered osmotic potential around the roots which restricts water uptake and transpiration (Martínez and Alvarez, 1997). High salinity of the soil or growing medium, above EC 0.3 mS cm-1 results in moderate incidences of SDCD. When EC exceeds 0.75 mS cm-1, SDCD incidence increases markedly (Martínez et al., 2014). In greenhouse trials, symptoms of SDCD appear at EC ~1.2 mS cm-1 (Santos et al., 2008). Ion toxicity occurs because ions such as Na+ and Cl- are taken up from the soil solution and accumulate in the leaves and fruit, causing phytotoxicity (Ran et al., 2022). Cations such as Na+ displace Ca²+ from cation-exchange sites and compete for Ca-permeable uptake, so that less Ca enters the transpiration stream (Hocking et al., 2016). The combination of osmotic and ionic stresses make xylem-dependent organs such as the calyx especially prone to desiccation and necrosis. This results in the dried calyx symptoms characteristic of SDCD, which are similar to tip burn sometimes observed in young leaves (Palencia et al., 2010; Melis et al., 2014). However, the underlying mechanisms are not fully understood, so further work is needed.
8.2 Factors and triggers
Environmental stresses such as low temperature and low light exposure are primary triggers for SDCD. The strawberry cv. ‘Festival’ was found to develop dried calyx within 10 days of a frost event or after several nights of low temperatures (≤3 °C) (Santos et al., 2008). Comparisons of strawberry production systems show that cooler and more-shaded environments lead to higher incidences of SDCD. Strawberry plants grown in a passive greenhouse (mean temperature 19.5 °C) had more fruit with SDCD (14.3%) than those under warmer low plastic tunnels (mean temperature 25 °C) (Santos et al., 2008). Further, SDCD occurs more frequently in high tunnels and fields in rows closer to entrances and at the ends of drip lines, where temperatures are likely to be lower (Santos et al., 2009). These observations suggest that SDCD incidence increases as growing temperature decreases.
Water management and salinity are intimately linked with SDCD. Insufficient irrigation and poor drainage may lead to salt accumulation in the root zone, especially under intensive fertigation regimes. High soil EC was found to be a consistent factor in farms with high incidences of SDCD (Martínez et al., 2014). Loamy soils that retain fertilizers show more problems than sandy soils, where leaching can remove excess salts (Santos et al., 2008). Palencia et al. (2010) found that tip burn incidences were more closely correlated with foliar K:Ca and K:Mg ratios rather than with absolute Ca levels. K:Ca ratios above 1.8 were associated with >50% tip burn. These factors may also account for SDCD.
Susceptibility to SDCD differs between cultivars, suggesting a genetic component to the disorder. For example, SDCD occurred earlier in the season in cvs. ‘Festival’, ‘Camino Real’ and ‘Palomar’ than in ‘Camarosa’ and ‘Candonga’ (Santos et al., 2008; Palencia et al., 2010; Martínez et al., 2014). The genetic basis of the difference in susceptibility is unclear. Potential traits include differences in root system vigor, Ca uptake efficiency, leaf transpiration rates, and cellular Ca handling. More vigorous, fast-growing genotypes appear more susceptible; probably because growth outpaces Ca supply to newly-developed tissues (Palencia et al., 2010).
8.3 Control measures
Agronomic practices that manage salinity have been successful in mitigating SDCD. Reducing the rates of fertilizer and leaching of the root zone by generous irrigation during the 3 to 4 day periods before and after a stress event (e.g., a frost) is useful for mitigating SDCD (Santos et al., 2008; Martínez et al., 2014). Balanced nutrition is also recommended. An oversupply of K should be avoided, and a sufficient supply of Ca should be maintained. Protecting plants from extreme cold can prevent the disorder. Growers use row covers or overhead irrigation during freezing temperatures to keep tissue temperatures just above the injury threshold. In Spain, the use of low plastic tunnels (which warm the air by a few degrees) was associated with significantly less SDCD than leaving plants fully exposed (Martínez et al., 2014). Lastly, due to the genetic component of the disorder, further research is warranted to identify molecular markers to allow the selection of SDCD resistant cultivars.
9 Conclusions and future research
This review summarizes the current state of knowledge on some common surface disorders of strawberry. From this review, we infer that water soaking, cracking and dried calyx are among the better understood disorders, with bronzing and albinism less well understood. For water soaking, the physiological mechanism and cultural countermeasures have been identified. Maintaining a dry fruit surface is essential. For cracking, a close relation with necked fruit is clear but the cause of neck formation remains uncertain. The fruit water relations and the lack of vascular bundles in the neck region and the resulting growth stress seem to be crucial. Dried calyx is a nutritional disorder, with appropriate fertilization and salinity management offering effective counter measures. However, the information on bronzing and on albinism is scanty at best. In these cases, hypotheses for potential mechanisms have been proposed and must now be tested experimentally. Future research should focus on identifying the physiological mechanism of these disorders. For all disorders reviewed here, differences in susceptibility between cultivars have been noted. This always indicates a genetic component in susceptibility. If these susceptibility differences can be shown to be consistent then cultivar improvement will be possible by breeding. A sound understanding of the physiological mechanisms, the potential triggers and their genetic bases is a prerequisite to developing countermeasures for culture and breeding. Systematic phenotyping across cultivars would uncover genetic components that can be used as molecular markers for developing less susceptible cultivars.
Author contributions
GH: Writing – review & editing, Writing – original draft, Conceptualization, Formal Analysis, Investigation, Visualization. MK: Writing – review & editing, Writing – original draft, Conceptualization, Funding acquisition.
Funding
The author(s) declared that financial support was received for this work and/or its publication. Open Access funding enabled and organized by Projekt DEAL. Funded in part by a grant from the Deutsche FOrschungsgemeinschaft KN 402/24-1.
Acknowledgments
We thank Dr. Alexander Lang for helpful comments on an earlier version of this manuscript.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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The author(s) declared that generative AI was not used in the creation of this manuscript.
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Keywords: albinism, bronzing, cracking, cuticle, Fragaria × ananassa Duch., microcracks
Citation: Hurtado G and Knoche M (2026) A review of surface disorders in strawberry: insights and challenges. Front. Hortic. 5:1779354. doi: 10.3389/fhort.2026.1779354
Received: 01 January 2026; Accepted: 29 January 2026; Revised: 28 January 2026;
Published: 16 February 2026.
Edited by:
Ozkan Kaya, North Dakota State University, United StatesReviewed by:
Hava Delavar, North Dakota State University, United StatesLeyla Demirsoy, Ondokuz Mayis University, Türkiye
Copyright © 2026 Hurtado and Knoche. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Moritz Knoche, bW9yaXR6Lmtub2NoZUBvYnN0LnVuaS1oYW5ub3Zlci5kZQ==