Rain Damage on Three Strawberry Cultivars Grown in Subtropical Queensland

Abstract

In Queensland, Australia, strawberry (Fragaria × ananassa Duchesne) is grown in open fields and rainfall events can damage fruit. Following a rain event, damage was evaluated on three commercial cultivars. ‘Rubygem’ (80%) had more fruit damaged than ‘Strawberry Festival’ (55%) and ‘Camarosa’ (61%). “Etch,” where the surface of the fruit is eroded and, consequently, the seeds are raised relative to the damaged surface, was the most frequently occurring (>80%) damage type and was distributed on the body and tip of the fruit, while some fruit (<16%) showed cracking. Fully mature fruit was damaged (>80%) more than partially mature fruit, which differed between ‘Strawberry Festival’ (16%) and ‘Rubygem’ (68%). Cultivars that are resistant to rain damage would reduce losses and lower risk for the growers.

Keywords:

• weather damage
• subtropical
• etch
• cracking
• Fragaria × ananassa

INTRODUCTION

In Queensland, Australia, 80% of strawberry (Fragaria × ananassa Duchesne) production occurs in the region between Brisbane and the Sunshine Coast (27°S, 153°E), with a major concentration in the Wamuran-Caboolture area about 50 km north of Brisbane. The fruit production season, from late autumn (May) through early spring (September), is characterized by moderate day (20 to 24°C) and night (9 to 16°C) temperatures and relatively low humidity (46 to 55%). Rainfall during the production season is typically low (average mean monthly rainfall 61 mm), but rainfall events are highly variable in intensity (highest monthly rainfall 626 mm, highest daily total 301 mm), duration, and timing but few commercial growers use covering structures.

The consequences of rain damage in strawberries can include: reduced marketable yield, increased harvesting, grading and packing costs, coupled with reduced consumer demand and, consequently, lower prices (Herrington et al., 2009). Rain damage may include water soaking, surface etching (i.e., where the surface of fruit is eroded and, consequently, the seeds are raised relative to the damaged surface), and cracking, in three general locations on the fruit—stem end (neck, shoulder), body, and tip (Herrington et al., 2009). However, a better understanding of the type and extent of rain damage and its variation among cultivars would provide information helpful in choosing options for evaluation and selection of protective techniques, including breeding resistant genotypes. We, therefore, conducted preliminary studies to describe the type and extent of damage to fruit following rainfall in southeast Queensland. The study reported here included evaluation of the severity, type, and location of damage on fruit of the three cultivars most commonly grown in the region and the effect of fruit maturity.

MATERIALS AND METHODS

To obtain a sample of fruit representative of the damage in the field (i.e., with minimal pre-assessment culling), we selected a farm where all the harvested fruit, except rotting fruit, are placed in the picking containers. Fruit on this farm is harvested and packed separately (as is common practice in Queensland). To provide the samples of fruit for assessment, we purchased fruit after harvest but before packing. Two 2.5-kg containers of ungraded, field-grown fruit of each of the cultivars, Strawberry Festival, Rubygem, and Camarosa, were purchased within 24 hr of their harvest and immediately following a rainfall event that had occurred over the three previous days. This rainfall event had registered 89 mm (58 [over the first 2 days combined] and 31 mm on the 3rd day) at Maroochy Research Station and 51.8 mm (11.8, 17.2, and 22.8 mm over the 3 days, respectively) at Maroochydore. These recording stations are 15 km west and 5 km east, respectively, of the farm. The approximate weighted average of rainfall from the two stations was 60 mm. Fruit were picked from first year plant crops.

The fruit were stored in polythene bags for 2 days at 4°C. From these bulk samples, 165, 93, and 108 regularly shaped fruit of ‘Strawberry Festival’, ‘Rubygem’, and ‘Camarosa’, respectively, were selected for evaluation. These fruit were sorted according to damage type, location of damage, and severity of damage. Also, the primary and secondary damage type and the location of damage on the fruit were noted. Fruit size was estimated using the average fruit size (total weight/total number) of eight random bulk samples each of about seven fruit in each cultivar.

The following terms were developed to describe the different types of rain damage and the primary location of damage on the fruit: “Etch,” the surface of fruit is eroded and consequently the seeds are raised relative to the damaged surface; “water-soak,” fruit tissue takes on a translucent “water soaked” appearance; “desiccated seed,” the fruit tissue immediately around a seed appears degraded and dry but the remaining fruit surface is intact; “star-crack,” longitudinal cracks in fruit flesh; “ring-crack,” transverse cracks in fruit flesh; and “body-crack,” oblique cracks in fruit flesh (Herrington et al., 2009) (Fig. 1). Damaged fruit of ‘Strawberry Festival’, ‘Rubygem’, and ‘Camarosa’ were randomly sampled to classify the damage types within and across cultivars. Also, 32, 66, and 58 “etched” fruit of ‘Strawberry Festival’, ‘Rubygem’, and ‘Camarosa’ were sampled to compare the location of the “etch”-type damage on fruit. The observed fruit numbers among categories with the expected fruit numbers were compared with numbers calculated from row and column totals using Chi-squared (χ²) values (with r-1, c-1 degrees of freedom) with the “Pearson method” selected within the “contingency table goodness of fit” module of GenStat 11th Ed. (GenStat, 2008). The three cultivars were assessed initially in a combined analysis and then pair-wise between cultivars. When the expected fruit number within a cultivar × damage type or location was less than five, indicated by n_E < 5 in the χ², this was noted in the initial analysis and, subsequently, the number of categories was reduced by excluding categories with all zero frequencies or pooling data across categories, beginning with the category of the lowest frequency, so that in many cases, the main location and type or incidence were compared with “all others” in a 2 × 2 contingency table.

FIGURE 1 Rain damage types on fruit. Upper Image: Cracking (Ring-crack; bottom left. Star-crack; bottom center. Ring- and Star-crack; upper). Etch (On tip; bottom right. On body; bottom center). Lower Image: Desiccated seed (i.e., achene) (color figure available online).

The estimate of severity of damage was categorized and quantified on a 1 to 8 scale. The severity of damage was first quantified and then fruit was sorted into categories based on the quantitative measure. The relationships between category and measurements were as follows: Where damage did not involve cracking, we used ‘x’ for the surface area damaged (mm²), estimated by visual comparison with square templates, where category 1 (x = 0), 2 (0 < x ≤ 10), 3 (10 < x ≤ 50), 4 (50 < x ≤ 100), 5 (100 < x ≤ 225), 6 (225 < x ≤ 400), 7 (400 < x ≤ 625), 8 (625 < x) mm². For the damage classified as cracking, we used ‘y’ length of crack where category 1 (y = 0), 2 (0 < y ≤ 3), 3 (3 < y ≤ 7), 4 (7 < y ≤ 10), 5 (10 < y ≤ 15), 6 (15 < y ≤ 20), 7 (20 < y ≤ 25), 8 (25 < y) (mm). Where more than one damaged section occurred on a fruit, we summed the estimated damaged sections score. Where more than one damage type occurred on a fruit, we visually compared the extent of damage between types and nominated the type with the larger score as the primary damage type.

With a 3- or 4-day interval between harvests, a range of fruit maturity is likely at harvest. As the effect of maturity on rain damage has not been documented, we made preliminary observations to determine the association between the severity of rain damage and fruit maturity. Samples of 74 ‘Strawberry Festival’ and 93 ‘Rubygem’ fruit from within the severity classes for rain damage were classified into two maturity groups, based on external color: a more mature group (M-0), where the entire fruit surface was red (full colored), and a less mature group (M-2), where the neck areas of the fruit lacked red color. Full-colored fruit (M-0) is considered “eating ripe” and the less mature group (M-2) 2 days before “eating ripe.” We counted fruit in each cell of the “damage severity” × “fruit maturity” × “cultivar” matrix and compared the observed with the expected fruit numbers using χ² values within the “contingency table goodness of fit” module of GenStat 11th Ed. (GenStat, 2008) initially in a combined analysis of 4 × 8 table and then pair-wise between combinations of cultivars and maturity.

RESULTS

The proportion of fruit of ‘Strawberry Festival’ and ‘Camarosa’ showing damage following the rain event were similar (55 and 61%, respectively (χ² _df=1 = 1.16, P = 0.28). However, in both of these cultivars, the proportion of fruit showing damage, i.e., ‘Strawberry Festival’, (χ² _df=1 = 16.12, P < 0.001) and Camarosa, (χ² _df=1 = 8.01, P = 0.005) was less than in ‘Rubygem’ (where 80% of the fruit was damaged). The cultivars differed from each other (χ² _df=14 = 66.92, P < 0.001), in their distribution among damage severity categories with ‘Rubygem’ being in the higher damage categories, ‘Strawberry Festival’ in the center, and ‘Camarosa’ spread across categories (Table 1). The fruit of ‘Strawberry Festival’ (average weight 21.3 g) and ‘Rubygem’ (23.8 g) were similarly sized and both smaller (Pr < 0.01) than ‘Camarosa’ (26.1 g).

TABLE 1 Number of Fruit in Damage Categories for Three Cultivars Following a 3-Day, 60-mm Rain Event Ending 23 June 2009 ^z

	Damage score ^z
Cultivar	1	2	3	4	5	6	7	8	Total
Strawberry Festival	75	10	12	16	30	15	6	1	165
Rubygem	19	5	8	3	9	14	16	19	93
Camarosa	42	7	13	9	13	6	10	8	108
Total	136	22	33	28	52	35	32	28	366
	Damage category
Cultivar	Undamaged			Damaged			Total
Strawberry Festival	75			90 (55) ^y			165
Rubygem	19			74 (80)			93
Camarosa	42			66 (61)			108
Total	136			230			366
^zFruit were separated into categories 1 to 8 according to severity of damage. Where damage did not involve cracking, category 1 (x = 0), 2 (0 < x ≤ 10), 3 (10 < x ≤ 50), 4 (50 < x ≤ 100), 5 (100 < x ≤ 225), 6 (225 < x ≤ 400), 7 (400 < x ≤ 625), 8 (625 < x), where ‘x’ is surface area damaged (mm^2). Where damage involved cracking, category 1 (y = 0), 2 (0 < y ≤ 3), 3 (3 < y ≤ 7), 4 (7 < y ≤ 10), 5 (10 < y ≤ 15), 6 (15 < y ≤ 20), 7 (20 < y ≤ 25), 8 (25 < y), where ‘y’ is length of crack (mm).
^yNumbers in parentheses indicate percent of total fruit.

“Etch” was the most frequently occurring damage type (Table 2) with similar percentages of damaged fruit affected as “etch” in ‘Strawberry Festival’, (84%), Rubygem (89%), and Camarosa (88%) (χ² _df=2 = 0.57, P = 0.75, n_E < 5). This primary damage described as “etch” was located on the body or tip of the fruit and the location of damage was similar (χ² _df=1 = 0.84, P = 0.36) in ‘Rubygem’ (74% body, 26% tip) and ‘Camarosa’ (67% body, 33% tip). The fruit tips in ‘Strawberry Festival’ (37% body, 59% tip) were more frequently damaged (Table 3) than those in ‘Rubygem’ (χ² _df=1 = 11.43, P < 0.001) and probably ‘Camarosa’ (χ² _df=1 = 6.36, P = 0.012). The “etch” damage was characterized by a reduction of the surface of fruit so that the seeds were raised relative to the damaged surface. In the lighter colored fruit of ‘Rubygem’, the areas affected were noticeably paler, whereas the color difference was less apparent in the darker colored fruit of ‘Strawberry Festival’ or ‘Camarosa’ (i.e., dry the affected area becomes straw colored and papery). The cuticle remained relatively intact but the underlying cells appeared ruptured.

TABLE 2 Number of Fruit in Damage Categories in Three Cultivars Following a 3-Day, 60-mm Rain Event Ending 23 June 2009^z

		Damage type ^z, ^y
Cultivar	Nil damage	Etch	Star-crack	Ring-crack	Desiccated	Total damaged
Strawberry Festival	29	32 (84)	4 (11)	2 (5)	0	38
Rubygem	19	66 (89)	6 (8)	1 (1.5)	1 (1.5)	74
Camarosa	42	58 (88)	0	8 (12)	0	66
Total	90	156	10	11	1	178
^zFruit were separated into damage categories (Herrington et al., 2009). “Etch” type is where the surface of fruit is eroded and, consequently, the seeds are raised relative to the damaged surface; “desiccated seed” is where the fruit tissue immediately around a seed appears degraded and dry but the remaining fruit surface is intact; star-cracks are longitudinal cracks in fruit flesh; ring-cracks are transverse cracks in fruit flesh. “Water soaked” and “body crack” types of damage did not occur and are not shown in the table.
^yNumbers in parentheses indicate percent of total damaged fruit.

TABLE 3 Number of Fruit with “Etch” Damage at Four Locations on the Fruit of Three Cultivars Following a 3-Day, 60-mm Rain Event Ending 23 June 2009^z

	Location of etch damage ^z, ^y
Cultivar	Tip	Body	Shoulder	Neck	Total
Strawberry Festival	19 (59)	12 (37)	1 (3)	0	32
Rubygem	17 (26)	49 (74)	0	0	66
Camarosa	19 (33)	38 (66)	1 (1)	0	58
Total	55	99	2	0	156
^zFruit with “etch” damage (Herrington et al., 2009) from a commercial field were separated according to location of the damage. Previous harvest occurred 4 days, 3 days, and 3 days earlier for ‘Strawberry Festival’, ‘Rubygem’, and ‘Camarosa’, respectively.
^yNumbers in parentheses indicate percent of total fruit.

Nine to 16% of damaged fruit showed some form of cracking as the primary damage type (Table 2). ‘Strawberry Festival’ (11% of fruit affected by star-cracking, 5% affected by ring-cracking) and ‘Rubygem’ (8%, 1%) had similar (χ² _df=2 = 1.76, P = 0.41, n_E < 5) frequencies of star- and ring-cracking, respectively. Together these differed (χ² _df=2 = 11.79, P = 0.003, n_E < 5) from ‘Camarosa’, which expressed only ring-cracking (12% of fruit affected). Ring-cracking and star-cracking occurred only on the neck of the fruit of ‘Rubygem’ and ‘Strawberry Festival’ but seven of the eight ring-cracks in ‘Camarosa’ were located on the shoulders of the fruit (data not shown).

Fruit in the most mature group (M-0), where the entire fruit surface was fully colored, were damaged more severely (χ² _df=1 = 36.14, P < 0.001) than those in the less mature group (M-2), where the neck areas of fruit lacked red color (Table 4). The cultivars differed in the association between damage and maturity. Similar proportions of the more mature and less mature fruit of ‘Rubygem’ were damaged, being 84 and 68%, respectively (χ² _df=1 = 2.83, P = 0.09). By contrast, a higher proportion (χ² _df=1 = 31.6, P < 0.001) of mature ‘Strawberry Festival’ fruit were damaged (81%) than were the immature fruit (16%). In the most mature group, damage on ‘Rubygem’ (84%) and ‘Strawberry Festival’ (81%) were similar (χ² _df=1 = 0.14, P = 0.70). By contrast, the less mature fruit of ‘Rubygem’ (68%) were damaged as much as (χ² _df=1 = 1.45, P = 0.23) the more mature fruit of ‘Strawberry Festival’ (81%). The distributions across the eight damage classes mostly reflected the previously described relationship between damaged/not damaged fruit although the ‘Rubygem’ and ‘Strawberry Festival’ distributions differed (χ² _df=7 = 27.7, P < 0.001) in the more mature fruit (M-0) group (Table 4). ‘Rubygem’ M-0 and M-2 distributions also differed (χ² _df=7 = 19.6, P = 0.006), although some expected that values in the χ² analysis were less than five.

TABLE 4 Number of Fruit in Damage Categories at Two Stages of Fruit Maturity in ‘Strawberry Festival’ and ‘Rubygem’ Following a 3-Day, 60-mm Rain Event Ending 23 June 2009

		Damage score
Cultivar	Maturity ^z	1	2	3	4	5	6	7	8	Total
Strawberry Festival	M-0	7	2	8	3	11	5	1	0	37
Strawberry Festival	M-2	31	2	2	1	0	0	1	0	37
Rubygem	M-0	11	4	7	2	5	6	16	17	68
Rubygem	M-2	8	1	1	1	4	8	0	2	25
		Damage category
Cultivar	Maturity	Undamaged			Damaged			Total
Strawberry Festival	M-0	7			30 (81) ^y			37
Strawberry Festival	M-2	31			6 (16)			37
Rubygem	M-0	11			57 (84)			68
Rubygem	M-2	8			17 (68)			25
^zFruit were separated into categories according to severity of damage (see Table 1) and stage of maturity as judged by external color. At maturity stage M-0, the entire fruit surface was red (full colored), and for M-2, the neck areas of fruit lacked red color.
^yNumbers in parentheses indicate percent of total fruit.

DISCUSSION

The results of these preliminary investigations into the intensity, type, and location of damage relative to maturity across cultivars were consistent with interviews of strawberry growers (M.E. Herrington, unpublished data) in southeast Queensland: following a rainfall event “more ripe” fruit have more damage than less ripe fruit and cultivars may differ in their response. Similar results have been found with other crops (Beyer et al., 2002; Granger, 2005) while Herrington et al. (2009) reported differences in the extent of damage among strawberry cultivars following rain events.

Surface etching appears to be related to water soaking. It is hypothesized that when fruit surfaces are moist for an extended period, water penetrates the surface and the tissue becomes water soaked. This leads to, and is perhaps accelerated by, the rupture of cells. The consequent lack of structural support, because of ruptured cells, lowers the fruit surface to produce “etching.” The etching is accentuated when the fruit surface dries and shrinks back on collapsed cells.

‘Strawberry Festival’ tips were more damaged than those of ‘Rubygem’ and ‘Camarosa’ (Table 3). The reason for this is not known but is possibly due to differences in fruit ripening characteristics, fruit shape, and/or plant architecture. Surface etching was the most frequent problem in the commercial cultivars (Table 2); therefore, focusing any future evaluation work on this type of damage seems justifiable. Additionally, the difference in surface etching among cultivars is consistent with there being genetic variability that can be used in selection for resistance to rain damage.

ACKNOWLEDGMENTS

We thank Horticulture Australia Limited (HAL) and Strawberries Australia Inc. (SAI) for funds partially supporting this study.