Investigation and Prevention of Cork Spot Disorder in ‘Akizuki’ Pear (Pyrus pyrifolia Nakai)

Comparison of fruit quality attributes between normal and cork spotted fruit.

The average fruit weight of disordered fruit was significantly higher than that of normal fruit (Table 1), which is similar to the report of Hayama et al. (2017). The diameter of fruit pedicels and the calyx concave depth in disordered fruit were also significantly greater than that of normal ones (Table 1); however, total soluble solids in each part of disordered fruit was significantly lower than that in the same part of normal ones (Table 2). In addition, there was no significant difference in titratable acidity and fruit firmness with the exception of that in the calyx end (Table 2).

Table 1.

Fruit characteristic of cork spotted and normal ‘Akizuki’ fruits.

Table 2.

Fruit quality of cork spotted and normal ‘Akizuki’ fruits.

We also determined the contents of vitamin C and carbohydrates, including soluble sugar, reducing sugar, and starch of different fruits. From the calyx end to the stem end, the content of soluble and reducing sugar, and vitamin C in normal fruit were significantly higher than those of disordered fruit (Fig. 2). Inconsistent with the report of Li et al. (1999), the starch content in our research was much higher in cork spotted fruits than normal fruit, but the difference was not significant. All the results presented previously indicated that cork spotted pear fruit had larger fruit size and decreased the contents of total soluble solids, soluble and reducing sugar, and vitamin C, which resulted in fruit quality deterioration.

Fig. 2.

Comparison of carbohydrate content of fruit flesh between cork spotted and normal ‘Akizuki’ pear. Error bars with different lowercase letters represent a statistical difference by Duncan’s multiple range test (P < 0.05).

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Fig. 2.

Comparison of carbohydrate content of fruit flesh between cork spotted and normal ‘Akizuki’ pear. Error bars with different lowercase letters represent a statistical difference by Duncan’s multiple range test (P < 0.05).

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Fig. 2.

Comparison of carbohydrate content of fruit flesh between cork spotted and normal ‘Akizuki’ pear. Error bars with different lowercase letters represent a statistical difference by Duncan’s multiple range test (P < 0.05).

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Analysis of the free Ca2+ localization in the flesh of cork spotted and normal pear fruit.

At harvest, the free Ca2+ localization was detected in the flesh cells loaded with fluo-4/AM. High levels of free Ca2+ were observed in the cell wall and intercellular space of cork spotted fruit flesh; however, lower free Ca2+ spread over all of the flesh cells of normal fruit (Fig. 4). This cellular distribution difference of free Ca2+ between cork spotted and normal fruit may be a reason for cork spot. In addition, the free Ca2+ in the cork spotted fruit was much greater than normal fruit, which was in accordance with the results of total Ca content (Fig. 3), and partially consistent with the results of Wang et al. (2018), who suggested that the free Ca2+ localization in the flesh cells of hard end fruit was greater than that of normal fruit at harvest (120 DAFB), whereas it showed an opposite tendency during ‘Whangkeumbae’ pear fruit development (75–105 DAFB) (Wang et al., 2018).

Fig. 4.

Localization of free Ca2 + fluorescence signals in the flesh cells of normal and cork spotted ‘Akizuki’ pear. (A–C) Free Ca2+ fluorescence signals in normal fruits by Argon ion laser excitation (A), light field (B), and merge field (C). (D–F) Free Ca2+ fluorescence signals in cork spotted fruits by Argon ion laser excitation (D), light field (E), and merge field (F). Scale bar is 250 µm for (A–C), and 100 µm for (D–F), respectively.

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Fig. 4.

Localization of free Ca2 + fluorescence signals in the flesh cells of normal and cork spotted ‘Akizuki’ pear. (A–C) Free Ca2+ fluorescence signals in normal fruits by Argon ion laser excitation (A), light field (B), and merge field (C). (D–F) Free Ca2+ fluorescence signals in cork spotted fruits by Argon ion laser excitation (D), light field (E), and merge field (F). Scale bar is 250 µm for (A–C), and 100 µm for (D–F), respectively.

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Fig. 4.

Localization of free Ca2 + fluorescence signals in the flesh cells of normal and cork spotted ‘Akizuki’ pear. (A–C) Free Ca2+ fluorescence signals in normal fruits by Argon ion laser excitation (A), light field (B), and merge field (C). (D–F) Free Ca2+ fluorescence signals in cork spotted fruits by Argon ion laser excitation (D), light field (E), and merge field (F). Scale bar is 250 µm for (A–C), and 100 µm for (D–F), respectively.

Citation: HortScience horts 54, 3; 10.21273/HORTSCI13775-18

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Effect of exogenous treatments on fruit quality in ‘Akizuki’ pear.

To further test the effects of exogenous treatments on fruit quality in ‘Akizuki’ pear, we investigated the fruit characteristics of treated fruit and unsprayed control. The results showed that all exogenous treatments had no negative effects on fruit quality attributes, especially for fruit size (Table 4). Fruit quality attributes were unaffected by P-Ca, similar to that found for ‘Anjou’ (Einhorn et al., 2014), ‘Bartlett’ (Elfving, et al., 2003), ‘Abate Fetel’ (Costa et al., 2004), and ‘Le Conte’ (Carra et al., 2016); however, it was not in agreement with the results in which P-Ca sprayed fruits were much larger than control fruits (Costa et al., 2001), or reports in which P-Ca has been reported to be associated with reduction of fruit size (Smit et al., 2005; Sugar et al., 2004). This may be due to the application time and concentration of P-Ca.

Table 4.

Effects of exogenous treatments on fruit characters of ‘Akizuki’ pear.

Furthermore, exogenous treatments helped to improve the development of fruit pedicels and the calyx concave: Ca(NO3)2 significantly increased the diameter and shortened the fruit pedicels, H3BO3 promoted the diameter increment of fruit pedicels, and P-Ca at different rates decreased the calyx concave depth (Table 5). Moreover, spraying of Ca(NO3)2 and H3BO3 solutions significantly reduced the titratable acidity, resulting in improved fruit flavor. For soluble solids, P-Ca with 1500 or 3500 times dilution did not differ from control (Table 5), which agrees with those found for ‘Rosemarie’, ‘Forelle’, ‘Packham’s Triumph’, and ‘Le Conte’ pear (Carra et al., 2016; Smit et al., 2005). P-Ca with 2500 times dilution significantly decreased fruit firmness and improved total soluble solids in the middle and the calyx of fruit (Table 5). To sum up, five sprays of P-Ca with 3500 times dilution was promise for inhibiting the incidence of cork spot without affecting fruit quality attributes, and could be used widely in ‘Akizuki’ pear cultivation.

Table 5.

Effects of exogenous treatments on fruit quality.