Calcium is regarded universally as the most important element for the maintenance of post-harvest quality. This applies not only to apple but to a range of other fresh horticultural crops where inadequate supply of calcium during development results in important economic losses.
The concentration of calcium required in apples at harvest to prevent premature senescence and the development of disorders such as bitter pit during storage have been defined for Cox and Bramley. Other physiological disorders associated with low calcium in the fruit include watercore, lenticel blotch pit, late storage corking and senescent breakdown.
Problems with calcium nutrition of apples generally relate to factors that affect the distribution of this element within the tree rather than a lack of uptake through the roots. However, it is advisable that growers ensure that the maximum amount of calcium is available for uptake by the roots in addition to the adoption of methods to supplement calcium nutrition.
It is particularly important to ensure that the acidity of the soil is optimal i.e. a pH level of 6-6.5. On a sandy soil improvement in the concentration of calcium in Cox apples was achieved by liming and by application of calcium sulphate to the soil. There was an additive positive effect of the treatments on fruit calcium concentration.
Calcium concentrations in bulked samples of apples are usually inversely related to the mean weight per apple. Therefore any management practice that increases mean fruit weight is likely to result in a lower concentration of calcium in the fruit. Practices that contribute calcium to the fruit without reducing fruit size need to be encouraged but often there is a requirement for directly supplementing calcium supply to the fruit by the use of orchard calcium sprays.
The application of calcium sprays should be considered as routine for cultivars such as Cox, Bramley, Egremont Russet and Spartan. Under UK conditions some cultivars, such as Gala, have very high calcium concentrations in the fruit and it is therefore questionable whether calcium sprays are warranted. There are some key points to consider with regard to the efficacy of calcium sprays.
- Calcium chloride is generally the preferred form of calcium for foliar application to UK cultivars. Although it is more likely to cause leaf scorch than the main alternative, calcium nitrate, it is less likely to damage the fruit.
- Calcium chloride may cause unacceptable leaf scorch on Egremont Russet and for this cultivar the use of calcium nitrate is advised.
- Recent research suggests that calcium nitrate sprays are effective in improving the firmness of Cox apples at harvest and during storage and may be preferable to calcium chloride sprays.
- However, the general use of calcium nitrate sprays is not advocated since a number of cultivars have suffered lenticel damage as a result. These include Discovery, Bramley's Seedling, Idared, Laxton's Superb, Spartan, Merton Worcester and Tydeman's Early Worcester.
- Proprietary forms of calcium chloride and other calcium salts are available to UK growers and these should be applied in accordance with manufacturer's instructions. However, it is important that growers ensure that spray programmes based on proprietary products provide an equivalent amount of actual calcium as a 'standard' calcium chloride programme (see below).
- Alternatively, where lower rates of commercial products are recommended, evidence should be provided of their greater efficacy.
Rate of application
- The major factor that determines the effectiveness of a calcium spray programme is the amount of actual calcium that is applied per hectare during the growing season.
- The minimum amount of calcium chloride flakes (78% calcium chloride) or calcium nitrate prills (79% calcium nitrate) that should be applied during the season is 72 kg and 110 kg hectare-1 respectively regardless of spray volume.
- Low volume (50‑100 litres hectare-1) applications of calcium chloride proved as effective as higher volume applications in raising calcium status and reducing bitter pit in Bramley.
- However, results also showed that no reduction in the amount of calcium chloride applied per hectare ws possible at reduced volumes of water. Low volume applications caused minimal leaf scorch and were not injurious to the fruit.
- Although no research has been done at East Malling Research on the use of ‘PreTect’, data provided by FAST Ltd indicate a significant improvement in the calcium content of apple fruits following foliar application at 14 and 28 days prior to harvest.
- A range of beneficial effects on storage quality of apples are reported by FAST Ltd in conjunction with the manufacturers (Plant Health Care plc).
Timing of sprays
- Increase in the calcium concentration in the fruit is affected by direct uptake of the spray solution by the fruit. Consequently as the fruits become larger during development there is a larger 'target' for the calcium sprays and, therefore, late sprays are generally more effective than early sprays.
- Spraying of early cultivars should commence in early June and for later maturing cultivars in mid-late June. It is important to continue spraying as near to picking as is practicable.
- Early sprays are often reduced to half-strength to avoid damage to young foliage. In this situation provision should be made to increase the frequency of application to ensure that the recommended minimum amount of calcium is applied during the season.
- The same advice applies where the risk of leaf scorch from calcium applied during warm weather (>21oC) necessitates half-strength application. It may not be necessary to be as cautious about spraying in high temperatures when applying calcium chloride in water volumes of 100 litres per hectare or less.