Relatively few types of sprays are applied to directly improve the storage quality of apples. Notable exceptions include the use of calcium and phosphorus sprays that are described elsewhere in this part of the Guide.
Most sprays are applied to protect the crop against the damaging effects of fungal diseases, insects and mites.
Bioregulant sprays are also applied to apple trees for specific purposes such as increased fruit set, fruit thinning, fruit shape, improved skin finish or control of shoot growth. Clearly bioregulant sprays are applied to induce a physiological response in the tree or fruit and are therefore more likely to affect the physiology of fruits during the storage period than protectant sprays.
However, a number of fungicides are reported to have side effects that influence storage quality. Recent evidence has shown that the susceptibility of CA-stored Cox apples to diffuse browning disorder (DBD) is associated with the application of triazole chemicals that include the commonly applied fungicide penconazole (‘Topas’ and ‘Topenco’).
There have been examples in the past where bioregulant sprays applied during fruit development have resulted in major adverse effects on the storage quality of the fruit. The most notable example was the use of daminozide (‘Alar’) to control vegetative growth in Cox trees that resulted in a marked increase in susceptibility of stored fruit to core flush and breakdown.
Bioregulants currently permitted for use in apple orchards in the UK do not appear to cause adverse effects on storage quality. However it is true to say that without trials dedicated to testing effects of compounds or mixtures of compounds on storage and eating quality we may be unaware of possible adverse effects in the future.
In early trials the effects of paclobutrazol on storage quality were generally positive whilst GA4+7 (‘Regulex’) appeared to have no effect. However, fruit-setting hormones (GA3 + 2,4,5-TP), recommended where frost has destroyed a high proportion of flowers, had major adverse effects on the storage quality of Bramley apples.
Fruit thinning sprays are expected to have generally beneficial effects on eating quality through establishing the desired leaf to fruit ratio. However, over-thinning will result in oversized fruit and a high leaf to fruit ratio that is likely to result in increased susceptibility to calcium deficiency disorders. Exilis (6-benzyladenine) and the photosynthesis inhibitor Brevis (metamitron) have recently come to the market as thinning agents for apple and have been tested as part of AHDB Project TF 225 to assess the effect of manipulating crop load on dry matter and storage quality.
Previous research projects have demonstrated that triazole fungicides promote DBD to a great extent. Restrictions on the use of triazole products are advised for Cox and Meridian. Other cultivars with Cox as a parent may also be at risk but generally none of these are widely grown in the UK.
Products have been developed that regulate the ripening of fruit on the tree. The ethylene antagonists ReTainR and a more recent formulation of 1-MCP (HarvistaTM) are registered for use in North America but not in Europe. HarvistaTM is undergoing product registration in the EU. SmartFreshTM application to apples in store is fully approved for use in the UK.