Rootstocks, pruning and shoot growth
Effects of hard pruning on apple quality and storage potential are similar to those induced by light crops, either naturally or by thinning. Vigorous extension growth competes with the developing fruit for available water and mineral nutrients although fruits are well supplied with carbohydrates.
A high leaf to fruit ratio can lead to competition for minerals such as calcium and consequently to a higher incidence of calcium deficiency disorders such as bitter pit.
It is also generally true that bitter pit incidence increases with increasing vigour of the rootstock. This is an important consideration when planting cultivars that are particularly susceptible to bitter pit such as Cox (various clones), Egremont Russet and Bramley’s Seedling.
Remedial measures such as calcium sprays and more stringent CA storage conditions (see Section 9) are less likely to provide a commercial level of control of bitter pit and associated disorders where vegetative growth is excessive.
It is important that considerations about the choice of rootstock take full account of the likely vegetative growth at any particular site and the likely impact upon the storage quality and strategy for marketing the fruit.
Pruning Cox trees in mid-August as opposed to the winter increased red colour development and drastically reduced the incidence of bitter pit in stored Cox apples.
Similarly late summer pruning (mid August) of Bramley (MM.106) trees reduced bitter pit incidence by 10% and 5% in air and CA storage respectively. Two consecutive years of summer pruning reduced bitter pit incidence by 50% and 69% in air and CA-stored fruit respectively.
Although heavy summer pruning may slightly depress fruit size, a yearly programme of heavy summer pruning (requiring only a minimal amount of pruning in the winter) is recommended for mature Bramley orchards with a recurrent bitter pit problem.
Orchard sprays of paclobutrazol (‘Cultar’) and more recently of prohexadione (‘Regalis’) are used commonly by growers to prevent excessive growth in apple trees.
Effects of single and multiple applications of paclobutrazol on the storage quality of Cox’s Orange Pippin and Bramley’s Seedling apples were investigated over a number of years. In general, single or split applications of paclobutrazol applied to the same trees over a number of years slightly reduced fruit size, increased firmness and reduced bitter pit incidence.
There was some evidence of increased scald development in Bramley apples from trees sprayed with paclobutrazol, possibly related to a retarded maturity at harvest. This is unlikely to present a commercial problem where post-harvest chemical treatments such as diphenylamine (DPA) or SmartFresh are applied (see Sections 7 and 8) or where other ethylene control strategies are applied (see Section 10).
There is evidence that the recent problem of diffuse browning disorder (DBD) in CA-stored Cox is associated with the use of paclobutrazol and of fungicides with a similar chemical (triazole) structure (see Section 13).
In trials done in commercial Cox orchards, repeated applications of paclobutrazol have sometimes induced DBD in the stored fruit. Paclobutrazol applied in combination with triazole fungicides was particularly conducive to DBD development.
Alternative growth regulators such as prohexadione should be considered for use in Cox orchards particularly where there is a history of a DBD problem.