Sprays of fertilisers, either major or trace elements, have often been tried in attempts to improve fruit set.
- Spring applications of nitrogenous fertilisers should be avoided where possible, as these can stimulate shoot growth, which in turn reduces fruit set (Hill-Cottingham and Williams, 1967).
Most treatments are focused on improving flower quality, however a few treatments, such as those with either boron or calcium, are aimed at directly stimulating the processes of flower fertilisation.
Studies conducted mainly on bulbous and herbaceous crops have shown that boron, and to a more limited extent calcium can aid the germination of pollen.
- Boron is always added to the sugar solutions or the Agar medium used to test pollen germination in the laboratory.
- Boron has been shown to stimulate pollen germination and also to speed up the growth rate of pollen tubes down the style (Brewbaker and Majumdar, 1961).
Many attempts have been made to replicate these boron effects on the flowers of apple trees growing in the orchard. Sprays of boron (e.g. boric acid) have been applied either prior to leaf fall in the autumn or in spring just before flowering. The aim in both cases is to increase the boron concentrations within the floral parts. The results of these experiments have proved very variable.
- In trials conducted at East Malling in 1973, three fortnightly sprays, commencing at 80% petal fall, of ‘Solubor’ (Na2B8O13.4H2O), were applied at 0.15% for the first and 0.25% for the second two sprays.
- The treatments increased the initial set and the set measured after June Drop of Cox’s Orange Pippin but by harvest the effect was not statistically significant (Yogaratnam and Greenham, 1982).
- In the subsequent year, initial set was increased but not final set.
- In neither year was crop weight per tree increased and effects on Discovery were also negative.
- A small but non significant increase in Cox fruit set was observed in one year of the same trials, following sprays of 0.5% urea at pink bud and petal fall.
- The conclusion must be that, although occasional benefits in terms of improved fruit set can be achieved using sprays containing boron, the effects are very inconsistent.
Sprays containing boron, applied at various timings between autumn and late spring have given very inconsistent effects on fruit set and retention of apples and cannot be recommended for this use on current evidence.