The harvesting period for any particular variety of plum or cherry is only one to three weeks. This causes problems in maintaining a consistent supply of UK-grown quality produce. The solution to the problem is to use a number of varieties with a suitable range of maturities, but this also has problems. It is not always easy to find varieties which have both high-quality fruit and an appropriate harvest period. Weather conditions affect maturity and can lead to varieties, which under most conditions provide a suitable sequence of cropping, overlapping or leaving a wide gap in supply. An alternative approach is to apply orchard treatments which slow the development of fruit. Thus, by using a mixture of sprayed and unsprayed trees, the harvest period can be extended under the control of the grower. Some preliminary experiments using this approach have been carried out at HRI-East Malling, using funding provided by government.

CHERRY

Gibberellic acid (GA), sprayed onto cherries even as late as 10 days before harvest, retarded development of skin colour, which is the main commercial criterion for cherry harvest maturity. Figure 1 shows that about 30 per cent of unsprayed (control) fruit had developed the dark colour associated with ripeness in Stella by July 7, whereas only 8 per cent of GA-sprayed fruits were in this category. One week later, 50 per cent of the control fruit were ripe, compared with 15 per cent of the GA-sprayed fruit, indicating that harvest maturity had been delayed. There were no adverse effects of GA on the amount of fruit discarded because of cracks and rots while size and soluble solids (sugar) content were only marginally lower in sprayed fruit (data not presented). Similar results were obtained in two years of these trials and in a number of earlier trials.

PLUM

The plum is a climacteric fruit, which means that ripening is initiated by ethylene. Inhibiting ethylene production should thus delay ripening. Aminoethoxyvinylglycine (AVG) has long been known as an inhibitor of ethylene synthesis and is now produced commercially as ReTain. This material should soon have clearance in the UK for use as a harvest and storage aid for apples. When applied to plums it delays colour development. The material was applied to Victoria plums on August 2, 2000 and the fruit was harvested on August 23 or September. 1 As can be seen in Figure 2, on the first pick 25 per cent of the control fruit was well-coloured, but only 2 per cent of the sprayed fruit. One week later the percentage of well-coloured control fruit had risen to 80 per cent, but only 13 per cent of the sprayed fruit was in that category. There was no effect of AVG on soluble solids content and, within a colour category, sprayed fruit tended to be slightly firmer than unsprayed.

CONCLUSIONS

These preliminary experiments demonstrate that it is possible to manipulate the harvest period for both plums and cherries. Both GA and AVG are naturally occurring compounds, GA being a plant hormone and AVG being produced by some fungi. Further work is needed to quantify the extension of the harvest period possible and to optimise the time and rate of application, but the potential benefits to the UK stone fruit industry could be large and could increase significantly the period that UK cherries and plums are available to the consumer.

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