It is recognised throughout the world that one of the most difficult and often most expensive decisions for commercial apple growers is whether or not to thin. Many factors must influence the decision including tree and variety characteristics, previous cropping history and the weather conditions before, during and after flowering.
Researchers in the USA have developed decision support systems (computer models) that take account of a multitude of climatic, cultural and application conditions to make recommendations on the use of chemical thinners.
- One such system, THIN-IT, developed in Washington State, produces a range of recommendations and is mainly appropriate for the dry arid climatic conditions of the Pacific North West of the USA (Williams and Wright, 1991).
- A slightly more sophisticated model has been developed in Pennsylvania and is incorporated into a larger suite of models distributed as the Penn State Apple Orchard Consultant (Crasweller et al., 1992).
- Unfortunately, although this system is more appropriate for maritime climates similar to the UK, it has little immediate value to the UK grower, as it focuses on use of products either not approved in the UK (NAA and NAD) or on products about to be withdrawn (Carbaryl). Another USA computer programme, focused on thinning apples has been developed for growers in New York State (Stover, 1992).
Researchers in Tasmania, Australia (Jones et al., 1987) have also developed computer models to aid growers in making decisions on apple thinning.
- The basis of the model is a mathematical model of optimum crop loading.
- Data from experiments over a twenty-year period have been used in this model.
- Factors, such as the scion variety, the rootstock, the thinning chemicals to be used, the age and size of tree and the fruit size required, are all taken into account.
- In addition, the model takes account of what are known as modifying factors; these are last year’s crop, rate of shoot growth and pruning severity.
- The aim is to deliver a best practice for Tasmanian orchardist.
- Unfortunately, this model, like those developed in the USA is not appropriate for UK use on account of the climatic differences and the reliance on use of ethephon as a thinning chemical.
- A full explanation of the Tasmanian model is given in a recent review paper (Jones et al., 2000).
- No models have yet been developed suited to UK conditions and utilising thinning strategies available to the UK grower.