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Fruit Thinning

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Trees of most apple cultivars begin to flower abundantly from an early age following planting in the orchard. Selected on fertility most cultivars demonstrate heavy fruit set leading to high number of fruits per tree that usually do not reach adequate size and quality at harvest. Simply because there are not enough leaves to ensure adequate fruit development. Also, excessive fruit set reduces the flower-bud formation which can lead to biennial bearing of trees. This alternating pattern of an overabundant crop one year, followed by a poor one the next year is, although it differs per variety, a natural tendency in uncontrolled trees. To overcome this problem and to balance the crop from year to year, some fruits needs to be removed at an early stage of development which also enables the remaining fruit to reach a larger size. Additional advantages of reducing the number of fruits are that picking, grading, handling and storage costs are significantly reduced.

Growers must find ways to reduce the crop load in an early stage and this is achieved by using various techniques of thinning. Strategies for thinning must produce the maximum crop of the optimum fruit size range which satisfies market requirements, without inducing biennial bearing. Targets for fruit size vary between cultivars and do not remain static with time. Each season can require a different fruit size outcome. Thinning programs must be flexible enough to react to these changes while maintaining a regular cropping outcome.

In apple, the use of thinning compounds is currently one of the most cost-effective examples of bioregulator use. In other fruit crops, like in certain cultivars of pear, but also stone fruits that become more self-compatible, the need for thinning becomes more important. Thinning by hand is still the most accurate and reliable method of thinning a fruit crop. It is an extremely expensive and time consuming exercise and usually not considered as cost effective. It is a skilled operation that is not suited to the casual labor force who normally undertake the job. Even in those parts of the world where labor costs are low it gets more and more difficult to find the people and to organize the operation. Another factor that rules out total hand thinning is the stage, around 6 weeks after full blossom. The fruits are easy to see but cell numbers have been determined which means that some limits have already been set on fruit size and quality. If thinning is delayed, the fruit remaining on the tree does not develop so well in size.

Because it is impractical for most growers to thin all their crop by hand, chemical thinning has become the preferred option. Chemical thinning agents can thin either flowers or fruitlets. Flowers can be prevented from setting fruits at the time of blossoming (primary thinning). Less risky is fruitlet thinning, fruit set can be judged and, if abundant, chemicals aimed at promoting the drop of young fruitlets can be used (secondary thinning). Especially in years with early frost during flowering or when conditions of pollination and fruit set are poor, secondary thinning is preferred.

The availability of chemical-thinning agents is constantly changing. Negative side effects of a few of the traditional thinning chemicals and their lack of perceived environmental compatibility has led to withdrawal of the approved use in many countries. New developed and registered thinning compounds are those that either occur naturally in plants, degrade rapidly into harmless compounds, disappear quickly to undetectable levels or are harmless for beneficial insects. Most of those compounds belong to the group of plant hormones and the level of efficacy is rather weather dependent. High temperatures and humidity around spraying time generally favors the effect of this group of thinners. More recently a new chemical fruit thinner for pome fruit, based on the active ingredient metamitron, has been developed and recently released in several countries. The compound inhibits the photosystem II of photosynthesis causing a reduction of photo-assimilates uptake and, therefore, determining correlative inhibition phenomena among fruits in the cluster and young growing shoots.

Many orchardists and scientists have puzzled over the problem of thinning and why it is so complicated. Thinning is not only an exercise of applying known facts but integrate them in a decision on commercial risk taking, which is an emotive situation.

To avoid economic loss by over-thinning and under-thinning a real thinning strategy needs to be adopted based on many factors that needs to be judged carefully. Having decided to chemically thin the next step is which chemical or series of chemical treatments would be most effective for the particular cultivar to be treated. Then consideration should be given to the prevailing weather and how to fit the spray program to these conditions.

A working group consisting of researchers from the leading European research stations is working on a model that reduces the undesired risks. The model includes climatic conditions before and after the applications with the aim of refining the practice of chemical thinning and make it safer, more effective and understandable.

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