The Importance of Thinning Apples ChemicallyThis article was reviewed by the editor of Orchard Network for technical accuracy and appropriateness. Apple producers are challenged this time of year with a number of decisions on how best to chemically thin their crop. The purpose of this article is to briefly discuss some thinning recommendations, particularly as they relate to Ontario, and to highlight the relationship between the number of fruit per tree, tree density, yield, and final fruit size. Apples (and other tree fruits) typically produce an overabundance of flowers each year, but only 6-10% of the flowers are required to set and produce fruit. If these are all permitted to develop, too many fruit will fail to develop acceptable market size. Excess fruit is removed commercially by chemical and hand thinning, however the latter is labourious and costly. In general, the number of fruit should be reduced to approximately 5 to 7 fruits per cm of limb circumference in order to achieve adequate size. All healthy, mature, bearing trees with sufficient bloom are candidates for chemical thinning. Trees that carried a light crop last year will have a heavy bloom and will need to be thinned extensively. Furthermore, trees with a light bloom this year will set a higher percentage of fruits than those with heavy bloom, thus need to be thinned lightly. Chemical thinning increases fruit size, return bloom, fruit colour, disease and pest control, and reduces alternate bearing, limb breakage, harvesting and handling costs, and fruit clustering. Although hand thinning is a fallback, don't rely on it exclusively. Inadequate thinning often results in depressed crop value and increased harvesting cost - much more than will be experienced by light to moderate over thinning. You should therefore consider every opportunity to thin your fruit chemically. Chemical thinning is a difficult process, often thought to be more art than science. Regardless, it is one that orchardists must master to be successful. Poor thinning results in the loss of more crop value than all other problems combined. To achieve successful chemical thinning, a grower is advised to keep accurate records, observe conditions in the orchard, and understand the factors, which affect the chemical thinning response. Such information can then be put together to select the best material, rate, and timing for the chemical thinning operation.
The relationship of fruit per tree, tree density, yield, and desired marketable fruit sizeRather than approach the subject of thinning (crop load adjustment) by relating how many fruit need to be removed from the tree, consider how many fruit should be left on a tree to obtain optimal fruit size. Growers can estimate this by estimating their yield (bins/ha) and dividing this by their tree density (trees/ha) and number of apples to fill a bin for a given minimum fruit size (Table 1). For example, if a grower has an orchard spaced at 6.5' x 13' (500 trees/acre), and an average yield of 1,000 bushels/acre, 251 fruit per tree would be required to produce a crop with an average minimum size of 3" (113 count) apples. If this calculation is reworked for a grower wishing to produce fruit with a minimum size of 2 ½", an additional 100 apples (total 362) should be left on the tree. It would be beneficial for growers to work through the exercise of estimating the maximum number of fruit required per tree to achieve the minimum size of fruit of fruit they are aiming to produce.
** - Remaining Rows - Target
Thinning checklistWhen it is time to chemically thin:
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For more information: Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca
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