Apples - Hand Thinning Strategies

Once the chemical thinning drop has finished, it is time to start the hand thinning programme. The order in which the hand thinning is done depends on the following factors:

1. Effectiveness of the chemical thinning programme. Where the chemicals have worked well and fruit numbers well spread out over the tree with crop load relatively close to the harvest target, the hand thinning can be delayed until defects such as russet and wind rub can be readily seen.
2. "Bunchiness" of the crop. Where flowering was light and fruit has set in large bunches, the hand thinning programme needs to begin early before the bunches close up and make pulling out the fruit very difficult. Cox's Orange and "off" crop Braeburn are examples of varieties which need to have their bunches broken up before the fruit size reaches the size that causes the bunches to close up.
3. Susceptibility to late drop. Some varieties, particularly younger vigourous Royal Gala, are prone to late natural fruit drop. Blocks with a history of heavy drops should not be hand thinned until the natural drop is complete. On the other hand, varieties which do not show late drops such as Fuji should be thinned ahead of varieties which do show late drop.
4. Susceptibility to sunburn. Varieties sensitive to sunburn injury such as Braeburn need to be thinned before the hot weather arrives.
5. Susceptibility to russet. Varieties prone to russet such as Pacific Queen are best left until fruit affected by russet becomes obvious so that blemished fruit can be thinned off.
6. Varieties showing high sensitivity to crop loading, particularly Pacific Rose which suffers from necrotic leaf breakdown, or "on" crop biennial bearing trees, need to be thinned early to reduce the adverse effects of too much crop on their general foliage health or return bloom.

Setting Optimum Crop Load

Getting the crop load right is the key to maximizing financial returns. Many varieties have large premiums paid for large fruit size. These varieties need to be cropped conservatively. Bitter pit sensitive varieties need uniform relatively large crops to minimize post-harvest disorders.

Fruit size at harvest is largely dependant on fruit numbers being carried and leaf to fruit ratios.

Favourable early season growing conditions increase fruitlet size at time of hand thinning, but where fruit loads are excessive, fruit sizing can stall near to harvest so the desired size is not achieved.

Historical cropping data and fruit size at harvest is a good guide to crop load levels required to achieve the targeted size range.

With varieties that are crop load sensitive to fruit size, altering fruit numbers by 20% will give a shift in fruit size by about 10%, ie, reducing historical fruit numbers by 20% will increase fruit size by 10%.

Setting crop loads by tree or branch size based on the diameter or cross sectional area of the branch is a good method for setting crop load and checking the quality of the hand thinning job. With young trees, setting crop load by trunk size works. In older trees, trunk size does not reflect cropping capacity well, so it is better to measure the crop loading on small to medium sized branches three to four centimeters in diameter.

For standard rootstocks, specific crop levels off 5 to 7 fruit per square centimeter of cross sectional area are considered optimum for most varieties.

Dwarf rootstocks can carry heavier crops of up to 10 to 12 fruit per square centimeter of cross sectional area.

Thin by Size and Position

For most varieties, highest quality and easiest harvest management occurs when the crop load allows the fruits to be thinned down to one per cluster, or in some cases spaced out in singles so the mature fruit does not touch. Unfortunately, this situation does not often occur, so it is necessary to leave more than one fruit per cluster in some positions.

As a general rule, strong sites with good bourse shoot development can comfortably carry two fruit where crops are light, possibly three or more.

Lateral bud of one year wood fruit is usually smaller and more russetted than terminal bud or healthy strong spur sites, so this fruit should be thinned off ahead of fruit on the older fruiting sites.

From about 60 days from full bloom, visible differences in fruit size among the fruit become obvious and this difference in fruit size relates to harvest size. Fruit thinning by fruitlet size enables smaller, low value fruit to be removed in the hand thinning.

At 65 days from full bloom, fruitlet size on Royal Gala in a big size year will average 45.5 mm, and in small sized years has averaged only 42 mm. At 65 days from full bloom, fruitlets capable of making 110 count size should be 44 mm in diameter.

Partial red varieties such as Fuji for which high colour is required need to be thinned to singles and spaced.

For sunburn varieties, thin off fruit from exposed sites with poor leaf cover.