HORT 413 Tree Fruit Production

Bud development, flower initiation and differentiation

Flowering is a prerequisite for fruit development. Since fruit trees are considered to be day neutral, flowering is initiated by a complex interaction of factors including environmental (but not daylength), cultural and internal chemical signals.

Flowering to fruit set can be divided into the following stages:

* Flower bud formation

* Pollination

* Fruit set

* Fruit development

Flower bud formation

Flowers are usually initiated from June-September during the previous year

An apple fruiting spur

Biennial (or alternate) bearing:

* what is it?

* what causes it?

* what's the solution?

Many factors influence flower development

• Light level
• Crop Load
• Rootstock
• Branch angle
• Ringing or scoring
• Root pruning
• Tree vigor

Pollination

Pollination and fertilization is required for fruit development of apples. Fruit trees have heavy, stick pollen which requires insects to disperse. In apple, the main pollinators are honey bees. Hives of bees should be introduced into the orchard over the flowering period to encourage good pollination. Generally, conditions which deter bees from flying are:

Rain

High winds

Temperatures below 50F

In addition pollination may be poor:

On shaded sides of trees

In hedgerow orchard layouts

When insecticides are used

When competition is present from other flowers

Flower morphology &endash; side working

Compatibility and Timing

1. Certain combinations of cultivars will not fertilize each other

2. Some cultivars are triploid or produce sterile pollen

3. Bloom time must overlap &endash; concept of effective pollination period

Pollinizer: the source of pollen

Pollinator: the agent of pollen transfer (usually a bee or other insect)

Fruit set and thinning

The tree produces many more flowers than it is capable of sizing &endash; in fact, only about 5% of the flowers are required to achieve a good crop. Due to the competition for resources (carbohydrates), the tree drops many of the fruit early in the course of development. This drop is seldom sufficient for stable commercial production, therefore additional fruit thinning is required. Thinning is carried out for 2 important purposes:

1. To ensure return bloom. Overcropping inhibits fruit bud initiation for the following years crop and can lead to biennial or alternate bearing. This is where a heavy crop is borne one year followed by a light crop the following year. Only thinning carried out early will help in reducing biennial bearing.

2. To encourage large fruit size. Heavy crops usually result in fruit of small size, which are of lower value in the marketplace. Thinning will improve fruit size if carried out during the cell division phase of fruit growth, about the first month after flowering.

Thinning may be by hand (very expensive) or using chemical thinning agents (higher risk). Chemical thinners are best applied during the period from flowering to 3 weeks later. The earlier thinning is done, the better the response will be in terms of return bloom and fruit size. However early thinning is more risky due to late spring frosts and unpredictable fruit drop. Therefore the fruit grower must try to balance the efficacy of early thinning with it's inherent high risk. Thinners usually work best in warmer temperatures, with temperatures in the 70's ideal. Applying chemical thinners in cool weather (below 60F) is unlikely to have any effect.