Age. Grafted fruit trees will normally begin bearing soon after they have become old enough to blossom freely. The length of time from planting to fruit bearing varies with the type of species as follows (in years): Apple: 2–5; Plum: 3–6; Pear: 4–6; Peach: 2–4; Fig: 2–3.
Excessive fertilisation or pruning. The onset of flowering indicates the beginning of the generative phase which can be delayed by heavy fertilization or excessive pruning.
Weather. Trees may flower but produce little or no fruit. One reason for this might be that unfavourable weather conditions prevailed during blossoming. Frost will kill the flowers, and low temperatures can adversely affect the pollen germination and consequently the fertilization of flowers, leading to non-viable seeds/fruits. Cross pollination, required by many fruit cultivars, is curtailed by heavy rain or strong winds, which affect the activity of pollinators. In addition, pollen may be spoiled or washed away. Flower drop has also been observed due to moisture stress.
Pollination. Most fruit trees need to be pollinated. Without sufficient pollination, they may blossom abundantly but bear no fruit. Some fruit species have perfect flowers. Both the anthers, which contain pollen, and the pistils, which develop into fruit, are located in the same blossom and will bear fruits by self-pollination. However, there are many types of fruit varieties that are self-infertile, i.e. they have perfect flowers that cannot produce fruit from their own pollen. In such cases, cross-pollination with the pollen from an entirely different variety of the same kind of fruit species is required. Self-infertility is very common and occurs in many varieties of apples, pears, plums, peaches and grapes.
Infestation. Another serious problem which reduces fruit set is the infestation of flowers with pests and diseases. Losses can be tremendous and only timely crop protection measures will save the crop.
Biennial bearing. Occasionally, certain fruit trees bear heavily in one year and sparsely the next. This is called biennial bearing. A particularly heavy crop in one year may prevent adequate flower bud formation in the following one. Biennial bearing is difficult to alter; the farmer can, however, induce a return to normal annual fruit production by early and thorough thinning during the year in which the trees produce their large yield.
Premature fruit drop. Farmers are often distressed by premature fruit drop, which occurs in definite waves and decreases their expected revenue. Fruit drop can be observed soon after fruit set and again later when the fruits have already reached a reasonable size (pre-harvest fruit drop). The cause of fruit drop in the first instance is the formation of an abscission layer at the point where the fruit attaches to the twig. Abnormally high pre-harvest fruit drop can be controled with chemicals. Before resorting to chemical means, however, growers should make sure that other factors like drought, over-bearing, pest and disease infestation, or adverse weather conditions, are not responsible.