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Unfruitfulness, alternate bearing and fruit drop

Why fruit trees fail to flower or set fruit, the causes and control of alternate bearing and fruit drop, and how to calculate an alternate bearing index.

Exam essential5 min readUpdated
On this page
  1. Unfruitfulness and its causes
  2. Alternate (biennial) bearing
  3. Fruit drop
  4. Key points
  5. What the exams ask
  6. Questions people ask
  7. Sources
  8. Related on Laisha
  9. Discussion

A healthy, vigorous fruit tree that flowers poorly, sets little fruit, or drops most of what it sets is a common and costly problem. Some trees crop heavily one year and almost nothing the next. These problems, grouped as unfruitfulness, alternate bearing and fruit drop, have many causes, from genetics and pollination to nutrition, hormones and weather, and their diagnosis is a classic part of pomology courses and exams.

Unfruitfulness and its causes

A tree is unfruitful when it fails to produce a crop it should be capable of. The causes are usually grouped as internal and external.

Internal causes

Genetic and reproductive

  • Self-incompatibility: many apple, pear, cherry, plum and almond varieties need pollen from a compatible variety.
  • Pollen sterility or poor pollen, as in triploid apples (Bramley's Seedling, Jonagold) and some grapes.
  • Dichogamy: male and female parts mature at different times, as in walnut, pecan and avocado, whose varieties are planted in complementary groups.
  • Dioecy: female papaya, date palm or kiwifruit plants need males nearby.
  • Embryo abortion, which causes fruits to drop or become seedless.
  • Imperfect flowers: some mango panicles carry a high proportion of male flowers.

Physiological

  • Juvenility: young seedling trees cannot flower.
  • Nutrition and the carbohydrate–nitrogen balance: E. J. Kraus and H. R. Kraybill showed in 1918, with tomatoes, that plants with abundant nitrogen but few carbohydrates grow vegetatively and fruit poorly, while a moderate nitrogen supply with good carbohydrate reserves favours flowering. Excessive nitrogen still promotes leafy growth at the expense of fruiting.
  • Hormones: gibberellins from developing seeds can suppress flower initiation for the next year.
  • Excessive vigour from heavy pruning, vigorous rootstocks or overfertilising.

External causes

  • Climate: insufficient winter chilling for temperate fruits (poor, uneven bud break), spring frost killing blossoms, rain, cold or heat at flowering reducing pollinator activity and pollen viability, and hot, dry winds.
  • Lack of pollinators, or pesticides that kill them.
  • Pests and diseases of flowers and young fruits, such as mango hoppers and powdery mildew on panicles.
  • Water stress or waterlogging during flowering and fruit set.
  • Management: wrong pruning that removes fruiting wood, nutrient deficiencies such as boron and zinc, which affect pollen and fruit set.

Alternate (biennial) bearing

Many fruit trees produce a heavy crop (on-year) followed by a light crop (off-year). It is especially pronounced in mango, apple (some varieties), mandarins, olive, pistachio, avocado, pecan and litchi.

Yields of two trees over six years: an alternate-bearing tree swings between heavy on-years and light off-years, about 120, 30, 110, 40, 100 and 35 kilograms, while a regular-bearing tree produces a steady 70 to 80 kilograms each year; total yield is similar but the alternate-bearing tree is harder to market and manage Yield per tree (kg) over six years Year 1120Year 230Year 3110Year 440Year 5100Year 635 alternate bearingregular Illustrative yields; the first tree's alternate bearing index is about 0.51 (see the worked example).
Alternate bearing compared with regular bearing. Total yield over several years may be similar, but the swings make marketing, labour and orchard management difficult.Diagram: Laisha, original.

Why it happens

  • A heavy crop uses carbohydrate reserves, leaving few for flower bud initiation for the next season.
  • Developing seeds in a heavy crop produce gibberellins that inhibit flower initiation in nearby buds.
  • In mango, a heavy crop delays new shoot growth after harvest, so shoots are too young to flower the next season.
  • A frost or other event that destroys one year's crop can start the cycle, and whole regions can then bear in step.

Management

  • Thin flowers or fruits in on-years, by hand or chemically (for example with NAA, carbaryl where permitted, or other thinning agents in apple), to balance the crop.
  • Prune to encourage regular new growth.
  • Balanced nutrition and irrigation, especially after harvest.
  • Growth regulators: in mango, paclobutrazol applied to the soil restricts gibberellin synthesis and promotes regular flowering, where permitted.
  • Choose regular-bearing varieties: mango breeding programmes in several countries have released hybrids that bear more regularly.
Worked example

Measuring alternate bearing

The alternate bearing index (ABI) compares each year's yield with the previous year's:

ABI = [Σ |Yi − Yi−1| ÷ (Yi + Yi−1)] ÷ (n − 1), where n is the number of years.

A tree yields 120, 30, 110, 40, 100 and 35 kg over six years:

  • 90 ÷ 150 = 0.600; 80 ÷ 140 = 0.571; 70 ÷ 150 = 0.467; 60 ÷ 140 = 0.429; 65 ÷ 135 = 0.481.
  • Sum = 2.548; ABI = 2.548 ÷ 5 ≈ 0.51.

An ABI of 0 means perfectly regular cropping and 1 means a crop only every other year. A tree yielding 75, 70, 80, 72, 78 and 74 kg has an ABI of about 0.05.

Fruit drop

Some fruit drop is natural: trees set far more fruits than they can mature. Drop is usually described in waves:

  • Post-bloom drop of unfertilised flowers and fruitlets.
  • Early (June) drop of young fruits, in apple and citrus, often linked to competition for carbohydrates and to poorly fertilised fruits with few seeds.
  • Pre-harvest drop of mature fruits, which causes direct loss and is common in apple, citrus, mango and litchi.

Causes include poor pollination, competition among fruits, water stress, high temperature, nutrient deficiency, pests such as fruit flies and borers, diseases, and hormonal changes, particularly ethylene and low auxin in the abscission zone.

Control

  • Good pollination and nutrition, regular irrigation, and pest and disease control.
  • Auxin sprays: NAA or 2,4-D at low concentrations, where approved, reduce pre-harvest drop in apple and citrus.
  • Ethylene inhibitors: aminoethoxyvinylglycine (AVG) delays pre-harvest drop of apples.
  • Avoid heavy nitrogen and sudden changes in soil moisture.

Key points

  • Unfruitfulness has internal (self-incompatibility, sterility, dichogamy, juvenility, C:N balance, hormones) and external (chilling, frost, weather at flowering, pollinators, pests, management) causes.
  • Kraus and Kraybill (1918) linked fruiting to the carbohydrate–nitrogen balance.
  • Alternate bearing comes from carbohydrate depletion and seed gibberellins after heavy crops; manage by thinning, pruning, nutrition, paclobutrazol (mango) and regular varieties.
  • ABI = mean of |Yi − Yi−1| ÷ (Yi + Yi−1); 0 is regular, 1 fully biennial.
  • Fruit drop occurs after bloom, in early summer and before harvest; auxins and AVG reduce pre-harvest drop.

What the exams ask

  • School-leaving and entrance papers: causes of unfruitfulness, what alternate bearing is, and examples.
  • Degree and research-fellowship entrances: internal and external causes, C:N ratio, chilling requirement, alternate bearing physiology and control, types of fruit drop, and growth regulators used.
  • Professional examinations: diagnosing poor cropping in orchards, alternate bearing index, and crop regulation strategies.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

My mango tree flowers but sets few fruits. Why?

Common causes are rain or cold during flowering, few pollinators, powdery mildew or hoppers on the panicles, or a high proportion of male flowers. Protecting panicles and encouraging pollinators often helps.

Does removing fruit really increase next year's crop?

Yes, when done early enough. Thinning reduces the carbohydrate drain and the gibberellin signal from seeds, allowing more buds to form flowers for the next season.

Why do my apple trees not flower after a warm winter?

Temperate fruit trees need a certain number of chilling hours to break dormancy evenly. After warm winters, bud break is poor and uneven, and flowering is reduced.

Sources

  1. Westwood, M. N. Temperate-Zone Pomology: Physiology and Culture, 3rd edition. Timber Press, 1993.
  2. Monselise, S. P. and Goldschmidt, E. E. "Alternate bearing in fruit trees." Horticultural Reviews 4, 128–173 (1982).
  3. Kraus, E. J. and Kraybill, H. R. "Vegetation and reproduction with special reference to the tomato." Oregon Agricultural Experiment Station Bulletin 149 (1918).
  4. Pearce, S. C. and Doberšek-Urbanc, S. "The measurement of irregularity in growth and cropping." Journal of Horticultural Science 42, 295–305 (1967).
  5. Litz, R. E. (ed.) The Mango: Botany, Production and Uses, 2nd edition. CABI, 2009.

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