Although flower or fruitlet thinning represents the most important strategy for improving final fruit size and quality, supplementary factors which influence fruit size and quality should also be considered.
If apple orchards are to be profitable it is essential that a high proportion of the harvested yields (>80%) should be of suitable size, appearance and quality to command the highest available market returns. It is therefore important to optimise all management inputs to ensure the best fruit size and quality at harvest and through storage. To achieve these objectives:
- Ensure that fruitlets set with adequate numbers of seeds.
- Encourage cell division and expansion in fruitlets.
- Prune and train trees so as to optimise exposure of the fruits to light.
- Maintain adequate and balanced supplies of water and nutrients to trees.
- Avoid use of crop protection or other sprays that may damage the skin finish of the fruits.
- Apply sprays of gibberellins to improve skin finish.
Ensure that fruitlets set with adequate numbers of seeds
Fruits with few seeds may, in some seasons, be slightly misshapen or smaller and it is always recommended policy to ensure setting of fruits with many seeds. Ensure that apple fruits are set with adequate seed numbers by:
- The planting of suitable pollinating varieties in adequate numbers within the orchard.
- The introduction of sufficient bee hives, each with ‘strong’ colonies of bees.
- The creation of suitable shelter within the orchard to aid bee activity and pollen germination and growth.
- Ensuring good pollination with the variety Braeburn so as to increase fruit seed numbers and help uptake of calcium into this bitter pit sensitive variety.
Encourage cell division and expansion in fruitlets by optimising orchard management
Apple fruits grow by cell division and cell expansion. Most cell division occurs in the first 3‑4 weeks following fruit set and by 7 weeks cell division has almost finished.
Thereafter, fruits grow by the cells expanding.
- The best large fruits, in terms of texture and storage potential are those with many cells rather than fewer larger cells.
- Increasing the size of fruits by applications of irrigation and nitrogen, so as to increase cell expansion, may produce fruits of poor texture, which store poorly.
The principal factors influencing cell division in young fruitlets are:
- Initial flower quality
- Crop loading
- Climatic conditions in the first month after fruit set.
Initial flower quality
- Poor ‘quality’ flowers usually result in poor quality fruits, which may be smaller than average and of poor texture.
- The production of flowers of high ‘quality’ is aided by:
– Optimising crop loads in the previous season.
– Maintaining a good balance of new shoot growth and flower production by appropriate pruning/training, nutrition and irrigation.
– Ensuring that the sites of flower production (e.g. the spurs) are well exposed to light.
– Ensuring that the concentrations of gibberellins in sprays applied to aid skin finish, are kept to a minimum.
- Biennial bearing varieties often produce poorer quality flowers, as well as fewer flowers in their ‘off’ year.
Crop loading can have a significant effect on fruit size and quality.
- Early thinning of apples by blossom thinning or very early fruitlet thinning will reduce the competition within the tree for assimilates (carbohydrates produced by photosynthesis) and improve cell division in the persisting fruitlets.
- Early thinning will improve fruit size and firmness and advance ripening.
- However, in some situations early thinned fruits may be more sensitive to storage disorders, such as senescent breakdown and core flush.
Climatic conditions in the first month after fruit set
Warm temperatures in the 4 to 6 weeks after fruit set will have a very beneficial effect in encouraging cell division in the young developing fruits.
- Such conditions will also aid the development of bourse and extension shoot leaves which will begin to contribute to the vital carbohydrates produced by photosynthesis which fuel the growth of young fruitlets.
- Endeavour to increase temperatures in orchards during the first six weeks following fruit set, so as to maximise cell division in the young apple fruits.
Prune and train trees so as to optimise exposure of fruits to light
Use of training systems which expose fruitlets to the full sunlight, such as Tatura or other trellis systems will also improve cell division.
- Prune and train trees so as to optimise exposure of fruits to light and achieve good colour development
- In UK conditions it is essential to ensure that young developing fruitlets and the leaf canopy which supports their expansion and growth are well exposed to sunlight. This is achieved by judicious use of pruning and training techniques.
Irrespective of the scion variety, the rootstock or the system of pruning and training chosen, it is vital in all instances to ensure good light penetration into the canopy and good exposure of the fruits to light throughout the growing season and especially in the weeks prior to ripening.
- Prune and train trees so as to maintain a balance of flowering spurs and new extension shoot growth.
- Stimulate growth on trees producing too little extension growth by making a few severe cuts back to vegetative buds in positions where new shoots/branches are desired.
- On trees with excessive shoot growth, reduce this by training shoots to the horizontal (or lower)
- Where summer shoot growth results in shading of fruitlets and poor fruit colour at harvest use summer pruning techniques to open up the tree canopy and expose the developing fruitlets to better light conditions.
- The need for summer pruning indicates that tree vigour is too strong.
- Review pruning and tree management and consider ways to reduce growth.
- On varieties such as Gala, prune so as to encourage the production of medium strong wood and reduce the production of weak ‘wispy’ shoots, which give rise to small fruits.
Maintain adequate and balanced tree nutrition and water supply
If fruitlets are to expand and develop into high quality large fruits at the time of harvest it is essential to maintain adequate supplies of water and nutrients to the tree throughout the growing season.
Water needs are estimated using calculations of water deficits based on evapotranspiration or are measured using various other systems (neutron probes, tensiometers etc.).
- Ensure that the trees are supplied with adequate water and nutrients so as to maintain the cell expansion and growth of apple fruitlets.
- Monitor water requirements regularly, using calculations of evapotranspiration or other equipment marketed to measure soil water levels.
- Check soil and leaf nutrient levels regularly and adjust nutrition accordingly.
Avoid use of crop protection or other chemical sprays which may cause damage to the skin of fruitlets (russeting)
Use of certain pesticides or other sprays may, on occasions, result in damage to the fruit skin and loss of quality.
- Adhere to label recommendations for pesticides, concerning rates, water volumes, compatible tank mixes and weather conditions, so as to avoid phytotoxicity damage and loss of fruit quality due to russeting.
- Avoid spraying within two days of a sudden change in weather patterns, particularly a rise in temperature, as the fruitlet will be very sensitive to any spray damage.
- It is known that sulphur and high rates of copper will cause damage to fruitlets of Cox.
- Full rate sprays of dithianon can cause russetting to Gala when applied at the fruitlet stage.
- Damage can be avoided by reducing spray rates and not spraying in stressful conditions.
- The addition of GA4+7 to sprays (see below) during this period can also reduce russetting.
Apply gibberellin sprays to improve final skin finish of fruits
Sprays of gibberellins (e.g. Regulex, Novagib) can improve the skin finish of varieties such as Cox at harvest time. GA4+7 is approved for use in the UK and trials have demonstrated that small but significant increases in % Class 1 fruit can be achieved.
- As fruitlets can be sensitive to russetting over a long period, a programme of sprays is regarded as better practice than just one or two applications.
- On varieties such as Cox and Discovery apply sprays of GA4+7 (e.g. Regulex or Novagib) at label recommendations (4 times at 5-10 ppm, at 10 day intervals commencing at first open flower) so as to improve skin finish at the time of harvest.
After fruit set, the growth of the persisting fruitlets is brought about by a combination of cell division and cell expansion. It is generally accepted that the majority of cell division takes place in the first 6 weeks ceases within 4 weeks after pollination and cell expansion continues throughout to harvest.
A number of factors, other than crop loading are known to influence cell division and expansion. These are:
- Photosynthesis and carbon supply to the fruitlets as influenced by temperature and light.
- Production and movement within the tree of natural plant hormones.
- Tree health and nutrition.
Photosynthesis and carbon supply to the fruitlets
The demand for assimilates (carbohydrates produced by photosynthesis) increases rapidly following bloom, peaking at about 4-6 weeks after full bloom and then remaining fairly stable through until harvest.
- There are two periods of potential limitation of carbon supply; one about 2-4 weeks after blooming and the other in the last few weeks prior to harvest.
Production and movement within the tree of natural plant hormones
Fertilised ovules (i.e. developing seeds) produce hormones.
- Gibberellins are found in apple seeds have are implicated in the growth of fruits.
- Gibberellins are also essential for the development of apples with good skin finish and freedom from russet.
The general health of trees is important and trees suffering significantly damaging pests or diseases will inevitably produce fruits of smaller size and quality at harvest.
- Most of the apple trees grown in the UK are free from the viruses and phytoplasmas that might influence fruit size and quality at harvest.
- However, care should be taken to ensure that any variety or clone used is as free as possible from these diseases.
- The disorder known as chat fruit reduces fruit size on some varieties, such as Lord Lambourne, and it is important that growers ensure that their young trees are free of this problem.