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Pollination and fruit set and preventing excessive drop of fruitlets

Introduction

 

Abundant flower numbers

If apple production is to be successful it is essential that abundant crops of large high quality fruits are set on trees, in every year. If the break-even point on capital invested in the orchard is to be achieved rapidly and the orchard is to move towards being a profitable investment, it is also essential that the trees begin cropping in the first two or three years following planting.

For these objectives to be achieved it is vital to ensure adequate pollination and fruit set.

The pollination and fruit set of apple trees can be influenced by many factors including:

  • The choice of site
  • Climatic conditions (and the provision of frost protection)
  • The planting of suitable pollinating varieties
  • The provision of bees in the orchard
  • Various management factors.

 

Choice of orchard site and its optimisation

 

Choice of a suitable site for apple production is one of the basic principles of fruit growing that appears in all standard textbooks and articles on apple production. Nothing has changed since this good advice was first put forward centuries ago and, if it is ignored, yields will be variable, especially from varieties that do not have a high setting capability.

Apple trees, provided with an appropriate choice of rootstock, will tolerate a range of site aspects and soil conditions whilst still growing and flowering sufficiently.

  • However, in many less favourable situations the trees will fail to be pollinated or the pollen will fail to germinate, grow down the style into the ovary and fertilise the female egg cell in the flower (the ovule).
  • Sites which are frosty, or simply cold and windy, at the time of blossoming are particularly unfavourable for pollination and fruit set.

In considering site influences on pollination and fruit set it is essential to pay attention to:

  • Site altitude and aspect
  • Provision of adequate shelter in the orchard
  • Avoidance of frost damage

Site altitude, aspect and slope

The following points should be taken into account when choosing a site for an apple orchard:

  •  Choose a site which is preferably between sea level and 125 metres above sea level.
  • Study the cropping history of previous orchards planted on the proposed site, or of neighbouring orchards.
  • If available, study meteorological records taken on the site in previous years.
  • Avoid sites prone to spring frosts or sites exposed to cold east or north winds.
  • Sites with a slight slope to allow the escape of cold air flows are to be preferred.
  • Ensure that there are no barriers (buildings or windbreaks), which impede the movement of cold air off the site and create ‘frost pockets’.
  • Apples can crop on north and east facing slopes but fruit size is likely to be maximised on warmer south facing slopes.
  • Sites close to large bodies of water tend to be slightly warmer and less sensitive to frost damage.
  • Choose a site which is sheltered from strong winds.

Provision of adequate shelter in the orchard

  • Plant windbreaks at regular intervals (every 100m) around sites, so as to provide adequate shelter for pollination, but not impede the escape of cold air flows during nights of radiation frosts.
  • Choose species such as alders or hornbeam, which are less competitive for water and nutrients than willows or poplars
  • Plant at spacings of 1.0m-1.75m apart in single or, if very good shelter needed, in double rows.
  • Trim windbreaks regularly and cut to approximately 7m in height.
  • Plant windbreaks several years in advance of planting the orchard trees to ensure adequate shelter when the young apple trees begin to flower.
  • Where living windbreaks are not available, use artificial windbreaks.

 

Protecting the flowers and young fruitlets from damaging climatic conditions

Avoidance of frost damage

In some seasons, considerable damage to flowers and young fruitlets is caused by frost. Desiccating winds at the time of flowering also serve to kill pollen and inhibit the activity of pollinating insects. Measures that can be taken to reduce potential damage from frosts:

  • Avoid sites at high altitudes or the tops of exposed hills where wind frosts are a likely occurrence.
  • Ensure flows of cold air down slopes and out of orchards in times of radiation frosts.
  • Do not plant windbreaks, which impede this flow.

Orchard management aids to reducing frost damage

Management of the soil beneath trees can help alleviate frost damage:

  • Most heat loss during radiation frosts (50%-80%) is from the soil surface.
  • To help in avoiding damage from radiation frosts, keep soil surface free of weeds and grass, keep soil compact and moist.
  • Although the use of canopies or polythene tunnels is widespread on other crops it has not proved economic yet on apples.

Installation of a frost protection system

Several methods of providing frost protection to orchard trees have been studied. All of these are used with varying degrees of success in different parts of the world. Only systems based on water sprinkling are currently recommended in the UK. Two types of sprinkler irrigation have been used: under tree systems with micro jets or over tree systems using impact spray nozzles. 

Under tree sprinklers

  • Low level, under tree micro sprinklers can reduce frost damage.
  • In calm (no wind) conditions applications of 2 mm water/hour to compact and previously moist soils can raise orchard temperatures by 1 or 2 degrees 2 metres above the soil surface.
  • Micro sprinklers cause no limb breakage, which is common following extended use of over tree sprinkler systems.

Over tree sprinklers

  • Over tree sprinkler systems, using impact type nozzles applying 2-3 mm of water per hour during frosts, can provide useful protection to the flowers.
  • The advantages of the systems are that they have low running costs and can be used for irrigation purposes as well as frost protection.
  • The disadvantages are the high installation costs, the potential for limb breakage due to ice loads on the tree and damage to the soil structure by the large amounts of water applied.
  • Water volumes applied can be reduced by use of a ‘pulsed’ system or by better targeting of the sprays, such that using mini sprinkler nozzles that hit only tree canopies and not the space between.

 

Influence of rain on pollination

Rain during the flowering period of apples reduces the potential for effective pollination and fruit set.

  • Firstly, the rain inhibits the foraging activities of all bee species and thus reduces pollen transfer.
  • Secondly, rain inhibits the germination and growth of pollen on the stigma and results in pollen death.

Use of chemical sprays designed to provide some protection from frost and winds

For many years, scientists have sought to identify products that, when applied to trees, improved the resistance of their flowers to frost damage.

  • Products that increase the concentration of dissolved salts of the cell sap, or that provide some physical type of protection to the cell, or that prevent cell damage following frosts, may have some benefit, however the results from these types of materials has been inconsistent.

 

Planting of suitable pollinating varieties in sufficient numbers

Most apple varieties are self-sterile and require pollen from another variety to achieve effective fruit set. Pollinating varieties, either other dessert or culinary varieties or ornamental crab apple varieties are planted in the orchard to provide this pollination.  Take account of the following when making decisions on choice and abundance of pollinators required:

Summary of pollination requirements of the principal commercial varieties

Varieties differ in their self-fertility and their pollination requirements as noted below:

Cox and Queen Cox                                         Weak self fertile but only at temperatures of

                                                                        20-25ºC

Q Cox clone 18                                                 Fully self-fertile              

Braeburn, Fiesta, James Grieve and Gala         Partially self-fertile, will set crops with their own pollen but a pollinator is recommended

Jonagold                                                           Has the ability to set fruits without pollination

                                                                        (therefore, seedless) given suitable weather.

Other varieties                                                  Require a pollinator

What constitutes an adequate supply of pollen therefore varies with the variety. It is also influenced by the season and the vigour of the orchard.

The importance of compatibility with the main commercial variety

It is wrong to assume that any two varieties of apple flowering at the same time will always be compatible with each other and capable of setting fruits when pollen is exchanged. Although, given favourable weather conditions during flowering this is mainly true, there are exceptions to this rule:    

  • Consider choice of pollinators carefully, avoiding unsuitable pollinators, especially on sites where conditions for pollination are less than ideal.
  • Do not use triploid varieties, such as Bramley’s Seedling, Jonagold or Boskoop as pollinators for other varieties. They produce only small amounts of pollen most of which is sterile.
  • Do not use Red Pippin as a pollinator for Elstar; unless climatic conditions are very favourable the varieties may prove incompatible.
  • Do not use Falstaff or Greensleeves as pollinators for Gala, unless climatic conditions are very favourable the varieties may prove incompatible.
  • Do not use Gala or Greensleeves as pollinators for Falstaff, unless climatic conditions are very favourable the varieties may prove incompatible.

Partial compatibility, where only 50% of the pollen is capable of growth down the style, may be experienced when using the following varieties as pollinators for Cox: –

   Alkmene, Elstar, Gala, James Grieve, Falstaff, Red Pippin.

Partial compatibility, where only 50% of the pollen is capable of growth down the style may be experienced when using the following varieties as pollinators for Gala:

   Alkmene, Arlet, Cox, Elstar, Golden Delicious, James Grieve, Red Pippin, Summered, Worcester Pearmain

Partial incompatibility is not a problem where ratios of pollinators is relatively high, blossom abundance on the pollinators and pollen supply is high and conditions for pollination are good.

Climatic influences on compatibility/incompatibility relationships

When weather conditions are very favourable during flowering some of the full and partial incompatibility mentioned above is overcome. Temperatures of more than 20 or 25oC will often be sufficient. Pollen quality and viability can clearly be affected by weather conditions.  

  • Pollen germination and growth down the style is greatly aided at temperatures of 15‑20oC.
  • At higher temperatures (e.g. 25oC) pollination efficiency is improved with variety combinations, which are normally incompatible or show only partial compatibility.
  • Winds cause pollen desiccation and often death.
  • Frost causes death of pollen and the female parts of the flowers. The damage is not always visible.

Self fertility

A few varieties of apple show either full or partial self-fertility.  This means that they can, given favourable climatic conditions during flowering, set fruits with their own pollen.

Self-fertile clones

Two varieties, Queen Cox Self-fertile Clone 18 and Cox Self-fertile Clone 8, both produced in the last 20 years, are fully self-fertile and can be planted without pollinators.

  • Self-fertile Queen Cox clone 18, which is available from UK nurseries, gives more reliable cropping than the traditional self-sterile clones in years unfavourable for pollen transfer between varieties by bees or other insects.
  • This self-fertile Queen Cox clone should not be used to pollinate other varieties as it produces insufficient viable pollen.
  • Growers considering purchasing self-fertile Queen Cox clone 18 are recommended to obtain this only from a verified source.

Planting partially self-fertile varieties without pollinators

A few varieties show partial self-fertility, especially when temperatures at the time of flowering are high.

  • Although several popular apple varieties, such as Red Pippin and Braeburn show a level of self fertility if climatic conditions at flowering time are favourable, this cannot be relied upon to ensure consistent and high yields of fruits on most UK sites.
  • Although Braeburn planted without pollinators will set good yields of fruits, these will contain few seeds and will have low levels of calcium.
  • The seeds are essential in the uptake of calcium into the fruits and the reduction in bitter pit incidence.
  • Varieties such as Gala and Golden Delicious also often set fruits with their own pollen when weather conditions are particularly favourable.
  • However, most of the self fertilised fruits usually drop off at the time of June Drop.

Pollination using ornamental crab apples or other Malus species

 

Ornamental Malus as a pollinator

Several species of ornamental Malus can prove effective pollinators for commercial varieties of dessert and culinary apples.

  • They have the advantage of taking up less space in the orchard than normal pollinating varieties.
  • The tried and tested species/varieties are M. hillierii, M. aldenhamensis, M. Professor Springer M. Winter Gold and M. Evereste.
  • Always plant several varieties of these ornamental crab pollinators in an apple orchard, not just one.
  • Their winter chilling and spring forcing temperature requirements are different from those of the commercial apple varieties and this often leads to lack of synchrony in flowering times.
  • Do not neglect the pruning and, where necessary, the thinning of ornamental crabs, or they may go biennial and fail to produce the required flowers in sufficient abundance.

Synchrony of flowering times

For effective pollination it is essential that the main and pollinating varieties flower at approximately the same time period in the spring.

  • Choose pollinating varieties which, according to local records, have flowering periods that overlap by a minimum of six days with the main apple variety in the orchard.
  • This overlap should be consistent and judged from records collected over a number of years at sites close to the intended site for the new orchard.

Production of adequate quantities of viable pollen by the pollinators

Pollinating varieties for use in orchards of self-sterile varieties of apple must produce adequate quantities of viable pollen.

  • The quantities produced are influenced by the scion variety chosen, the rootstock used, the crop loading (on the fruiting crabs), the orchard climatic conditions, the density of pollinating varieties planted and their management.

Scion varieties and their production of viable pollen

Take account of the pollen producing potentials of the varieties chosen as pollinators.

  • Varieties such as Golden Delicious produce copious quantities, whilst Cox and its clones produce much less.
  • Triploid varieties, such as Bramley and Jonagold produce almost no viable pollen and should not be used as pollinators.
  • Do not choose the self-fertile clones of Queen Cox or Cox as pollinators for self-sterile varieties; they produce too little viable pollen.

Rootstock influence on the production of viable pollen by scions

  • Pollinating varieties grown on dwarfing rootstocks, such as M.9, produce more flowers per unit tree size than the same variety on a more invigorating rootstock.
  • Trees on dwarfing rootstocks also take up much less valuable space in the orchard.

Crop loading and its influence on pollen production by scion varieties

  • Manage the crop loads on the pollinating varieties so as to avoid overset and the establishment of a biennial pattern of cropping.
  • Only if thinned well will the fruiting pollinating varieties produce abundant supplies of flowers and pollen on a consistent seasonal basis.

The orchard environment and its influence on pollen quality

  • Avoid frost damage to pollinators by good site selection and, where possible, use of frost protection measures.
  • In the event of frosts after green cluster, check the pollen viability using simple pollen germination tests.

The ratio between the pollinating variety and the main variety

Generally the pollination ratio should be 1 in 8 to 1 in 10, but in extremes i.e.

  • Poor setting, cool sites with excess vigour 1 in 4 to 1 in 6 will be necessary .
  • Weak trees of a partially self fertile variety on a good site 1 in 12 to 1 in 15 would be adequate.

Take advice from your local advisor before choosing a ratio of pollinator to main variety for planting in a new orchard. This ratio will be influenced by:

  • How favourable the orchard location is, in terms of temperatures and shelter      from winds.
  • The populations of bees, either wild or introduced in the orchard.
  • The pollen producing abilities of the pollinating varieties chosen.
  • The propensity of the main commercial scion variety to set abundantly or           lightly.
  • The compatibility (full or partial) of the chosen pollinating varieties with the main variety.
  • Avoid varieties exhibiting a degree of incompatibility as these will give poor results in marginal site/weather conditions.

The management of the pollinating variety [hyperlink 9] in the orchard

The management (pruning and training) of pollinating varieties should aim to stimulate renewal growth and adequate production of quality flowers.

  • Apply water and nutrients to pollinating varieties so as to sustain their growth and flowering.

Providing ideal conditions for pollen transfer

Transfer is by insects and to a small extent by wind.  Best practice is to achieve an orchard environment which:

  • Encourages a wide range of natural insect vectors especially bumble bees by leaving (or creating) grassy sheltered banks and alternative food sources.
  • Creates adequate shelter, reduces wind speeds to encourage insect flight.
  • Does not reduce wind speed too low, creating stagnant air and no wind transfer of pollen.

Introducing bees where needed

Where these natural levels of insect activity are low, or where pollinator numbers are low and tree vigour is high, pollination can be supplemented by importing hives of honey bees or by encouraging the establishment of bees in the orchard.          

  • Honey bees will forage more successfully on clear days and when temperatures are above 12oC.  Rent healthy, well- stocked (>15000 bees) hives and shelter the hives from cool winds.
  • Introduce hive or bumble bees to orchards only when 20% of the flowers are open.
  • Introduction earlier may lead to the bees seeking food supplies on other crops growing nearby.
  • Once habituated to another crop it is often very difficult to attract the bees back into the apple orchard.
  1. ·        Remove (by mowing or use of herbicides) weeds or other species that are flowering in the orchard at the same time as the apples.   
  • These may prove more attractive to the bees than the apple flowers.
  • Avoid broad-spectrum insecticides during blossom

Improving conditions for pollen germination and pollen tube growth

Germination of pollen

The fertilisation process begins with the pollen falling on the stigma and germinating.  Germination requires adequate but not excessive moisture as the pollen grains need to take in water in order to germinate, drying winds will reduce viability. 

  • Pollen grains lose viability rapidly once wetted so rain will significantly reduce pollination. 
  • Germination is temperature dependent with optimum temperatures being between 15ºC and 25ºC.
  • Some laboratory tests have indicated that certain pesticide sprays can reduce pollen germination. 
  • The relationship between these tests and the effect in the orchard is not known at today’s spray volumes. 
  • Dinocap was the most damaging with Captan and sulphur also having some reduction on pollen germination.
  • Sprays of penconazole (e.g. Topas) and chlorpyriphos (e.g. Equity and Lorsban) have been shown to reduce fruit set in some seasons if applied during flowering.
  • Laboratory tests have shown that boron and calcium can aid pollen germination but field experiments have given very variable results.
  • Various plant growth regulating chemicals have been trialled aiming to improve the fruit set and retention of apples, with varying levels of success.

Best practice to increase pollen germination

Pollen tube growth on stigma and style

  • Aim to create a sheltered orchard environment in order to lift average temperatures and reduce desiccation from drying winds.
  • Avoid any sprays especially on those warm days when germination is the most likely to proceed.
  • Avoid spray volumes which thoroughly wet the flowers.
  • Provide plenty of pollen as there is evidence that increasing the number of grains on the stigma seems to stimulate germination.
  • Ensure that tree nutrition is adequate and they are not deficient in boron or calcium.

Growth of the pollen tube

Following germination on the stigma the pollen tube must grow down through the style to reach the ovule in order to fertilise it.

  • Pollen tube growth is almost entirely dependent on temperature.
  • Where temperatures are low and the tube does not grow down the style within 2-4 days fertilisation may not occur.
  • Some varieties, notably Falstaff and Redsleeves, have pollen that is able to germinate and grow at much lower temperatures.

Best practice for encouraging growth of pollen tube

  • Create a sheltered warm environment in the orchard.
  • Consider using Falstaff or Redsleeves as pollinators, especially in sites which have marginal spring temperatures.

Successful fertilisation of the ovule

The Effective Pollination Period (EPP) is the number of days after the flower opens during which time it can receive pollen and still set a fruit. 

Growth of the pollen tube

  • Some varieties have shorter periods than others. 
  • Measurements of Cox showed it to be both shorter than other varieties and more variable from year to year. 
  • The EPP combines the time taken for the pollen to germinate, the pollen tube to grow and the time during which the ovule remains viable. 
  • How the EPP is influenced is not understood but aiming for best practice in all the above areas will induce stronger flowers, more viable pollen, better pollination conditions and a higher success rate at fertilisation.

 

Supplementing pollen supply in the orchard using floral bouquets

In situations where the orchard has insufficient pollinators or these trees have become biennial in cropping it may be necessary to supplement the supplies of flowers/pollen in the orchard.

  • Where pollen supply in the orchard is inadequate due to biennial flowering of the pollinators, growers should consider placing floral bouquets in the orchard during the flowering period to supplement pollination.
  • Where inadequate pollination is a more consistent problem (i.e. in every year), growers should either interplant the trees with additional pollinators or graft branches of these pollinating varieties into some of the existing trees of the main commercial variety.

 

The effect of vigour and branch orientation on fruit set and retention

It is clear that training branches of the variety Cox’s Orange Pippin to the horizontal results in better fruit set. 

  • Trials indicate that the horizontal orientation is important not just to flower development but to the success of pollination/fertilisation itself. 
  • Management practices, which reduce the vigour of shoot growth, generally result in improved fruitlet retention.