Suitable pollinating varieties

Most commercial varieties of apple are self-sterile and require pollen from another variety in order to ensure adequate fruit set. Self-fertile clones of Cox are available and a few other varieties show a level of self-fertility, if given favourable climatic conditions.

The attributes of an ideal pollinating variety are:

• Compatibility with the main commercial variety.
• Synchrony of flowering time with the main commercial variety.
• Production of adequate quantities of viable pollen.

Another important factor is the influence of temperatures and stylar receptivity on pollination efficiency.

A few varieties of apple are self-fertile and able to set a crop of fruits with no need for cross pollination. Also, in a few special circumstances, (usually very high spring temperatures during flowering time), certain varieties of apple which are normally self-sterile exhibit a level of self compatibility and are able to set fruits with their own pollen.

Similarly, some varieties of apple are able to set fruit without the need for fertilisation of the female egg cell (ovule) by pollen. Fruit set of this type is known as Parthenocarpic Fruit Set.

Compatibility with the main commercial variety

All commercial varieties of apple are of the species Malus pumila Miller (also sometimes referred to as Malus x domestica Borkh.). All the cultivated varieties are very similar and probably the same species as the wild apples found in the forests of several Eurasian countries. These are often referred to as Malus sieversii.

The majority of varieties are self-sterile (cross compatible) i.e. the pollen of one variety is capable of germination, growth and fertilisation of the flower ovules of the other variety.

There are however some significant exceptions to this rule:

• Triploid varieties (e.g. Bramley’s Seedling and Jonagold).
• Varieties exhibiting cross incompatibility.
• Varieties that are semi- or partially cross incompatible.

It is also necessary to consider the climatic influences on the compatibility/incompatibility relationships.

Triploid varieties of apple

All varieties of apple which have three sets of chromosomes (triploids), rather than the normal two sets (diploids), produce almost no viable pollen and should not be planted as pollen donors for another variety in the orchard.

• In most cases, these triploids can be pollinated efficiently by most diploid varieties.
• However, recent results would suggest that Golden Delicious could prove a very poor pollinator for Jonagold, Crispin and Belle de Boskoop, whilst Summered will also prove a poor pollinator for Jonagold.

Taking account only of compatibility, and not considering synchrony of flowering times, the best pollinators for Jonagold (amongst those so far characterised) appear to be:

• Alkmene, Arlet, James Grieve, Idared and Tydemans Early Worcester.
• Many other varieties, such as Golden Delicious, Cox, Fiesta, Gala, Summered, Falstaff and Worcester Pearmain, exhibit partial compatibility with Jonagold.

Pollen compatibility characterisations on Bramley’s Seedling are still incomplete. However, preliminary evidence suggests that no variety is fully incompatible with it. Partial incompatibility with Bramley is shown by:

• Idared, Golden Delicious, Fiesta, Elstar and Kent.
• Full compatibility with Bramley is shown by Braeburn, Alkmene, Arlet, Cox, Delbard Jubilee, Gala, James Grieve, Worcester Pearmain and Falstaff.

Some popular triploid varieties of apple

• Do not use triploid varieties such as Bramley’s Seedling or Jonagold (or its various sports) as pollinators for other varieties.
• Do not choose Golden Delicious as a pollinator for Jonagold.

Varieties exhibiting cross incompatibility

Most varieties of apple have two sets of chromosomes (diploids) and are capable of pollinating/fertilising each other, providing they flower at approximately the same time and produce viable pollen.

• However, studies at East Malling characterising what are known as the ‘incompatibility alleles’ in apple varieties, have shown that a few varieties are, in most environmental conditions, incompatible with each other.
• This means that in most circumstances when planted together they will fail to pollinate each other effectively.
• This phenomenon has been recognised in sweet cherry varieties for many years, but it is only recently that it has been noted with apples.
• When pollen from one variety is transferred to the stigma of another with which it is incompatible, the pollen tubes fail to grow down the style into the ovary and no fertilisation of the ovule and fruit set is possible.
• This work also shows that some varieties may prove only partially efficient as pollinators for each other.

Definitive proof of the incompatibility of apple varieties in the orchard situation is, as yet, lacking.

• However, the research suggests that Elstar and Fiesta, in some environmental conditions, will not be able to pollinate each other efficiently.
• The same is true for Falstaff, Gala and Greensleeves. Golden Delicious and Jester will also not fertilise each other efficiently.

Even in apparently incompatible combinations fruit set is often able to occur.

• For instance, trials have shown that when Fiesta received pollen of another variety, with which it should be incompatible, it was able to set a reasonable crop of fruits.
• However, when the reciprocal cross was made, using pollen from Fiesta, no fruit set occurred.

Avoid using ‘incompatible’ varieties to pollinate each other:

• Do not choose to plant Elstar and Fiesta (Red Pippin) with the aim of them pollinating one another effectively.
• Falstaff, Gala and Greensleeves are not fully compatible and in situations unfavourable to pollination and fruit set should not be planted with the aim of mutual pollination.
• Only in situations of warm temperatures at blossom time will the above combinations be successful in pollinating each other.

Varieties which are semi or partially cross incompatible

With any two semi-incompatible or partially compatible varieties, only 50% of the pollen grains produced are able to germinate, grow down the style and set fruit when pollen is transferred between the two varieties.

Cox’s Orange Pippin and its sports are only partially compatible with Alkmene, Elstar, Fiesta, Gala, Kent, James Grieve, and Falstaff.

• However, as several of these varieties have been used quite successfully as pollinators in Cox orchards.
• This partial constraint is probably only critical when conditions for favourable pollen production and transfer are severely limited by unfavourable weather conditions.

Similarly, Gala and its sports are only partially compatible with Alkmene, Arlet, Cox, Elstar, Fiesta, Golden Delicious, Summered, James Grieve and Worcester Pearmain.

• In conditions very unfavourable for pollination these varieties may be slightly less efficient pollinators than fully compatible varieties.

Semi-compatibility usually presents little or no problem to the majority of apple growers.

• Studies have shown little difference between the pollen tube growth and fruit set of fully compatible and semi-compatible combinations (Alston, 1996) given reasonable environmental conditions at the time of flowering.
• Indeed, semi-compatible combinations of apple varieties have been recommended for planting in areas experiencing warm temperatures and other favourable climatic conditions at flowering time, as they can help avoid overset and biennial bearing and may reduce the need for fruit thinning (Alston and Tobutt, 1989).

The potential incompatibility relationships between all the popular varieties of apple have not been characterised. For instance, further research is needed to characterise Braeburn and Bramley’s Seedling.

• Some apple varieties are only partially compatible with each other.
• In situations unfavourable to pollen transfer and germination (cool windy sites) these varieties should not be planted together in low ratios of pollinator to main variety for pollination purposes.

Climatic influences on compatibility/incompatibility relationships

Temperature at the time of flowering can have a significant influence on pollen germination and at high temperatures (>25^oC) incompatible combinations may set fruits quite satisfactorily and many varieties become partially self-fertile. One example is the variety Fiesta, which exhibits increasing levels of self-fertility as temperatures increase up to 15^oC (Petropoulu, 1985).

• Frost damages both the male (pollen) and female (stigma, style and ovary) parts of the flowers.
• Improving shelter in orchards and raising ambient temperatures at flowering time can influence the type and number of pollinators required in an apple orchard.
• Pollen germination and growth down the style is greatly aided at temperatures of >15^oC.
• At high temperatures (e.g. 25^oC) 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.

Influence of temperatures and stylar receptivity or pollination efficiency

Low temperatures (<10^oC) can seriously reduce the effectiveness of pollen germination and speed of pollen tube growth down the style (i.e. stylar receptivity).

• Pollen of varieties such as Redsleeves gives high germination percentages and speed of growth, even at temperatures of 8-10^oC, whilst the pollen of Spartan and Falstaff are also good in this respect (Petropoulu and Alston, 1998).
• At these same low temperatures, the pollen of Cox and Fiesta germinated poorly.
• At temperatures as low as 5^oC, Redsleeves and Spartan were very effective pollinators for Cox, whilst many other varieties performed very poorly.
• Although Fiesta pollen grew poorly at low temperatures, the styles of flowers of this variety were very receptive to pollen of other varieties applied at low temperatures.
• At 15^oC, flowers of Fiesta exhibit partial self fertility.

In orchards where temperatures at flowering time are frequently less than ideal for pollination, choose pollinating varieties such as Falstaff or Redsleeves, which produce pollen that can germinate and grow at quite low temperatures.

Synchrony of flowering times

If pollinating varieties are to be efficient, it is essential that their flowering periods overlap sufficiently with that of the main variety planted in the orchard.

• In several countries, varieties of apple have been divided into three categories of early, midseason and late flowering types.
• The assumption usually made is that varieties in the same grouping pollinate each other best, there being no other constraints on pollination (e.g. compatibility).

It has also been assumed that varieties in the early and midseason groups, and also in the midseason and late flowering groups, overlapped each other sufficiently to ensure adequate pollination and fruit set.

• These assumptions are not always true.
• In some seasons certain varieties classified in the early and midseason categories do not overlap sufficiently; the same is true for varieties in the midseason and late categories.
• It has been suggested that, unless the flowering date ranges of any two varieties overlap by a minimum of 6 days, they should not be considered as suitable pollinators for each other (Kemp and Wertheim, 1992).
• Therefore choose pollinating varieties which, according to records, have flowering periods that overlap by a minimum of 6 days with the main apple variety in the orchard.
• This overlap should be consistent and judged from records collected over a number of years.

The average dates of full bloom since 1936 at East Malling for the scion varieties Cox’s Orange Pippin and Bramley’s Seedling are displayed in a graph.