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:
- Scion varieties and their production of viable pollen.
- Rootstock influence on the production of viable pollen by scions.
- Crop loading in the previous year and its influence on pollen production by the scion.
- The orchard environment and its influence on pollen quality.
- The number of trees of the pollinating variety that are planted in relation to the number of trees of the main variety.
- The management of the pollinating variety in the orchard.
Scion varieties and their production of viable pollen
Varieties such as Golden Delicious have been shown to produce abundant quantities of viable pollen. This is a combined influence of the high numbers of flowers produced and the large quantities of viable pollen produced in each flower.
- Cox and its clones are poorer in this respect, producing fewer flowers per tree and less viable pollen per flower.
- The self-fertile clones of Cox and Queen Cox produce even less viable pollen per flower and should not be used as pollinators in orchards where the main variety needs pollination.
- Triploid varieties, such as Bramley’s Seedling, Jonagold and Crispin, produce almost no viable pollen and are, therefore, of no value as pollinators for other varieties.
- 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.
Rootstock influence on the production of viable pollen by scions
Most apple varieties and crab apples planted only for the purpose of pollinating the commercial apple variety are planted on dwarfing rootstocks, so as to take up the minimum land space within the orchard. This choice of rootstock has the additional advantage of inducing precocious and abundant flowering on the pollinator.
- The increased flowering induced usually also equates with the production of more viable pollen.
- There is no reliable evidence suggesting that rootstocks influence the percentage viability of the pollen produced by flowers of scions worked upon them.
- 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 in the previous year and its influence on pollen production by scion varieties
It is essential that pollinating varieties are not allowed to crop excessively.
- Excessive set and retention of fruits on pollinating varieties can lead to reduction in the number of flowers produced in the subsequent season.
- Occasionally, the flower quality, including pollen viability, may also be influenced deleteriously.
- In extreme cases, the pollinating varieties may go into a cycle of biennial bearing.
- Where this is a potential problem take time to thin the pollinating varieties.
Research conducted on the variety Golden Delicious growing in the Czech Republic (Blažek) showed that, where pollen donors were abundant, seasonal cropping was inconsistent.
- In contrast, where the distances between the Golden Delicious trees and a source of compatible pollen were as much as 500–1000m, cropping was lower but quite consistent, season to season.
- The Czech research suggested that a critical limit, above which symptoms of biennial bearing were noticed, was 5000 to 6000 apple seeds per 100 cm of trunk cross sectional area.
- For many varieties of apple this would mean approximately 1000 fruits per 100 cm of trunk cross sectional area.
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
Pollen quality is an important factor in successful pollination. Poor weather conditions, especially frost, can damage the pollen during its development and result in a large proportion of it being non-viable (dead). Such damage often occurs at the early stages of budburst when frost damage is not visible and is, therefore, often overlooked by the apple grower.
- Careful monitoring of frost is essential from bud burst onwards.
- Where frost damage to flowers and their pollen is anticipated, growers or advisors can check on the viability of the pollen using a simple test.
- 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.