In an apple or pear orchard, one can readily observe a great variation in spur quality: from strong, healthy spurs to weak, non-productive ones.
There are many factors that can cause this, such as weather–related factors, the distribution of light within the tree canopy, pruning and training, nutrition, genetics, water management, viruses, pests and diseases.
Strong fruiting spurs produce fruit of high quality. Apple and pear spurs vary in their number and size of leaves.
"A spur is a compressed leafy shoot with either a floral or vegetative terminal bud.
Spurs develop from axillary buds on two-year-old shoots, and often flower the following year.
Axillary buds on a spur can give rise to shoots or to new spurs.
A spur complex or branched spur system, forms after several years when new spurs originate on older spurs.
Spur-bound is a condition of a weakly growing branch or tree with abundant spurs and with little or no annual shoot growth."
Spur types & age
The buds on spurs can be classified as either vegetative (resting buds) or reproductive (fruit buds).
The key to good fruit tree management is to understand the role each spur type plays and then create a balance between the two types.
It is well-known that fruit tree productivity, especially fruit size and percentage packout, declines as spurs age.
A study in New Zealand has shown that two-year-old spurs on Golden Delicious, Braeburn, Granny Smith and Gala apple trees, had consistently larger fruit than on three-year-old and older spurs.
Fruit set data for different ages of fruiting wood of Packham’s Triumph pears in South Africa revealed that spurs on two-year-old wood set 83 fruit per 100 clusters, and spurs on older wood set 57 fruit per 100 clusters.
These are just a few examples that the highest quality fruit is borne on two-year-old spurs on replacement branches. The terminal buds of laterals (one-year-old wood) also had good quality fruit in these studies.
How to create and maintain young spur wood
Here is how you can create and maintain young spur wood in your apple and pear trees.
Vegetative spurs
Vegetative spurs form on one-year-old or older wood. They are vegetative because they have no flowers.
They form terminal buds which will flower and bear fruit the following year (they ‘rest’ for one year).
Sometimes a terminal resting bud decides to become a shoot extending the spur.
Vegetative spurs have typically 6 to 20 leaves. Flower bud formation normally starts 80 to 90 days after full bloom and may continue very late into the season.
Studies in England with apple trees have indicated that if trees lack vigour, shoot growth stops early and flower bud formation occurs very early in the summer on vegetative buds. The next spring, these flowering spurs appear ‘old’, have small leaves, short flower stems and set small fruit.
The opposite happens if the trees are excessively vigorous or carry a very heavy crop. Flower bud formation is then delayed and takes place in the autumn.
The following spring these ‘young’ flowering spurs have very large leaves, incomplete flower clusters with long stems and generally will not set much fruit.
In a well-managed orchard, flower buds will form during mid-summer. As winter approaches, you should see strong buds on these vegetative spurs that are soon going to become reproductive spurs. At the same time, you should see a new lot of resting buds.
In spring, the vegetative spurs that went through the resting stage and formed flower buds, now produce a full cluster of five or six flowers each, together with strong, medium sized leaves.
A strong relationship exists between the size of the bud formed on the vegetative spur and the ability of the spur to set fruit the following spring.
As bud diameter increases so does the potential for this bud to increase fruit set. A big bullet-shaped bud of 4.6 mm diameter in winter has a full set of flowers, which have a 65 per cent chance to set fruit.
Excessively vigorous trees or weak trees, trees with a heavy crop load or poor nutrition, will have small pointy buds of about 3.5 mm diameter, which will have only an 18 per cent chance of setting fruit.
Think a year ahead
You have to think at least one year ahead.
It is easy to think about the current season’s crop, but while the fruit for this year is sizing, the flowers for next year are developing.
In any one year you manage two crops—this year’s and next year’s.
Vegetative spurs need about 100 to 150 cm² of spur leaf area during the last 90 days of the season to form flower buds and complete their development.
If more than 50 per cent of this leaf area is removed or damaged from January onwards, it could dramatically reduce flower formation and flowering the following year.
High insect pressure (especially from mites), excessive tree shading (yellow leaves), water problems or poor nutrition, can cause premature leaf drop and poor spur quality.
Although these problems may only slightly affect the fruit in the current season, next year’s crop may be more adversely affected.
Reproductive spurs
Reproductive spurs have gone through the vegetative or resting stage as explained.
They are formed on two-year-old and older wood. There are typically 6 to 10 primary spur leaves which form a whorl below the flower cluster of five to six flowers.
One or two small lateral shoots can grow out of this whorl of spur leaves. These are called the bourse shoots. They have 5 to 20 leaves and grow 250 mm or more in length, depending on variety, rootstock, crop load, nutrition etc., or they may merely form another whorl of leaves which end up in bourse buds.
In most cases, these bourse buds are resting buds and will not flower the following spring. A bourse is the thickened base of an inflorescence, which is the base of the flower cluster.
Importance of primary spur leaves
The primary spur leaves are very important for fruit set and early fruit development.
Most of the photosynthates (carbohydrates, sugars) needed by the flowers and developing fruitlets come from photosynthesis of these leaves.
If these leaves are damaged or shaded, the result is poor fruit set and small fruit.
They are also important for the uptake of calcium by the young fruit. Since calcium is needed in large amounts during the cell division period, it will ultimately determine fruit quality, especially fruit firmness.
The leaf area on a spur during flowering relates to long-term tree productivity. Trees with small spurs and small spur leaves have low yields. Trees with large spurs tended to have good yields during the life of the orchards. There is no substitute for good consistent tree management.
Early in the season, the developing bourse shoots may compete with the flowers for the photosynthates produced by the spur leaves. If bourse shoots grow too vigorously due to high nitrogen status or due to light fruit set, they will compete with the fruitlets for carbohydrates and nutrients.
Unless the fruitlet has seeds (a pear needs five or more seeds), the chances are that the fruitlet will lose the battle, die and drop off. This competition is also strong among flowers and fruitlets within the same cluster.
In apple, the strongest flower in the cluster (king bloom) usually sets and its demands for carbohydrates and nutrients increase daily. As time goes on, the spur leaves can hardly keep up with the demands and the bourse shoot leaves will then provide additional assimilates to the fruit for continued growth.
Thus, a strong reproductive spur will have good spur leaf growth during flowering and strong, but not excessive bourse shoot growth.
Six weeks later
About six weeks after flowering, cell division ends, the bourse shoots have succumbed to the strong competing forces of the developing fruitlets and stopped growing.
Apples may have to be chemically thinned during this period. This early burst of vegetative and fruit growth during the first 40 to 45 days after full bloom is extremely important and critical and will determine the size of your crop this year and most likely next year as well.
Cell enlargement
After this period, the apples and pears turn over and start the long process of cell enlargement until they are harvested.
Cells of fruit flesh continuously expand and fill with water and sugar.
Spur leaves and bourse shoot leaves are now not the only leaves which the fruit needs to satisfy its requirements for sugars. Nearby shoot leaves will have to assist.
There is increasing evidence to suggest that after mid-summer, shoot leaves contribute more to fruit growth than spur leaves. Spur leaves develop earlier than shoot leaves, but they also senesce (drop) earlier in the season. This may explain why spur-bound trees have trouble sizing their fruit.
The photosynthates produced by shoot leaves move only about 1–1.8 m within the tree. Therefore it is important to have a balance of spur leaves and shoots along the length of each branch and on the fruiting units as well, in order to produce the large high quality fruit which trees are capable of carrying.
Fruiting unit
A fruiting unit is a piece of one, two and three-year-old bearing wood, that is attached to a branch.
A fruiting unit is continually renewed to keep the bearing wood young, and is the foundation of the bearing capacity of an apple or pear tree.
The importance of shoot leaves in maintaining the exponential increase in fruit growth during the latter part of the season should not be underestimated.
In many fruit growing regions around the world it has been reported that shading shoot leaves or heavy summer pruning late in the season, can result in small fruit with decreasing soluble solids at harvest.
Only thick, parasitic sucker growth in the tops of the trees or inside the Vs of Tatura Trellis trees, should be removed in the summer.
A shoot on an apple or pear tree continues to grow and develop spurs each season if left untouched. After several years, spurs grow on many ages of wood. What growers should keep in mind is that as leaders and branches age, fruit set and fruit size decrease.
Age of spurs
It was found that when apple spurs age, the number of leaves per spur increased. But two-year-old spurs had more leaf area per spur, higher leaf efficiency and spur efficiency than spurs on either younger or older wood.
Leaf efficiency is used to describe specific leaf weight, because it is correlated with the rate of photosynthesis, the ‘factory’ that produces carbohydrates for fruit and tree growth.
Spur efficiency is used to describe total leaf dry weight per spur, which is a measure of the ability of all the leaves on a spur to provide photosynthates for fruit and bud growth.
When spurs had become four years old, fruit set and average fruit weight was significantly reduced.
Spurs and branch positioning
Spur quality can also differ on individual branches.
Spurs at the tip of a branch tend to be larger and more vigorous than those at the base. But spurs in the middle section tend to have a higher percentage fruit set than those at either end of the branch.
The spurs at the tips form flower buds last in the previous season (‘young’ spurs). Often fruit will not set, instead, lateral extension shoots will develop, especially if a heading cut is made close to a spur.
Basal spurs have ‘old’ flowers, which are under the control of apical dominance. The buds are small and have less leaf area than spurs in the middle.
The effect of position on a branch becomes even more pronounced as the wood ages. Spurs at the base of older branches can become weak and useless. They rob the tree of valuable reserves, fail to set fruit or produce small fruit.
Detailed pruning is necessary to keep an orchard highly productive.
Spurs and branch orientation
Spur quality is also affected by branch orientation.
On vertical branches the effect of spur position as discussed above, is much more pronounced than on horizontal shoots.
Horizontal branches have more spurs per length, the spurs are more uniform and the percentage fruit set is higher than spurs on vertical wood.
Over time, horizontal branches can develop pendant, downward-pointing spurs on the undersides. These spurs have fewer leaves, small leaf areas, lower leaf efficiencies and spur efficiencies and the fruit is always small and has poor quality. Detailed pruning will take care of these useless spurs.
Part one of two (see part two here)
The purpose of this first part of pruning apple and pear trees is to understand the factors which influence spur development and spur quality.
Fruit quality starts in the orchard with good spur quality. In the second part (starting page 21) you will read ways in which spurs can be managed to maximise their productivity.
You will take advantage of the productive potential of young spurs by using renewal pruning and don’t allow fruiting wood to become older than three years.
For more information and images of spur types, download the August 2012 issue of Tree Fruit.
