Managing orchards for fruit sizeResearch & Extension
Authors: John Wilton and Ross Wilson, AgFirst, New Zealand.
Using two Focus Orchards in Victoria and South Australia as case studies, the Future Orchards® team shows how site specific management can be used to optimise apple size.
There is an old saying in the orchard industry that says “keep doing the same practices and the results will remain the same.” Within reason, unless it is a very different growing season, this saying holds true. So, if you have orchard blocks that fail to perform well, continuing with the same husbandry practices will not change their performance. The reasons for their mediocre performance need to be identified and then crop husbandry management can be adjusted to overcome the limiting factors to improve performance.
Three Bridges, Victoria
The first Future Orchards® Focus Orchard case study we want to highlight concerns the size of Royal Gala apples on the Three Bridges property in southern Victoria (Table 1). ‘Royal Gala’ on this orchard have had a history of small fruit size. The average fruit size of the property’s Focus Orchard blocks are well below average when assessed using AgFirst’s OrchardNet™ block analysis report. For instance in 2013, the OrchardNet™ average ‘Royal Gala’ fruit size was 160g and the upper quartile average was 175g, but the Focus blocks on this orchard were:
- Royal Gala M (2D single row): 146g
- Royal Gala J (V net block): 129g
In 2012, the fruit size of Australian ‘Royal Gala’ apples was generally larger, with an average fruit size of 168g, but the two Three Bridges Focus Orchard blocks averaged 123g.
Always check out the obvious first before searching out more exotic solutions.
Managing crop load
As there is a very strong negative relationship between crop load and fruit size, one would expect the likely culprit for small fruit size to be excessive crop load. A comparison of yield data for these two blocks indicated that, relative to the OrchardNet™ database, crop loads were not excessive. In general crop loads were only average, which indicates other aspects of orchard husbandry must be responsible for the chronic small fruit size problem.
Reducing water stress
As water stress is a major limiting factor on many Australian orchards this was an obvious place to look. A comparison of applied water with evapotranspiration requirement for the 2012-13 season indicated that the Royal Gala orchards were being under watered and therefore suffering from water stress. For the 2013-14 season irrigation rates were lifted to bring them into line with evapotranspiration requirements to minimise the impact of water stress on fruit sizing.
The production policy for Royal Gala on these orchards had been to focus on supplying early fruit, by advancing bud break and flowering with dormancy breakers to bring harvest forward. For their particular orchard’s micro climate it was determined that bringing flowering forward brought it into a cooler weather window. This could have adverse affects on the tree’s response to chemical thinning and lead to poorer growth over the critical post-bloom cell division period. Both are important factors in causing poor fruit size.
It was decided that for the 2013-14 growing season no dormancy breaker would be applied so that the flowering and critical chemical thinning period would occur later in the spring when more favourable warmer weather was likely.
Fruit size monitoring this season shows that Royal Gala M achieved 76mm, while the southern Victoria average was 71.2mm at 143 days after full bloom (Figure 1). Royal Gala J (V net block) achieved 69mm compared to the southern Victoria average of 67.8mm at 136 days after full bloom. Not only has the average fruit size improved but the total harvested yield has also increased substantially. This strategy for increasing Royal Gala has certainly worked at Three Bridges and is all the more impressive when you consider that for Australia, 2014 has been a small fruit size year.
Oakleigh Orchard, South Australia
The orchard block chosen for the second case study was a block of ‘Galaxy’ on Oakleigh Orchard, South Australia. At pruning time, the block was left with a few too many buds, which would not have been a problem if there had been good spring weather conditions for chemical thinners. However, spring weather conditions were poor and consequently chemical thinners failed to remove sufficient fruit. The higher fruit numbers at the beginning of the season, and cool weather over the first 42 days after full bloom depressed fruit sizing and this, together with some very high summer temperature periods, all impacted on the early fruit growth rate. Fruit size monitoring using the OrchardNet™ fruit size tracking tool indicated fruit size on this block would be too small if harvested in the normal ‘Galaxy’ harvest window.
‘Galaxy’ on this orchard block would normally have been harvested around 136 days after full bloom when historical fruit size for the block has been in the vicinity of 74mm. This season, measured fruit size at 136 days after full bloom was 64.9mm some 9mm smaller than normal (Figure 2).
As fruit continues to grow through harvest, delaying harvest will increase fruit size so the strategy to delay harvest was adopted. This was achieved by two applications of Retain®, the first at an estimated 28 days prior to harvest, which is around 110 days after full bloom, followed by a second before the effects of the first one began to run out.
This strategy pushed harvest out to 147 days after full bloom when measured fruit size had reached 68.1mm. This gave an average harvested fruit weight of 150g, which was at least one size count larger than had the fruit been harvested as normal (Figure 3). This is an example of proactive crop management based on access to real time data being used to overcome a potential fruit size and marketing problem. If it wasn’t for the bird damage the yield would have been excellent and the fruit size very acceptable.
Post harvest orchard husbandry
Once harvest is over, preparation for next year’s crop commences. Flower bud initiation and differentiation for next year’s crop has already occurred so it is too late to influence the quantity of return bloom, but it is not too late to improve its quality.
The post harvest period is when the tree makes one of its two root growth flushes for the season. The other one commences in late winter and extends into late spring, slowing right down once the crop is set and growing.
The autumn root growth flush is very important for nutrient uptake to replenish nutrient reserves that have been depleted by the crop that has just been picked. The spring growth flush, including fruit set, is very dependent on mobilised nitrogen reserves from within the tree.
Straight after harvest is a key time to apply fertiliser, particularly nitrogen. This is the most important nutrient when it comes to setting up heavy regular crops. The trick is to maintain sufficient nitrogen levels to set the crop and drive enough leaf and shoot growth to support the crop without lifting the nitrogen status into the range where fruit colour development is compromised. Incidentally, tree vigour is not a good indicator of nitrogen status because trees with marginal nitrogen levels often do not set good crops, and trees without crops grow more than trees with crops.
Several foliar nitrogen applications over the post harvest period are an effective way of lifting nitrogen reserves while minimising the risk of excessive nitrogen levels that could affect fruit colour.
If soil conditions are dry, the orchard needs a good irrigation straight after harvest to allow good root growth to occur because roots cannot grow in dry soil.
Post harvest through to dormancy is also a major bud development period. The harvested apple fruit needs about 23 cell divisions to achieve its full potential and only two and a half to three of these cell division cycles occur after pollination. The other twenty occur prior to pollination with a significant number of these occurring over the post harvest period. It is therefore important to retain good leaf quality over the post harvest period to build up carbohydrate reserves and drive bud development. Trees under water stress tend to shed leaves early.
Disease and vigour management
Phytophthora root disease can be active once soil temperatures cool down in the autumn. Phosphorous acid foliar sprays are good insurances against this disease.
Where there are problems with excessive vigour and tree containment issues, taking out strongly growing branches that are responsible for shading immediately after harvest can make these trees easier to manage and lower their vigour response to pruning. This reduces the amount of photosynthates transported from these branches to elsewhere in the tree, particularly their roots, where they would otherwise fuel future vigour. Another benefit is improved light levels into the remaining canopy which will strengthen their buds.
Post harvest vegetative growth flushes can also be a problem in some early harvest varieties, notably the ‘Royal Gala’ group. This new growth is highly susceptible to both scab and powdery mildew infection, making it a potential source of disease infection for next season’s crop if the diseases are left uncontrolled. Post harvest fungicide applications may be necessary in orchards where the diseases are causing major problems.