AgFirst’s John Wilton looks at what to do now to prepare for the 2018 crop covering thinning, setting crop loads, summer pruning and tree vigour management.
By now it should be possible to see the level of fruitset for the coming crop and determine how effective chemical thinning programs have been. There may also be an indication of the level of irrigation water available to grow the crop.
Last year, the growing season was fairly mild by Australian standards and there was adequate water, so tree condition and bud quality coming into this spring should have been very favourable for setting up a good crop. Because much of the cell division in the apple fruit occurs before flowering and last year’s weather was kind, there should be good fruit size potential for the coming crop.
Historical data indicates that, in general, Australian orchards have become more stable in their cropping and their average production per hectare has been increasing. In recent years, Australian apple growers have moved to intensive orchards on precocious dwarfing rootstocks. Many of these orchards are now approaching their full yield potential so this will be one of the reasons for ongoing increasing average yields.
There are still many older orchards planted on semi-dwarfing rootstocks, too. When well managed with good vigour control these plantings are also high-yielding. With certain scions, such as Fuji, these older orchards can become prone to biennial bearing so need careful pruning and thinning is needed to minimise this problem.
Once the chemical thinners have done their work and the fruit drop is coming to an end, hand-thinning needs to start as soon as possible. Market expectation in regard to fruit quality, particularly colour, continues to rise. Crop load and fruit distribution have a huge impact on fruit quality, including fruit size, fruit colour, harvest maturity, flavour and sugar levels. It is not possible to grow a high-quality fresh market crop without careful attention to hand-thinning and crop load.
As a general rule, the earlier you can bring the crop load down towards optimum levels, the better the fruit size and, for biennial-bearing-prone varieties, the higher the likelihood of a good return bloom. In hot climates where there is a high risk of sunburn, it is critical that early thinning to break bunches down to singles or certainly no more than twos is done before the hot weather arrives. Once apples are singled, further thinning is possible with minimal sunburn risk.
With large-fruited, partial red varieties, such as Fuji, Envy™ and Kanzi™, spaced singles are necessary to achieve high-specification fruit. With these varieties, trying to compensate for inadequate fruit numbers for a full crop by leaving fruit in bunches is false economy and usually leads to problems with fruit quality, particularly colour at harvest. If you are looking for an easy harvest, thin these varieties down to singles.
If thinning to singles is not giving enough yield then the solution to this problem is growing more canopy with sufficient fruiting sites to allow thinning to singles.
Set hand-thinning priorities
Varieties that have short stems need to be thinned first because bunches in these varieties soon become tight with increasing fruit size, making hand-thinning much more difficult. Fuji and Kanzi clearly belong to this camp, as do Jazz™ and Envy. Royal Gala (which is prone to heavy late fruitlet drop), Cripps Pink and Granny Smith can be left until later.
It is difficult to set precise crop loads with early thinning due to the fruit being small and hard to see at this stage of development, so for high-value varieties with substantial price premium for quality, a second thinning pass later once the fruit is more visible is usually necessary to groom the crop and make it ready for harvest.
Trying to do a perfect early thinning job can slow up hand-thinning, making it difficult to get across the orchard in the time available. By planning a second pass, it is possible to speed up the first pass so your thinning team does not get bogged down too much in a difficult block and run into problems on other blocks later.
The objective should be to get across the whole orchard to bring the crop down to singles and the occasional double before the really hot weather arrives.
Setting crop loads
Before hand-thinning begins, it is necessary to evaluate what level of thinning will be required by variety and block.
Set crop loads required for satisfactory crop performance. There are a number of methods that can be used to determine what the crop load should be. The focus has to be on growing a quality crop fit for market requirement. With mature orchards, historical performance in regard to fruit size, quality and yield is a good guide.
Trunk cross sectional area
In developing orchards, setting crop loads is a little more tricky. Where the canopy has not filled its allotted space, crop load calculated on trunk cross-sectional area (TCA) is an easy way to set fruit numbers after thinning. We have done many studies on the effect crop load has on tree development and have generally found that where trees are growing normally, it is possible on dwarf rootstocks to begin cropping in the second leaf and then gradually build crop load as tree size increases.
Our experience suggests that for the first few years of the planting, crop loads in the order of 10-12 fruit per cm² TCA are possible. In the harsher Australian climate this level of cropping may be too high so maybe it should be dropped back to perhaps 6-8 fruit per cm² TCA. I would base this decision on how well the trees are growing. Where trees are making good annual extension growth on dwarfing rootstocks, 10-12 fruit per cm² TCA should be possible.
By the time of hand-thinning, most of the harm overcropping will do to tree growth will have already occurred. Heavy flowering and allowing too many growing tips does most of the damage. Where young tree growth is too weak, it is usually caused by a site-related stress problem such as poor drainage, drought stress or low soil fertility. These problems need to be identified and overcome, otherwise the trees will always do poorly irrespective of crop load.
Branch cross-sectional area
Once trees begin to fill their allotted space, TCA tends to over-estimate cropping potential unless the specific crop load is pared back. This is because canopy volume becomes the limiting factor to cropping capacity but each year TCA increases due to trunk thickening. In this situation, using branch cross-sectional area (BCA) to set crop loads is a much better tool. Branches in the range of 2-2.5cm in diameter require about four fruit per cm² BCA. Using crop loads on branches, it is also very easy to monitor the thinning job without having to count whole trees.
It is necessary to count a few whole trees to check that the post-thinning crop load is satisfactory. If you are looking for a specific harvested fruit number, it is necessary to allow for the fruit that disappears between thinning and harvest and that which may not be harvested. For most varieties, something like 10-20 per cent of fruit after thinning does not make it into the bin.
Within block variation
In most orchard blocks there is a range of fruitset level so some trees will require more hand-thinning effort than others. For this reason, the hand-thinners need careful supervision to make sure they are ending up with a uniform crop load across the block.
Generally, hand-thinners never take enough fruit off weaker-growing heavy-fruitset trees. This is because these trees set more fruit than more vigorous trees and because of their smaller tree size (often caused by years of excessive crops) do not have the same yield capability of larger trees. Where large teams of hand-thinners are working, watch out for crop load differences across the rows, particularly where the work is being done by contract rates.
While thinning is by far the most important growing-season crop-husbandry task, there are many other tasks in the orchard that need attention.
Where tree vigour is excessive and light penetration into the fruiting zones is becoming a problem, some late-spring shoot ripping or summer pruning may be required. This is best done early in the growing season before the weather gets too hot. The best technique is to pluck unwanted shoots out, rather than cut them out, because with plucking shoots the adventitious buds at their base are removed too so regrowth is less likely.
I am not a strong advocate of summer pruning and consider that if too much is necessary it is an admission of tree management failure. It is best to address this problem by identifying the branches likely to produce excessive annual shoot growth and pruning them out during winter.
Tree vigour management
Once full canopy volume has been established, the most efficient cropping trees are those that just make sufficient new foliage to support the crop. This usually requires annual shoots to grow only 20-30cm in length and form terminal buds by mid growing season (usually early to mid December). Shoot growth in excess of this length represents wasting photosynthates that should have been diverted into fruit rather than unwanted shoot extension growth.
For varieties that are susceptible to bitter pit and blotch, high vigour is a key contributing factor. Scifresh (Jazz™), Kanzi, Granny Smith and Braeburn all suffer from this calcium-related disorder. I have also seen problems with it in Fuji and Royal Gala as well if summer temperatures are high and tree vigour strong.
Regular full crops of fruit are your best vigour-control agent. Where tree vigour becomes excessive and there is not the crop load for its control, it may be necessary to implement other vigour-control tools such as growth regulators, reduced deficit irrigation (RDI), trunk girdling or even a program of root pruning to bring tree vigour back into balance.
The Future Orchards® website library contains numerous articles describing these vigour-control techniques and is available on APAL’s website.
Bitter pit and blotch control
In addition to good vigour cropping, balance control of these calcium-related disorders requires an effective foliar calcium spray program. Most varieties, including those with low pit and blotch risk, will benefit from a foliar calcium program. Apart from on Granny Smith (which requires calcium nitrate because calcium chloride is phytotoxic to its foliage and it generally benefits from the extra nitrogen), formulations containing calcium chloride are the most effective foliar calcium sprays as they don’t apply excess nitrogen.
Varieties with low pit and blotch risk should have a minimum of 4-6 foliar calcium sprays, whereas those prone to problems require a much more intensive application program, usually 15-20 or more.
Foliar calcium spraying should begin when fruitlets are about 20mm in diameter and continue right through to harvest. For varieties susceptible to lenticel blotch pit – for example, Scifresh (Jazz) – it is the foliar calciums near harvest that do the most good.
Managing water stress
Irrigation water supplies in Australia can become marginal so it is necessary to have a plan for managing the orchard should irrigation water become limiting. Established apple trees, even on dwarfing rootstocks, are quite resilient when it comes to water stress. Younger trees are less so. In the first year after planting, trees are very sensitive to drought stress and will actually die if the water stress is too severe.
Recently planted trees do not need a lot of water to keep them going but maintaining their water supply is probably the highest priority for water when it becomes short.
High soil temperatures adversely affect apple tree root growth so mulching to lower soil surface temperatures and reduce soil surface evaporation losses is a very effective method to reducing the effects of water stress. There is good data to show that an organic mulch can double the irrigation interval. Mulching lighter soil areas that are prone to water stress is a good way to even up tree growth across a block.
If it looks as if irrigation will need to be severely reduced, use of the limited water supply will need to be carefully planned. As mentioned above, recently planted trees should remain top priority. Next in priority should be the high-value varieties that contribute most to your cashflow.
Early-harvest varieties may need priority for water two or three weeks before harvest because excessive water stress at this stage can interfere with maturity, particularly colour. These varieties will not need further irrigation over harvest but may need some water after harvest just to maintain foliage health and prevent premature leaf fall.
Marketable fruit size can be maintained in trees under water stress by dropping crop loads to approximately 40 per cent below normal. This is best achieved by selectively thinning by fruit size, taking out the smallest fruit, any damaged fruit and fruit growing in parts of the canopy where there is inadequate light for good colour.
Sunburn is always a risk with late thinning and this is why it is important to try to single bunches in the main hand-thinning pass earlier in the growing season.
Future Orchards® is a strategic levy investment under the Hort Innovation Apple and Pear Fund. It is funded by Hort Innovation using the apple and pear levy and funds from the Australian Government, and is delivered by APAL and AgFirst.