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High orchard performance depends on good thinning

Research & Extension

The 2014/15 season was a very good time for pome fruit across Australia. The positive results were partly due to a kinder than normal climate for growing apples but this appears to have been a minor factor.

These second leaf red strain of Cripps Pink have failed to colour well due to over cropping. Crop load is estimated to be 20 fruit/cm2 TCA.

OrchardNet™ shows that the increased crop performance was mainly in those varieties that have been planted to modern intensive growing systems in recent years. Comparing 2015 variety yields with five years ago (2011), shows that the older varieties now have upper quartile orchard performance falling from 63.1t/ha to 54.9t/ha for Jonathons, and 59.2t/ha to 50.0t/ha for Red Delicious.

Also in the last five years, the average of the upper quartile gross production has lifted between 16 and 30 per cent for the main varieties of Cripps Pink, Gala, Fuji and Granny Smith. Similar productivity lifts have occurred in average variety yields.

The emphasis has been placed on the average of the upper quartile performance because the key to lifting orchard performance lies in demonstrating to orchardists what is possible in the way of yield.

Table 1. Change in productivity (2011 to 2015) for the main apple varieties.

Increasing Scifresh yields

Well thinned Fuji – early thinning to spaced singles will ensure good colour and return bloom.

In recent years in New Zealand, we had a good example of increasing the performance with the apple variety Scifresh (that is sold under the trademarked name of Jazz™).

Seven or eight years ago the yield ceiling for this variety was thought to be in the region of 60t/ha. At that time, an extension program for that variety involving discussion groups, orchard walks and grower meetings was implemented with the objective of improving its performance as well as solving fruit condition and other problems that may not appear until the fruit has reached the market. Today, average yield for Scifresh exceeds 60t/ha by a comfortable margin. Lifting grower perception of what is possible was one of the key factors in pushing up Scifresh orchard performance.

Our Australian database shows the top performing blocks are now producing in the region of 100t/ha or more. The challenge now is to lift more of them into that zone and achieve yields of this order on a regular basis without the problem of biennial bearing.

Thinning is critical to high orchard performance

These Fuji were well thinned with BA and have high fruit colour.

By now the chemical thinning response should be well under way and for very responsive varieties the drop is probably complete. It is very unlikely that the chemical thinning program will have removed the need for hand thinning or, if it has, this suggests it was a bit on the aggressive side.

Thinning, in my opinion, is the most critical orchard husbandry task when it comes to maximising yield and quality. The sooner the crop load is brought down to optimum levels, the closer you will be to capturing the genetic potential of the crop in regard to yield, fruit size, colour and quality. The level of return bloom is also determined by the time the crop load is brought down to optimum levels.

To minimise biennial bearing risk and maximise the potential of the crop, optimum fruit numbers need to be achieved within about six weeks after full bloom for varieties with strong biennial bearing tendencies. Varieties particularly prone to biennial bearing include Fuji, Braeburn, Golden Delicious, Scifresh and, to a lesser extent, Granny Smith. Cripps Pink and the Gala group are relatively forgiving when it comes to cropping and are much less likely to go biennial, but will under tough growing conditions subjected to high summer temperatures. Trees on dwarfing rootstocks tend to be less prone to biennial bearing but, if they happen to go biennial, are very difficult to bring back into regular cropping.

Know the numbers

These Fuji were not chemically thinned and have a heavier crop and much poorer fruit colour than those shown in the photo to the left.

Fruit and cluster counts on a few trees in each block are necessary to determine a thinning strategy.  Counting whole trees is time consuming but necessary to obtain a picture of the overall fruitset level.  Once a few trees have been counted, reverting to just counting fruiting branch units in the vicinity of 2 to 3cm in diameter at random across the block will give a good indication of how fruitset and therefore thinning requirements will vary across the block.

We do not have good Australian data for optimum fruit number per cm² branch cross sectional area (BCA) but I suspect it will be in the three to five fruit per cm² BCA range depending on variety.  Royal Gala and Cripps Pink will be at the upper end of that range while large fruit varieties such as Fuji will be towards the lower end of the range.

With mature canopies, previous yield and fruit size history can be a good guide to the level of crop load the block can carry. With younger trees that have not filled their allotted space trunk cross sectional area (TCA) can be used. It is not as reliable as BCA and the fruit numbers per cm² TCA need to be adjusted downwards as trunk size increases. While trees are small with trunk diameters up to about 5cm, crop loads can be in the region of 10 to 12 fruit per cm² TCA, dropping back to around six fruit per cm² TCA for trunk diameters of 7 to 8cm.

Thinning priorities

These Fuji have been well thinned and while their fruitlets were small, the fruit is largely in singles and is colouring well.

Once fruit numbers are known, a block by block hand thinning strategy can be developed for each variety.  A hand thinning priority programme also needs to be worked out.  ‘On’ crop and heavy set areas need to be hand thinned first with particular emphasis placed on getting to varieties such as Fuji, which have strong biennial tendencies as soon as possible in the hand thinning program.

The effectiveness of the chemical thinning program will also determine the order of hand thinning.  Where the chemicals have worked well and there has been good fruit shedding, particularly out of clusters, hand thinning of these blocks can be left until later.

Bunchy crops and those varieties with short stems need to be hand thinned early with the objective of having them thinned before the bunches close.  They are easy to do before this stage and become very difficult once the bunches have closed.

The pre-thinning fruit and cluster counts will show how many fruit need to be thinned off to bring the crop down to optimum levels and whether or not it will be necessary to leave doubles on some of the better sites.  Where fruit distribution and numbers allow it, the best strategy is to thin down to spaced singles.  Often there are not enough fruiting sites to do this and still achieve desired crop load so some doubles will need to be retained on strong buds.  Tip buds with bourse shoots handle multiple fruit numbers better than normal spurs.

The poorest fruit is found on auxiliary buds of one year old fruiting laterals. Sizing potential of this fruit is up to 30 per cent less than tip or spur buds and for varieties susceptible to stem end russet, these fruit have a high incidence of russet.

Table 2. Optimum fruit number per branch.

Monitoring the thinners

Fruitset and chemical thinner response is generally determined by tree vigour.  High vigour trees shed fruit readily and will require less hand thinning than lower vigour trees.  Where there is a wide variation in tree vigour within a block, it is a real challenge to get sufficient hand thinning on the weaker trees.  This is where setting thinning specifications by branch size really pays off.  Monitoring the hand thinners is quick and easy.  Adjusting their level of thinning is simple and once you get a picture of the fruit density on these branch units it is relatively simple to assess the quality of the thinning job.

Although branch unit counts are the main thinner assessment tool it is still necessary to do some post-thinning, whole tree fruit counts to make sure that the crop load is where you want it.  This is particularly important for weaker growing, heavy fruitset areas where it is very difficult to get thinners to remove sufficient fruit.  Incidentally, there is often significant fruit drop between hand thinning and harvest so target crop loads at hand thinning time need to be 10 to 15 per cent higher than the number you expect to harvest.

With high value, partially coloured varieties, crop load has a huge impact on harvest fruit quality.  Too much crop will markedly suppress colour development.  With these varieties a second thinning pass towards harvest to remove poorer quality fruit can pay big dividends in terms of lower harvesting and handling costs as well as lifting fruit value through better colour.

A second thinning pass is only possible where the crop is already largely in singles, otherwise sunburn injury following late thinning can be a big problem.  The first thinning pass should aim to bring crop loads down to 110 to 120 per cent of final fruit numbers.

Biennial bearing

The thinning strategy and particularly the time of thinning is an important strategy for managing biennial bearing. There is good data to show that the chances of a return crop are greatly improved if excess crop is thinned off within four to six weeks after full bloom.

Along with a good chemical thinning program that employs products like ethephon, ATS and benzyladenine (BA), early hand thinning to remove excess crop is a major step in overcoming biennial bearing.  Where biennial bearing is entrenched, thinning alone is usually insufficient to bring the crop back next season.

So in addition to a robust thinning program, other measures need to be implemented as well.  Where vigour is high, getting vigour under control will help. The most effective tool at this time of the season for vigour control is trunk girdling done in mid to late November once the fruit drop is underway. In winter or early spring, pruning out strong, high vigour branches to stack the tree canopy with weak pendant branches is a good way to work the vigour out of the tree.

Root pruning can also be very successful but has to be done in the late dormant, early bud break period to avoid excessive water stress problems. Growth regulators also play an important role in managing the biennial bearing problem.

Sequential sprays of ethephon at low rates over the main shoot growth period in late spring/early summer can be quite effective on some varieties for both vigour control and improving return bloom. There is also research data to show that adding ethephon to the later prohexadione–calcium vigour control sprays improves return bloom.

There has been a lot of trial work done in north America with sequential sprays of NAA at 5ppm through the mid growing season commencing five or six weeks after full bloom to stimulate flower development. Usually three applications are made in the cover sprays at 10 to 14 days apart. These recommendations have now been written into many of their state pome fruit production manuals.

We have adopted this strategy here in New Zealand and have found it very effective on our biennial varieties. The application window for us is late November through December. We are also aware of this program being used in Australia with good results on Fuji.

Summer pruning

Summer stubbing of annual shoots is an effective way to turn excess vigour into useful fruit bud.

Where annual growth is causing shading problems in the canopy, stripping the offending water shoots out in late spring, while they are still soft and can be easily pulled out, is an easy way to deal with this problem and minimises regrowth. Incidentally, their removal improves visibility within the canopy and makes hand thinning much easier because you can see the fruitlets much better.

Late spring shoot tipping somewhere in the 7 to 10 open leaf stage of growth is also a good vigour control measure and can turn excess vigour shoots into useful fruiting sites

About the author

John Wilton was a Horticultural Consultant at AgFirst – New Zealand.

biennial bearing crop load and thinning Future Orchards vigour

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