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Spring crop management

Research & Extension

This tree flowered for about six weeks. At the time the photo was taken the flowering sites ranged from tight cluster to early stages of petal fall. The chemical thinners lifted fruitlet rather than reduced it.

As we head towards Spring, AgFirst’s John Wilton shares his advice and expert tips to help get your crop off to the best possible start.

Spring is a very important period for the pome fruit grower. It is over this period between late dormancy and about 42 days after full bloom, which is usually the end of cell division in the developing fruitlet, that the potential of the coming crop is determined. I believe this critical period should begin in late dormancy because it is at this stage that you have to give serious thought to thinning, and pest and disease control strategies.

For instance, Venturia (scab or black spot) fungicide protection needs to commence prior to any bud break. In localities where bud break and blossom period tend to be slow and drawn out, consideration needs to be given to applying dormancy breakers to compress bloom.

Bud break and flowering period compression is the key to increasing the effectiveness of a chemical thinning program. In addition, the resultant crop has more uniformity in regard to maturity and fruit size. This simplifies harvest management and results in more uniform fruit quality.

A well-timed dormancy breaker has compressed bloom on the row on the left. Compact blossom leads to better chemical thinning.

Work on chemical thinners we did some years ago showed that both blossom and post-blossom thinning programs were more effective where the blossom period had been compressed with dormancy breakers. Dr Sally Bound(Tasmanian Institute of Agriculture) has had similar results in her work with chemical thinners in Australia.

Vigour management

Judging tree vigour behaviour is critical to obtaining high orchard performance, as is growing a uniform fruiting canopy. Once a full fruiting canopy has been achieved, optimum shoot extension growth needs to be in the 20-30cm range, preferably terminating in a fruitful bud. Where tree vigour exceeds this level, which frequently happens in netted orchards because of their much kinder growing environment, measures to control excess tree vigour are necessary. Once full canopy is reached, low vigour trees give more regular crops, usually better fruit quality with much lower pit and blotch risk, and better fruit colour.

The row on the right has been root pruned where as the one on the left has not. Root pruning could be a useful technique for calming down over vigorous trees growing under hail net.

There are a number of tools for managing excess tree vigour. These include root pruning, which is best done in the late dormant period, growth regulators applied before shoot growth gets more than about 5cm in length and there is also the trunk girdling option.

Chemical thinning

Chemical thinning plays a major role in modern orchard crop management. There are already a large number of very good articles written on chemical thinning that are stored on APAL’s Future Orchards®website.  The key ones include:

Regular trunk girdling will keep excess vigour under control and improve fruitset, colour and quality.

Dr Sally Bound has also written some good material over the years, some of which can be found on the Future Orchards website too. And a reminder – if you need access to this site contact APAL.

Spray coverage is very important for chemical thinners and often implicated in situations where thinning results have been unsatisfactory. David Manktelow provided excellent notes on sprayers for the November 2014 Orchard Walks titled: Sprayers and sprayer performance – optimisation, coverage, volumes, chemical rates.

Chemical thinning is as much art as science and relies on very careful judgement of all the factors that may influence the result. These are well documented in the above references. Some of the key factors include:

Netted orchards often develop excess vigour problems as this one shows.

  • tree vigour and branch inclination
  • weather conditions, particularly post thinning spray application
  • variety
  • provision for cross pollination – a major factor in netted orchards
  • soil water logging – increases natural fruit drop
  • canopy shade
  • duration of flowering period
  • biennial bearing phase – ‘on’ crops are very hard to thin
  • nitrogen nutrition status

Many chemical thinning programs fall short of expectation because growers fear excessive response so take too timid an approach to the chemical thinning program. With the possible exception of misjudging the temperature and ethephon rate when used in early bloom stages, over thinning with our present available chemical thinning tools is rare. Nine times out of ten excess thinning responses will be due to other factors. Nitrogen deficiency, soil water logging, high tree vigour, shading within the canopy and poor provision for cross pollination will be more likely reasons for poor fruitset than the chemical thinning program alone.

Mature intensive orchards on dwarfing rootstocks become difficult to this due to their lack of vigour.

Where established chemical thinning strategies are working well, I would be reluctant to move away from them without careful testing of the other strategies under consideration.  Spray water pH and surfactants can also be a major factor in situations where chemical thinning programs fail. NAA and the benzyladenine (BA) products are sensitive to high pH. Surfactants also are very important for BA thinning. They become very important if the application period was preceded by cool dry weather conditions because under these conditions tissues develop much stronger cuticles than under warm humid conditions.

With blossom period thinners such as ethephon and ammonium thiosulphate (ATS), flowering stage is very important. NAA, Carbaryl and the BA thinners have much wider application windows. With these thinners it is much more important to get the weather conditions around and after application right, rather than target a precise fruitlet development stage.

There are now increasing areas of maturing intensive orchards planted on dwarfing rootstocks beginning to appear in Australian orchards. Because of their high precocity and significantly lower tree vigour than the old standard vigour rootstocks, such as MM106 and M793, these maturing intensive orchards will be more difficult to chemically thin than the older high vigour orchards they have replaced. On the positive side, their smaller, more compact canopies will make it much easier to obtain uniform spray coverage.

Fruitset in the image on the left is poor because there are no pollinators nearby. (Lt) Abundant fruitlet near to an adjacent variety.

Biennial bearing

The few ‘on’ crop trees in this largely ‘off’ block were blossom thinned and girdled to even them up with the remainder of the block. Production this year was 12t/ha, with the three subsequent crops 116, 143 and 123t/ha.

The spring period is an important time to begin addressing biennial bearing. Over the years I have found the best approach to biennial bearing is to avoid it in the first place. Your biennial bearing avoidance program should begin the first time the young orchard shows significant blossom. For trees planted on precocious dwarfing rootstocks this is usually in their second leaf. Our experience has been that where we have run an aggressive post-blossom chemical thinning program such as BA plus either NAA or Carbaryl, or even Carbaryl plus NAA, we have been able to avoid biennial bearing in varieties that are extremely prone to it. A robust ATS program over the flowering period would probably also achieve the same objective provided there is a compact blossom period.

Once established, it is difficult to break out of biennial bearing. However, there are established crop husbandry strategies, which if diligently followed, can be used with success to farm our way out of biennial bearing.

One of the key crop husbandry practices to reduce biennial bearing lies in bringing down excessive crop loads rapidly, preferably within three to five weeks of full bloom. Where flowering in a block is erratic with only a small proportion of the trees, biennial clearing two out of every three flowering sites on the ‘on’ crop trees prior to fruitset is a good starting point. This will bring their return bloom into line with the remainder of the block.

In general our experience with biennial bearing is that where it is entrenched, thinning strategies alone are insufficient to overcome the problem, but they are a good starting point. The thinning strategies have to be used in conjunction with vigour management, often trunk girdling and growth regulator programs known to stimulate flower bud development.

About the author

John Wilton is a Horticultural Consultant at AgFirst – New Zealand and can be contacted at hawkesbay@agfirst.co.nz or +64 6 872 7080.

Tagged:
Biennial bearing crop load and thinning Future Orchards Vigour

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