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Working with trees delivers ASE results for Allan Brothers

Industry Best Practice

Understanding tree physiology and the complexity of natural plant hormones is key to managing crop load.

Key points

  • Crop load plays key role in quality. 
  • PGRs are imprecise and reliance on them is impractical. 
  • Learn to read signals trees provide. 
  • Precision pruning eliminates need for chemical thinning and simplifies hand thinning. 

Switching from a reliance on plant growth regulators (PGRs) for managing crop load to working with the trees’ own physiology has delivered a more consistent and higher quality crop for Washington State-based apple growers Allan Brothers.  

The business, which grows 500ha of apples among other crops in the Yakima Valley, switched to the use of artificial spur extinction (ASE) in 2018, focusing on pruning surplus floral spurs prior to bud break to deliver earlier – and importantly more precise – crop load management. 

Allan Bros. R&D manager Suzanne Bishop shared her insights into PGRs as a guest speaker on the northern loop of the recent Future Orchards® spring walks. 

Suzanne Bishop, R&D Manager, Allan Bros., Washington, talks to growers at Fonlea Orchards, Stanthorpe, as part of the Future Orchards® spring walk series.

She told attendees that, frustrated by the unpredictability of PGRs and the challenges of the many variables impacting their performance, Allan Bros. had gone back to the drawing board to find a more exact?/reliable? way to manage crop load. A key part of that process had been going back to basics and building a better understanding of the physiological processes of the tree and the complexity of the natural plant hormones. 

“We had an incomplete understanding of plant hormones in general, so we were using them to kind of force the physiology of the trees,” Suzanne said. “The vision for our company now it to reduce reliance on plant growth regulators. 

“It’s learning to read the signals as a tree provides and then what we can do is use plant growth regulators to support the physiology of the tree and not force it.” 

No silver bullet

Plant growth regulators are very effective tools in the orchard toolkit used for a wide range of applications, depending on timing, including flower thinning, stimulating return bloom, vigour control and functioning as a stop drop agent. 

Suzanne said it was this wide range of action that limited the ability to get a predictable outcome from PGRs, as well as their complexity and the fact they were: 

  • non-selective 
  • weather-dependent 
  • dependent on tree physiology 
  • not always able to give a single response  
  • able to mask a bigger problem. 

“How many times do we actually have an instance where we’ve got the perfect physiology, and the weather cooperates perfectly where we can get the ideal application rate? It’s pretty rare, at least for us,” she said. 

“Then, sometimes we have the expectation of a single response, the silver bullet to our problem, but that isn’t always what happens. Or sometimes it hides a bigger problem.” 

As flower induction for the subsequent season’s crop starts in mid-to-late spring, Suzanne warned that applying PGRs post-harvest for [ dormancy breakers? What would you be applying PGRs post harvest for] could also impact the subsequent crop. 

“We’re trying now to reduce the impact of chemical thinners during this critical time of cell division,” she said. “So we’re shutting down photosynthesis with our post-harvest thinners. What happens is we’re really depriving some of these buds in developing for next year as well. 

“If we can get our crop load right, we don’t really need to rely on plant growth regulators quite as much.” 

Crop load  

As part of the T&G program – growing Scilate (Envy™) and Scifresh (Jazz™) – Allan Bros. was able to work with the New Zealand T&G team and physiologist Dr Stuart Tustin, a long-time proponent of using crop load (rather than chemicals?) to regulate growth. 

In a 2018 trial of ASE on third leaf Scilate, Suzanne said the difference of just one-to-two fruit per branch made a significant difference in size and colour of the final crop, with positive implications for packout and returns.

Effect of artificial spur extinction (ASE) on crop load of third leaf Scilate (Envy™).

“You’ve got one-to-two more fruit on the high crop load than on the medium crop load and that’s enough to change that background colour and size profile of the fruit,” Suzanne said. 

“So, it’s amazing just the size difference and the colour difference that we can see, just by changing crop loads. I think what’s even more interesting is the fact that the difference between medium and high crop loads is just two-to-three fruit per branch difference.” 

Suzanne said understanding the competition for resources going in the tree shed light on why the relatively small change in crop load had such an impact on fruit size and colour. 

“It all goes back to light and carbohydrates,” she said. “Leaves intercept light for energy, energy is then converted into carbohydrates, which can end up in two places – one is the apple and one is the woody tissue or leaves. If we’re talking about vigour, we’re talking about the woody tissue or leaves.  

“Then there’s competition for those carbohydrates. So, if we have high crop load or too many apples, we’re going to get competition between fruit, so fruit-to-fruit competition. In that case, we’re going to have small fruit, lack of colour, lack of taste, lack of nutrients. On the other extreme, if there’s not enough fruit, you’re going to have competition from shoots. You’re going to have those young shoots, young growth, so you’re going to have a lack of calcium, maybe some high potassium, but you’re also going to see poor cell structure as that apple starts to get really big.” 

As developing seeds produce gibberellins (GA) that among other things decrease return bloom, Suzanne said crop load could also impact return bloom and the subsequent season’s crop load.  

“If we go back to the example of too many apples, what’s happening is the seeds in each of the fruit gives a GA signal response, which signals to the branch ‘Hey, let’s not put out as much fruit next year’,” she said. “So that’s where we get into that biennial bearing pattern.” 

As flower initiation occurs at different points for different varieties, Suzanne said thinning timing was important. 

“Gala is easier to thin and does not have an issue with biennial bearing because you’ve got a longer timeframe to get your crop load right,” Suzanne said. “Whereas with Honeycrisp, that window is much shorter. So, if we don’t get in there and thin, there’s a lot more GA response getting sent back to the tree and saying: ‘Hey, we’re going to go biennial this year’. 

“So crop load is important because it regulates size, we can work with the tree to manage growth, to get consistency in return bloom, improve light infiltration, enhance colour and, if you have a nice crop load, you have a better chance of harvesting at optimal maturity. 

“A perfect horticultural program is not complete without a perfect crop load.” 

Precision pruning 

Artificial spur extinction (ASE) is a crop management technique that reduces floral bud density by removing floral spurs to a target number, using branch size to determine the number of fruit each branch can comfortably carry. Applied after winter pruning, but before bud break, ASE aims to mimic the trees’ natural bud abortion process, conserving tree resources for remaining buds, supporting regular bearing and strong fruit buds. 

“All of us kind of do ASE in some way, shape or form just by pruning,” Suzanne said. “So now, if we’re actually taking measurements, we’re being more precise about how we’re pruning and understanding the physiology behind it. 

“By using the size of a branch to determine the number of fruit it can carry, we can create more uniformity in the fruit across the tree.” 

Target fruit number is determined by 

  • measuring the branch cross-sectional area (BCA) of each branch 2–3cm from the base using a branch size gauge such as a Mafcot wheel (see image).  
  • adding the branch BCAs together for a total tree BCA 
  • multiplying the tree BCA by a pre-determined crop coefficient (roughly 5–6 fruit/cm2 BCA) for the target number of apples. 

Using the branch size, measured with a MAFCOT wheel (pictured) or other branch gauge, to set the target number of fruit it can carry creates more uniformity in fruit across the tree.

Suzanne said the crop coefficient would depend on variety, past performance and planning. 

To set the target buds to produce a Pink Lady apple crop on a tree with a total BCA of 26cm2, Suzanne used a coefficient of 6 to calculate a fruit target of 156. She stressed this was the fruit target, not the bud target.  

As each bud could potentially produce five flowers/fruit, if bud numbers were below the target fruit number the discrepancy could easily be addressed at thinning. 

“We can hang a couple of doubles and everything will be okay,” she said. 

Problem trees to ‘fit’ trees 

In the orchard, Suzanne said the precision pruning ASE approach had allowed Allan Bros. to deal effectively with ‘problem tree’ issues such as low numbers of large fruit in the lower canopy and high numbers of small fruit at the top and knock-on issues of variable maturity and quality. 

By reducing the quantity of large and small fruit, a higher percentage of the crop hit the ‘sweet spot’ demanded by retailers, and packouts and returns were improved.  

Suzanne said removing surplus spurs had the additional benefit of removing excessive leaves, which increased woody growth, decreased nutrients to the fruit and shaded the apple’s primary energy source, the leaves immediately behind the apple itself. 

“So, it’s a paradigm shift. Instead of having a bunch of excessive leaves that are really just giving a lot of carbohydrates to the tree to grow more vegetation, we’re taking that out and trying to create a fit tree,” she said. “It’s making sure that the right amount of energy goes to the fruit in order for that fruit tree to excel.” 

Letting plenty of light penetrate to the lower canopy is a priority for Suzanne and she advocates removing excess vegetation that is shading fruit and also ensuring there are plenty of ‘light windows’ in the canopy to let light through, particularly as rows come closer together, increasing shading. 

“Instead of being possessive of fruit and potential yield, let’s actually be possessive of our light windows that feed the buds down below,” she said during the Goulburn Valley orchard walk at Duffy Orchards. 

“So that way, everything has the right amount of carbohydrates, you get good fruit size and colour, but you are also feeding the buds that are coming up for next year.” 

Light windows provide the light essential for fruit size, colour and return bloom, says Washington physiologist Suzanne Bishop.

What’s next? 

Suzanne said the ongoing focus was finding the right blend of orchard management, tree structure and record-keeping to drive optimal decision-making. 

“That is not only being more precise, with our pruning, but maybe using some lime sulphur to help knock down the side limbs a little and still using the mafcot to help us get our green fruit thinning numbers done more accurately earlier in the season,” she said. 

“Then finding a better tree structure and making sure our timing of our green fruit thinning is optimal, not letting it go too late in the season.  

“And also improved record-keeping. We’ve learned it can be really hard to record where your targeted crop load should be and then what it actually was and then what you’re dealing with. So we’re working on that process just trying to get better record-keeping all the way through. 

“So, we’re just trying to be more precise to make the rest of the downstream life much easier and sustainable as well.” 

Further reading 

“Lessons with plant growth regulators”, Suzanne Bishop’s presentation slides from the Future Orchards spring walk series 2023, https:/apal.org.au/wp-content/uploads/2023/11/Suzanne-Bishop-Nov23.pdf  

“Spur extinction – a natural process leading to a new crop management technology”, AFG, June 2014, https://apal.org.au/wp-content/uploads/2019/08/2014_Tustin-et-al_Spur-extinction_AFG-Jun14-Vol-8-5.pdf  

 

This article was first published in the Summer 2023/4 edition of AFG.

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