Jack Hughes, Fruition Horticulture (First seen in The Orchardist, August 2015)

This work has been led by two researchers who can easily be likened as the Lennon and McCartney of the apple physiology research world. Alan Lakso, who has recently retired, carried out the more theoretical side of the work while Terence Robinson operates at the industry or applied end. These guys are, of course, backed up by many other research associates and extension agents and together they have completed an enormous amount of original work.

The New York climate appears to be a lot more demanding of fruit growers’ skills than ours. Their northern latitudes result in rapid change of season which means little time after harvest for trees to recover and accumulate reserves. Equally, in Spring, crop phonological development is rapid, bloom duration is typically half ours. Weather conditions also vary a lot meaning that conditions for fruit set can swing from favourable to unfavourable from week to week and season to season.
As a result, the NY industry is very conscious of achieving fruit set as close as possible to final targets. This is not just because of immediate hand thinning cost implications but because they know that failing to get within a fairly narrow band of acceptable fruit set level will often have serious downstream implications, i.e. biennial bearing.

The methodical and scientific way that the NY industry deals with figuring out the ‘unknowns’ of fruitgrowing is impressive. It takes resources and dedication but their knowledge base derived from replicated trials repeated over numerous seasons is impressive.

Pruning Strategy
An example of this is a study, now in its 10th year, of the effect of pruning technique on fruit size and yield of Tall Spindle Gala and Honeycrisp. Three pruning treatments are being compared – Minimal Pruning (complete removal of 1-3 of the largest limbs in the tree), Spur Pruning (minimal pruning plus manual removal of 1/3rd of all fruiting spurs) and Stubbing Back Pruning (minimal pruning plus shortening back by 1/3rd of all horizontal and pendant branches).
They found that spur extinction pruning and stubbing back pruning had very similar effects on flower bud load, fruit set, fruit size, yield and crop value in both varieties. Both treatments improved fruit value compared to minimal pruning which supports the case for more aggressive pruning. However, the researchers have concluded that while the benefit of pruning comes from reducing bud numbers, the style of doing so is not important as both methods achieve a similar result. And a result, spur pruning (or bud extinction to use the French terminology) has not been advocated in NY. It is seen as a more time consuming and expensive way of achieving a similar result to conventional pruning.

Bud Numbers
Related work has investigated pruning intensity in terms of the number of fruit buds carried as a ratio of final crop load. A survey of NY apple growers was carried out in 2013 and found that there was a wide range of pruning severity. The ratio of floral bud numbers to final target fruit numbers varied from 1.1 to 5.8 in Gala and Honeycrisp. Average survey values were 2.4 and 3.0 respectively. I imagine we’d see a similar range in New Zealand
The researchers accept that there is a risk factor with reducing bud numbers close to final crop targets (poor bloom, pollination, set, frost). They currently recommend lower bud numbers than that surveyed – 1.5 flower buds for each final fruit for Gala and 1.8 for the weak growing and more biennial Honeycrisp. The reason for this is that chemical thinning studies have shown that the higher bud numbers are, the more difficult it is to get numbers down to target.

Chemical Thinning
Variability of chemical thinning response has been a problem for apple growers everywhere for years. There are many sources of variability – over 20 factors have been identified by various reviewers. The main two sources of variation identified by Stover and Greene were spray chemical uptake and the sensitivity of the tree.
The team at Geneva ran a series of experiments over a number of years to test the source of variability from chemical uptake. They found that the time of day that thinner sprays were applied had little effect on final uptake. It was concluded that temperature and humidity largely compensate for each other in affecting drying time and uptake. In other words, warm temperatures and low humidity promote faster drying and more rapid chemical uptake. Lower temperatures, higher humidity give slower drying and rate of uptake uptake but the final outcome is the same. Nice to have that old chestnut cleared up. Nevertheless, temperatures over the next few days after application does have a big effect on the activity of most thinners.
Figuring out the second major source of variation, the sensitivity of the tree, is a much bigger deal. Alan Lakso spent decades creating an apple simulation model called MaluSim. It was originally aimed at quantifying seasonal growth and development but found a commercial application in helping to predict chemical thinner response for NY growers.

Carbohydrate Model
The model uses both temperature and sunlight to predict carbohydrate supply and demand. The model estimates photosynthesis (carbohydrate supply) and respiration (demand) of the different parts of the apple tree (fruits, leaves, roots and structure). A schematic flow diagram shows how the fruit set part of the model is constructed.

The model has been validated with greenhouse studies of mature potted trees over three years. When fruit reach 10mm diameter, rapid fruitlet growth creates a carbohydrate demand that may not be met by photosynthetic supply. If the carbohydrate balance is negative, fruit will abscise. Validation studies also showed that thinning responses could be predicted by the model. For example, cool temperatures with high sunlight after thinner application resulted in less thinning because carbon supply was high and respiration demand was low. Conversely, high temperatures, especially at night, combined with cloudy conditions resulted in much greater thinning efficacy of similar chemical applications.
The Carbohydrate model is now available as a web based tool for NY growers to use to help with chemical thinning decision making. An example of the output of the model for the early part of bloom in Geneva in May 2015 is shown in Figure 2.

The Carbon Balance is expressed in terms of the tree’s daily carbohydrate production from green tip. In this plot, a deficit accumulates until the 12 of May, the date of full bloom. Cloudy weather from then on again contributes to an increasing deficit. As deficits increase, the model recommends decreasing chemical thinner rates. If deficits are large no thinning is recommended. In carbohydrate surplus situations, the model recommends increasing rates to achieve required thinning responses.

Fruitlet Model
Further work by Lakso & Robinson along with colleagues Duane Greene and Phillip Schwallier examined young apple fruitlet growth rates and their consequences for fruit survival or abscission. They found that fruitlets had to sustain a growth rate to survive and if the growth rate of any fruit fell below 50% of the fastest growing fruits, those fruit were destined to drop (Figure 3).

This relationship holds true for most varieties although harder to thin varieties like Gala might persist at lower growth rates (say 40%). This work has led to the development of a Fruit Set Model4 that is used to give an early prediction of chemical thinner response. Detailed measurements 4-8 days after thinner application can assist in an early prediction of the extent of thinning that has occurred and is just beginning to be expressed. This information supports early and informed decision making on whether to repeat thinner application while fruitlets are still within a susceptible size range.

Combining the Tools

Combined use of these tools – precision bud counts, the carbohydrate model to guide chemical thinner dose rates and the fruitlet model to determine whether subsequent zone targeted applications are justified are improving the ability of NY growers to achieve accurate crop load adjustment. Achieving target numbers early improves fruit yield and value and reduces the risk of tipping into biennial bearing. Pretty cool science, I reckon.

Acknowledgement:
The generous sharing of information and ideas by Alan Lakso, Terence Robinson and their colleagues is gratefully acknowledged.
References
1 Robinson et al. Pruning strategy affects fruit size, yield and biennial bearing of Gala and Honeycrisp apples. New York Fruit Quarterly, Vol 22 No.3, Fall 2014.
2 Greene et al. Development of a fruitlet thinner model to predict thinner response on apples. HortScience 48(5): 584-587. 2013
3 Robinson & Lakso. Advances in predicting chemical thinner response of apple using a carbon balance model. New York Fruit Quarterly, Vol 19 No.1, Spring 2011.
4 Stover & Greene. Environmental effects on the performance of foliar applied plant growth regulators: A review focusing on tree fruits. HortScience 15: 214-221. 2005.