Avocado fruit drop investigations

Applied fruit drop investigations in avocado

BY DECLAN MCCAULEY RESEARCH OFFICER, DPIRD

It has been suggested avocado yields could reach 32.5 tonnes per hectare but are currently not achieving this limit (Whiley, 2013).

Factors limiting yield include pollination issues, insufficient disease control, rootstock choices, and management relating to pollination and fruit retention. Fruit retention in particular is a serious problem. Avocado trees regularly set many fertilised fruits which then begin to abscise for unknown, biologically complex reasons (Gün et al., 2008) (Sawicki et al., 2015).

In order to investigate fruit retention a method was chosen whereby different chemicals were trialled to attempt to reduce fruit drop.

Funding for these trials came from Hort Innovation as part of AV17006 Avocado capacity building WA. This work was done in conjunction with CSIRO who are investigating the underlying mechanisms of fruit drop. Three chemicals were used Glycine betaine (GB), Aminoethoxyvinylglycine (AVG) and Maxcel ® . GB is a compound that occurs naturally in plants and is naturally accrued to provide stress protecting effects. Trials with GB in South Africa have been promising so it was decided to trial GB in our experiment to test its effectiveness on fruit retention (Blakey, 2015). The product used in the South African trials that contains GB is called Greenstim ® . Both Greenstim ® and purer lab grade GB were used in our experiments. AVG is a well-known ethylene biosynthesis inhibitor and since ethylene has been suggested as a regulator of fruit drop (Sawicki et al., 2015) it could have an application for avocados. Maxcel ® is a synthetic cytokinin normally used in the apple industry to induce fruit drop. Cytokinins are plant hormones with roles in regulating cell growth and the development of various organs in the plant, such as the roots. They also control leaf and flower drop (Raines et al., 2016).

In order to evaluate these chemicals two approaches were taken. One approach was to apply GB by spray to trees under induced drought stress. The drought stress was induced to try and force some fruits to drop. As defoliation is known to cause massive fruit drop, the other approach was to defoliate some trees and dip individual fruits with either AVG, GB or Maxcel ® .

Fruit retention is a seriousproblem.

3 DEMONSTRATION of how the fruits were dipped.

Method

For the defoliation approach with AVG, Maxcel ® , and GB six ‘Hass’ and ‘Zutano’ trees growing in a Mediterranean climate in South West Western Australia were selected for defoliation. The trees were completely defoliated of leaves and shoots on the 21st of January. On the 22nd of January 40 ‘golfball’ sized fruits on each tree were tagged as: • 10 fruits tagged as the untreated control 10 fruits tagged as the GB treatment (13.5g L -1 ) 10 fruits tagged as the Maxcel ® treatment (1980ppm) 10 fruits tagged as the AVG treatment (200ppm). On the 22nd of January the fruit were dipped up to the centre of the fruit pedicel. The fruits were dipped a second time on the 29th of January. The fruits were checked to see if they had abscised a total of four times on the 29th of January, 31st of January, 3rd of February and the 5th of February. Fruits were checked by gently ’wobbling’ each fruit in the hand for a few seconds to confirm if the fruit was going to abscise or not. Two approaches were taken for the drought stress trial which was done at the same location as the defoliation trial. The first drought trial was started on 15th of January with disconnection of the sprinklers and the selection of trees and fruits as follows: • Control trees (normally irrigated): four trees with 20 fruits tagged • Drought trees: five trees with 20 fruit tagged

Drought plus GB trees: five trees with 20 fruits tagged, GB was applied on the 16th of January. The GB was applied with a packpack sprayer at a rate of 5g L -1 on the 16th of January. A wetting agent was mixed in at 0.6g L -1 with the GB to maximise penetration into the plant. The spray was applied to the point of runoff. The sprinklers were reconnected on the two weeks after the sprinklers had been disconnected as the trees were severely stressed. Fruit diameters were measured four times; on the 15th of January, the 21st of January, the 29th of January, and the 5th of February. The diameter of the tagged fruit were measured with digital calliper. The abscission of a fruit was noted at each of the four measurement times. The second drought trial was started almost as soon as the first drought trial finished. Considerably more trees were chosen with six trees per treatment (28 trees total). The treatments started on the 5th of February were as follows: • Drought control: six trees with 20 tagged fruit • Drought plus GB: six trees with 20 tagged fruit, spray applied on the 5th and again seven days later • Drought plus Greenstim ® : six trees with 20 tagged fruit, spray applied on the 5th and again seven days later. Greenstim ® was included here to compare it to the purer lab grade GB. Both GB and the Greenstim ® were applied at 5g L -1 with the same amount of adjuvant as the first trial. The sprinklers were reconnected after one week had passed.

As per the first trial 20 fruit were tagged on each tree. Instead of measuring the fruit diameter they were instead only counted for presence absence. Fruit counts were done once a week on the 5th, 12th, 19th, 28th of February and the 4th of March. All data was analysed with a chi-squared statistical analysis to find significance.

Results and discussion

The drought stress trial showed treatments of ‘drought’ and ‘drought plus GB’ cause a significant reduction of fruit on avocado trees in comparison to

100.0 **

FRUIT PROPORTION REMAINING (%) 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 * Control Drought Drought plus GB X * X X 0.0 * 10 Jan 15 Jan 20 Jan 25 Jan 30 Jan 4 Feb 9 Feb 14 Feb 19 Feb 24 Feb TIME FIGURE 1 THE PROPORTION OF FRUIT RETAINED OVER TIME OF THE FIRST TRIAL OF TREES SUBJECTED TO DROUGHT AND TREATED WITH GB WITH STANDARD ERRORS SHOWN The X, *, and ** indicate treatments that are significantly different to other treatments, but only at the time point it is marked at.

100 90 80 70 60 50 40 30 20 10 0 30 Jan

Greenstim ® GB Drought (Control)

9 Feb 19 Feb TIME

X

X

29 Feb 10 Mar

FIGURE 2 THE PROPORTION OF FRUIT RETAINED OVER TIME OF THE SECOND TRIAL OF TREES SUBJECTED TO DROUGHT AND SPRAYED WITH GB AND GREENSTIM ® WITH ERROR BARS The X and * indicate treatments that are significantly different to other treatments. FRUIT PROPORTION REMAINING (%)

100 90 80 70 60 50 40 30 20 10 0 20 Jan

MaxCel ® AVG Control GB

22 Jan 24 Jan 26 Jan 28 Jan 30 Jan TIME

1 Feb

X

3 Feb 5 Feb 7 Feb

FIGURE 3 THE PROPORTION OF FRUIT RETAINED OVER TIME IN THE DEFOLIATION TRIAL WITH ERROR BARS The X and * indicate treatments that are significantly different to other treatments.

the control (Figure 1). At the end of the experiment the control retained 75% of fruit, while the drought retained only 3% and the drought and GB retained 7.1%. The control treatment exhibited a slight reduction in fruit number with 75% of fruit remaining by the end of the experiment, which is likely due to late summer fruit drop. The greatest drop happened after the 29th which was after the sprinklers had been reconnected and was probably due to the shock of being irrigated again. Or possibly the accumulated stress had finally become high enough to cause fruit drop even after the drought. While both drought and drought plus GB decreased after the 29th there were significantly fewer drought fruits than the drought plus GB fruits. The fruit diameter measurements (not shown) of the control consistently increased over the time period while the drought measurements reduced from an average diameter of 40mm on the 21st of January down to 34.2mm by the 5th of February. The drought plus GB treatment slightly increased from 40.8mm to 42.6mm by the 5th of February. By the 19th of February the surviving fruits of the drought and drought plus GB treatments had resumed growth with the control being 51.4mm, drought 46.8mm, and drought plus GB being 48.5mm. Higher fruit growth in the drought plus GB treatment compared to the drought treatment, and the slightly higher fruit retention of drought plus GB compared to drought may be attributed to the protectant effects of the GB, even if fruit retention was much lower than the irrigated control. The second GB trial showed that most fruits dropped after the 19th. Fruit drop was highest in the drought control and Greenstim ® treatment (Figure 2). The GB treatment retained the most fruit at the end of the experiment (65.8%) while the drought retained 45.8% and the Greenstim ® 44.2%. The overall level of fruit drop was less than in the first trial and is probably because the drought stress was applied for a shorter period of time. The GB did make a difference in this trial, perhaps applying it twice improved the response. Furthermore, because the stress was applied for a shorter period the plant may have had a better response to the GB. The Greenstim ® did not reduce fruit drop compared to the drought control. While the Greenstim ® does contain GB it may be that the lower purity, and hence lower effective concentration of GB, of this product as compared to the lab grade GB used meant the effective concentration was lower. Therefore GB was shown to be effective in reducing fruit drop in this situation. The AVG and control treatments used on the fruits in the defoliated trees failed to show a significant difference over time (Figure 3). Maxcel ® showed a significant difference on the 3rd of February and GB showed a significant difference to the other treatments on the 31st of January and the 3rd of February. GB causing more fruit drop is unexpected and requires further investigation. Despite these slight differences overall fruit drop in all treatments were very similar and were all very close to losing all fruit at the end of the experiment. We found the defoliation treatment to be harsh and was probably too powerful to have an effect. Defoliation itself is successfully used in plant studies to understand physiological mechanisms but perhaps for applied studies it is less useful or needs to be changed to partial defoliation (Pastore et al., 2013; Srisook et al., 2015). An applied approach to investigate fruit drop with drought and defoliation showed that GB was able to reduce fruit drop in a manipulated drought stress situation. Defoliation was probably too powerful for the applied system used. AVG did not show difference compared to the control, however this may be due to the effect of suppression by the defoliation system. Further trials with GB will be done.

ACKNOWLEDGEMENTS

I would like to acknowledge the considerable assistance of Dr Amnon Haberman from CSIRO.

MORE INFORMATION

Please contact Declan McCauley, email declan.mccauley@dpird.wa.gov.au, or go to

https://dpird.wa.gov.au

Department of

Primary Industries and Regional Development

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