ISOL Iodine as a post-harvest treatment for avocados

Materials and Methods

Abstract

ISOL Iodine was evaluated as a post-harvest treatment on Hass avocados at Avowest.  ISOL was used as a post-harvest dip immediately after picking wet avocados.  Avocados allowed to ripen naturally and assessed for body rots showed no significant effect of ISOL upon body rots.  The Iodine treatment did however significantly decrease the ripening time compared to the control.  When used as an ethylene scavenger in sealed trays, ISOL iodine had no significant effect on body rots or the ripening time.  The fruit quality of Hass avocados stored for one month at 5^oC was excellent. 

Introduction

It is generally recommended not to pick avocados in wet weather as this can increase lenticel / spotting damage ^{(Duvenhage 1993: Everitt et al, 2008)} and potentially increase body rots ^{(Pak et al, 2003)}.  The Australian Avocado Best Practice resource indicates that harvest should stop if it rains.  At a practical level this is very difficult to do, given Hass is harvested at Avowest in late winter / early spring.

To control body rots at Avowest a combination of pre and post-harvest fungicide applications are used, along with cool storage.  At Avowest fruit is harvested into bins which are cool stored at 5^oC until ready for packing.  This usually occurs less than 24 hours after harvest but can take as long as 2-3 days.  Because there may be some delay between harvest and the post-harvest application of Graduate A Plus (Azoxustrobin + Fludioxonil) fungicide an additional post-harvest treatment was considered.  ISOL is a “molecular iodine” formulation.  This trial was to evaluate the effect of dipping “wet picked” avocados in ISOL on post-harvest rots.

Materials and Methods

Trial 1 (ISOL Dip)

Hass avocados were picked on a wet day (6mm rain) on 1/9/2021.  From one bin, two 22L tubs of avocados were randomly selected from the top of the bin at 3pm.  One became the control.  The other was dipped in a 300ppm solution of ISOL for 60 seconds and then allowed to drain.  Both tubs were then stored overnight in a coolroom at 5^oC.  The next morning at 7am both treatments were run through the Graduate A Plus application in Avowest’s packhouse.  Graduate A Plus was applied at a rate of 125mL / 100L and an exposure time of 30 seconds.  The Control fruit were run through first to avoid any contamination with Iodine residues from the Treatment fruit.  The time between treatment with ISOL and treatment with Graduate A Plus was 16 hours.

Following treatment both boxes of fruit were stored in room conditions with temperatures ranging from 5 – 25^oC.  Avocados were assessed daily.  They were cut open and assessed 2-3 days past optimal eating condition when the skin was dark and dull.  The colour of the skin was purple / black to even brown.  Previous assessment of library trays at Avowest usually indicates a low level of body rots when assessed at optimal ripeness.  Allowing the avocados to progress well beyond optimal ripeness was to maximise the opportunity for post-harvest rots to occur.

When cutting open avocados, the fruit was inspected for external evidence of body rots.  Where this was seen the fruit was cut open in that area to expose the maximum amount of body rot.  Fruit rots were assessed on a 1-5 scale (see Appendix 1). Please note, we could not bring ourselves to show a Scale-5 fruit, it upsets people.

Fruit rots – scale 1-5

1. No rots evident
2. 10% body rots or vascular browning
3. 20-30% body rots or vascular browning
4. 40-60% body rots or vascular browning
5. More than 60% body rots or vascular browning

Trial 2 (ISOL in tray)

A second trial to assess the affect of ISOL Iodine on post-harvest life of avocados was carried out.  The Iodine was alleged to have ethylene scavenging activity.  This was tested by soaking paper towelling in ISOL Iodine (200mL in a single layer tray) which was placed underneath the liner in the tray.  Two trays of Class 1 avocados (1-2 cm² of blemish) that had been through Avowest’s packing line were trialled.  Each tray contained 28 avocados.

 These were picked and packed on the 3^rd September 2021.  One tray was control and the other tray included paper towelling soaked in ISOL Iodine.  Each tray was wrapped in a garbage bag and sealed up.  The trays were then placed in one of Avowest’s coolrooms.  This was set to 5^oC but had warm fruit going into it from our orchard regularly.  There was no temperature logging in the coolroom but it was highly unlikely to have remained at a steady 5^oC. 

The trays stayed sealed in the coolroom until 29^th September (26 days of storage).  Upon inspection they were still very green (Image 1).   The trays were resealed and put back in the coolroom.  On the 4^th October (31 days of storage) both trays were removed and allowed to ripen at room temperature (Image 2).  As with Trial 1, “room temperature” was highly variable.  Unlike Trial 1, the avocados were cut open when they were deemed to be optimally ripe for eating.  The skin at the neck yielded to a gentle pressure and the colour of the skin was usually olive brown.  The same scale (1-5) was used to rate the avocados for rots.  Avocados were cut open and assessed from 11^th October (38 days from harvest) to 14^th October (41 days).

References

1. Duvenhage, J.A. 1993.  THE INFLUENCE OF WET PICKING ON POST HARVEST DISEASES AND DISORDERS OF AVOCADO FRUIT.  South African Avocado Growers’ Association Yearbook 1993. 16:77-79
   
2. Kerry R.Everett^a, Ian C.Hallett^aJonathanRees-George^aRobert W.Chynoweth^aHenry A.Pak^b  Avocado lenticel damage: The cause and the effect on fruit quality.  Postharvest Biology and Technology, Volume 48, Issue 3, June 2008, Pages 383-390
   
3. H.A. Pak1, J. Dixon, D.B. Smith, T.A. Elmsly and J.G.M. Cutting 2003.  Impact of rainfall prior to harvest on ripe fruit quality of Hass avocados from New Zealand.  Proceedings V World Avocado Congress (Actas V Congreso Mundial del Aguacate) 2003. pp. 629-634.

I ISOL Iodine treatment – Index