John Lopresti, researcher at Agriculture Victoria, discusses table grapes rot risk assessment at harvest, part of the Serviced Supply Chain Project

Video transcript: Table grapes rot risk assessment at harvest

The idea behind the rot risk assessment is that you can determine the post-harvest rot risk of rot development when you're actually harvesting the fruit, which allows you to potentially assign the fruit at different markets, depending on the likelihood of rot development after harvest. For example, if you've got, you do it rot risk incubation at harvest, and you find that there's a lot of rot development in those incubated samples, then you might assign a high risk to that batch or consignment of fruit, and you may market it domestically rather than market it or send it for export because that risk, that fruit has a relatively high risk of developing rots. Whereas if you do the incubation, conduct the incubation test, and find that there's very little rot development, then you can feel assured that you can export that fruit. And where exports taking three to four weeks of sea freight, then another period of up to three or four weeks in the importing countries, so you're looking at eight weeks that the fruit has to remain in good quality. So, you want to have some assurance at harvest that will be the case in combination with using sulphur dioxide pads, as well as for further insurance.

In the Australian table grape industry, it's standard commercial practice to treat harvested fruit with sulphur dioxide pads, and that's been done for over 40 years. And in general, these pads control rot development, fungal rot development very well during subsequent cool storage or sea freight export. And in most seasons growers and exporters won't have a problem with rot development. For example, after sea freight and importer storage, and then retailing. That said, over the last five years we've been working on a harvest rot risk protocol, where we incubate a sample of grape bunches at harvests to determine the rot risk potential of a fruit consignment prior to it being exported or marketed. And we found that it can be quite a useful tool for growers and exporters to assign rot risk to a consignment. This assignment can be either high risk, intermediate risk or low risk, and this is based on incubating a sample of bunches from the vineyard at harvest. The incubation usually takes place at 18 degrees, so it can be done by a grower quite easily at room temperature, and usually the fruit's placed in plastic bags and incubated for about five days and the grower or exporter could monitor the incubation, and after four to six days can assess the level of rot development in each of those samples. And that will provide them an idea of the risk of the fruit consignment that sample's attached to, that harvested fruit. It provides them an idea of the rot risk during subsequent storage, export, and marketing.

The potential for grapes to develop rots is also determined by the specific cultivar and other factors, such as vineyard management and control of diseases in the vineyard during fruit development. Rot risk assessment is just one component of controlling post-harvest rots as is a sulphur dioxide treatment, which generally works, but in, in difficult seasons with high rot risk in the vineyard, it may fail. And when we we're looking at rot incidents after coal storage or after export, we are talking about high rot levels for the fruit to become unmarketable. So generally, it's, if a bunch has to more than 2% rot incidence by weight, then generally it can be considered. So, it's very low level, low levels of rot incidents that can cause problems in direct marketing.

So, the idea is between SO2 to treatment, harvest rot risk assessment, and good vineyard management, we can prevent rot development that will cause problems in export and domestic markets most seasons.

Why rot risk assessments are important

  • Most decay caused by fungi in stored table grapes results from latent (underlying) infections not evident at harvest.
  • Botrytis cinerea, the cause of Botrytis bunch rot, is the main pathogen causing decay, especially after periods of high humidity or rainfall.
  • B. cinerea infects senescing flowers and young fruitlets, with infections remaining dormant (latent) in green fruit. Further infections can occur in pre-harvest period when fruit is ripening.
  • Other decay causing fungi (saprophytic and wound infecting pathogens) present on the orchard floor can also cause fruit infections during the pre-harvest period.
  • Fumigation of table grapes with sulphur dioxide surface-sterilises fruit and thereby reduces losses from decay during storage/sea freight, but it does not eliminate decay completely because the gas does not kill fungi that have already entered berries before harvest.
  • The incidence of latent botrytis infection can be determined following surface-sterilisation of the berries by fumigation with sulphur dioxide and incubation at room temperature under sterile conditions for 7-10 days.
  • Simulated storage and sea freight experiments showed a good correlation between incidence of botrytis infection measured at harvest and botrytis bunch rot that developed in grapes from the same fruit batches after cold storage and marketing.
Therefore, the pre-harvest incubation method (listed below) could be used to predict the decay likely to develop during cold storage and marketing, both in fumigated and organic fruit.

Method for determining rot risk potential of table grapes during cool storage

This rot risk protocol can be used both prior to harvest and at harvest, although preharvest rot risk assessment will allow for changes in marketing and export strategies if fruit is found to be of medium or high risk.

1. Bunch sampling: Collect 10 to 20 representative bunches from each block 7 to 10 days before harvest (or at harvest if preharvest sampling is not possible), place each bunch in a separate bunch bag, number each bag, and tag location of sampling in the vineyard. If rain occurs after first rot risk assessment, then a second assessment should be conducted. Bunches should be cleaned and trimmed as per commercial practice.

2. Bunch preparation: In a clean work area trim off two clusters of at least 50 berries from each bunch, placing one cluster (A) back in its bunch bag, and the other cluster (B) in a sealable plastic bag (i.e., large sandwich bag). Clusters in plastic bags (B) should be stored at 0-2°C until other clusters (A) have been surface-sterilized. After cluster removal the remaining bunch can be discarded.

3. Surface sterilization: To determinethe level of latent botrytis infection in bunches place each fruit cluster (A) in its bunch bag within a plastic liner inside an export carton with a sulphur pad as per commercial practice. After lidding, store carton at 0-2°C for 24 hours so that clusters are surface-sterilized.

4. Fruit cluster preparation: After sterilization remove each cluster (A) from its bunch bag and place in a sealable plastic bag so that for each bunch sampled in the vineyard there are two clusters in separate sealable bags, sterilized cluster (A) and unsterilized cluster (B) (ensure that each bag is marked with the cultivar, block, bunch number, and either A or B).

5. Fruit incubation: Place bagged clusters flat on a bench at room temperature (at approximately 20°C), spreading the berries from the outside of each bag so that they remain loose without damaging them, and incubate clusters for up to 7 days, ensuring that bags remain sealed to maintain a high humidity environment.

Bagged table grape clusters for rot risk assessments

Bagged clusters of grapes for assessments.

6. Berry assessment: The fruit can be examined at regular intervals (i.e., 3, 5 and 7 days) for rot development. If surface sterilization was properly conducted, then the number of berries per cluster with botrytis rot should be a measure of latent infection. Unsterilized clusters can also be examined for rot development that is likely to indicate the level of fungi on the surface of bunches at harvest.

7. Interpretation of rot risk results: The incidence (number) of infected berries per cluster will likely represent the percentage of rots that will develop during cold storage or sea freight. Research has shown that 1-2 berries per cluster infected by botrytis or other rots after incubation is enough to cause considerable losses of fruit due to rots after 3 to 4 weeks of cold storage in fruit without sulphur dioxide treatment. Cool storage below 2°C will delay both surface rots and latent botrytis infection but as fruit quality begins to deteriorate (e.g., berry skin cracks, berry softening, loosening of berries from stem etc.) rot development in medium to high risk fruit will likely develop, particularly during fruit marketing.

8. Fruit batch rot risk: Although many vineyard and postharvest factors will determine the level of rot development during cold storage or sea freight in general a fruit batch can be categorised as follows based on a pre-harvest botrytis rot risk assessment of sterilized clusters:

Low risk fruit batch

Up to 3 infected berries per 10 clusters

Medium risk fruit batch

4 to 10 infected berries per 10 clusters

High risk fruit batch

More than 10 infected berries per 10 clusters

Table grapes on assessment trays comparing SO2 and nil chemical applications Preharvest rot risk assessment - sterlized fruit using a sulphur pad (A) versus unsterlized fruit (B), after incubation at 18ºC for 7 days.

Pathogens causing quality loss during cool storage and marketing

Images of typical fungi that cause rot development during cold storage and marketing, as an image of medium-risk fruit after incubation.

  • Botrytis (grey mould)
  • Penicillium expansum (blue mould)
  • Alternaria alternata (A. rot)
  • Aspergillus sp (A. rot)
  • Rhizopus sp (R. rot)
Botrytis (grey mould) Botrytis (grey mould)
 Peniciliium expansum (blue mould) Penicillium expansum (blue mould)
Alternaria alternata (A. rot) Alternaria alternata (A. rot)
Aspergillus sp (A. Rot) Aspergillus sp (A. Rot)
Rhizopus sp (R.rot) Rhizopus sp (R. rot)

Other pathogens isolated:

  • Cladosporium cladosporioides
  • Ulocladium atrum
  • Epicocum nigrum
  • Fusarium sp
  • Mucor sp
  • Truncatella angustat
In many cases Penicillium and other pathogens were as important as Botrytis in reducing quality during long-term storage.

John Lopresti, researcher at Agriculture Victoria, discusses validating the rot risk protocol, part of the Serviced Supply Chain Project

Video transcript: Validating the rot risk protocol

Once we developed the rot risk protocol to be used at harvest, we attempted to validate the protocol to determine if assigning a risk rating to fruit at harvest, gives a good indication of what's actually likely to occur during cool storage and / or exports in terms of rot development.

And what we found was that the rot risk protocol was quite robust and was quite accurate in determining how likely it was for fruit consignment to develop, develop rots post-harvest, whether that's after cool storage or after export.

Project acknowledgement

The Serviced Supply Chains project is funded by the Hort Frontiers Asian Markets Fund, part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation with co-investment from: Department of Agriculture and Fisheries, Queensland; Department of Jobs, Precincts and Regions, Victoria; Manbulloo (mangoes); Montague (Summerfruit); Glen Grove (citrus); and the Australian Government plus in-kind support from The University of Queensland and the Chinese Academy of Sciences.