Biology comes to the Postharvest Rescue of Avocados

The elicitors option

Elicitors are biological compounds that trigger an immune response in a plant by activating a signal cascade. They can be derived from pathogens and applied to the plant, or be released in the plant by the action of the pathogen.

Elicitors are not a new discovery and several products have already been registered for commercial use on a variety of fruit crops. ChitoPlant® (CHP), for instance, is a pre-harvest, water-soluble formulation with a track record of improving the postharvest quality of table grapes, sweet cherries and strawberries when applied as an external elicitor.

Drawing on this established practice, one approach for avocados could be to replace the postharvest Prochloraz® application with a pre-harvest application of CHP.

Prochloraz® is not the only treatment problem avocado producers have to solve. Current practice includes the application of 1-methylcyclopropene (1-MCP) to early-season fruit. However, particularly in the Hass cultivar, 1-MCP causes uneven ripening and poor fruit colouration in avocados destined for the ready-to-eat ripening programme.

Although it is known that the postharvest use of methyl jasmonates (MeJa) alleviates chilling injury at quarantine treatment temperatures, this elicitor has not yet been explored for South African avocados.

Therefore, a study conducted over two seasons – 2019 and 2021 – set out to determine the extent to which CHP and MeJa could improve the postharvest health of locally produced avocados.

Study design and methodology

Under the leadership of Prof Dharini Sivakumar of the Department of Crop Sciences at the Tshwane University of Technology, the project was designed to meet specific objectives:

• Investigate the influence of commercially registered CHP and MeJa on postharvest decay control in inoculated fruit from the Fuerte and Hass cultivars.
• Investigate the underpinning biochemical mechanisms to explain the mode of action of the two elicitors.
• Investigate the influence of commercially registered CHP and MeJa on postharvest decay control and overall quality in naturally infected Fuerte and Hass avocados.
• Conduct commercialisation trials that will allow the best-performing product to be registered as a biofungicide in SA.

The first field trial took place in May and June 2019 on the Lombard farm near Tzaneen in Limpopo. Fruit was sprayed with different dosages of CHP (0.5% and 1.5%) and MeJa (0.1%, 0.2% and 0.4%), two and four weeks before harvest respectively.

After harvest, the fruit was split in two lots. The first was stored in a cold room at 6.5°C for 28 days and observations were recorded on the incidence and severity of anthracnose and stem-end rot. Physiological disorders were also recorded.

The second set of fruit was further divided into two sub-lots and inoculated with anthracnose and stem-end rot and incubated at 25°C to simulate shelf conditions.

Study results

An application of 0.1% MeJa or 1.5 % CHP two weeks before harvest delivered significant results: the incidence of both anthracnose and stem-end rot was reduced to 20% in Fuerte and less than 20% in Hass.

The naturally infected fruit of both cultivars, sprayed with the same application within the same timeframe, had the lowest incidence of anthracnose (<5%) and stem-end rot after cold storage and on the fifth day of the simulated shelf conditions. MeJa promoted early ripening but was discarded as an option, as the fruit was very soft.

Therefore, the 2021 trials focused on a pre-harvest application of 1.5% CHP two or four weeks before harvest. Anthracnose was detected in 5% of Fuerte avocados and not at all in Hass. Conversely, stem-end rot was found in Fuerte at 15% and again, not in Hass.
ChitoPlant® forms a semipermeable film on the surface of avocados that may reduce the respiration rate, decrease water loss and inhibit fungal mycelial growth. The result is delayed ripening and fruit decay.

In terms of biochemical mechanisms, the study affirmed that a 1.5% pre-harvest application of CHP on infected avocados results in higher epicatechin biosynthesis, which plays an important role in the defence mechanism of the fruit against anthracnose.

The upregulation of phenyalanine lyase genes (PAL), chalcone synthase (CHS), and flavanol synthase (FLS) increased phenolics and epicatechin in the avocado pericarp, thereby resulting in an increase in antifungal activity and reactive oxygen species in the fruit tissue.

Fruit pericarp epicatechin content is controlled by lipoxygenase (LOX) activity, which is involved in the degradation of persin and the antifungal compound AFD. Because CHP downregulates the LOX gene, levels of epicatechin are maintained in the pericarp.

The study also found Avfad1 and Avfae12-3 gene expression was upregulated by a pre-harvest application of CHP, compared to the Prochloraz® treated and untreated fruit.

Conclusion

The 1.5% CHP and 0.1% MeJa solutions proved promising in controlling both anthracnose and stem-end rot. By alternating the two products, fruit colouration during ripening can also be improved.

Moreover, the study suggests that avocados sprayed earlier than two weeks before harvest with CHP are highly resistant to especially anthracnose infection after cold storage. This is probably due to the presence of epicatechin in the pericarp and the stimulation of the production of antifungal compounds.

The second trial conducted in 2022 confirmed these trends, indicating that a compelling case can be made for the elicitors CHP and MeJa to replace Prochloraz® – a finding that should please producers and consumers of South African avocados.

The project's economic impact

Using the CHP solution is a key mitigator against trade barriers.

Project information

• Project title: Impact of commercial elicitors on postharvest quality
• Principal investigator: Prof. Dharini Sivakumar
• Duration: 01/03/2019 – 31/03/2022
• PHI Programme and Industry Contribution by PHI: R365 372
• Lead institutions: Tshwane University of Technology
• Beneficiary: The South African Avocado industry
• Human resource development: 1 MSc student
• Focus area: Alternative control, anthracnose, stem-end rot, physiological disorders, induced defence mechanism, ripening, skin colour