What was it all about?

The development of rots and decay (such as green mould) after harvest can be a serious problem for growers and packers, particularly in fruit for export markets where produce is stored and transported over many weeks.

The Australian Citrus Postharvest Science Program ran from 2017 to 2019 to develop best practices to manage current fungicides and sanitisers to control decay, and to ensure Australian citrus remains clean and green with ultra-low residues.

There are several challenges for the Australian Citrus industry in consistently delivering high quality, safe and nutritious fruit to domestic and export markets. New fungicide chemistries and sanitisers are opening alternate avenues for controlling postharvest decay in citrus.

However, with increasing demand for reduced Maximum Residue Limits (MRLs) in fruit, this project worked towards postharvest solutions to meet the Australian citrus industry’s ‘ultra-low’ residue goals.

Postharvest decay control

The major postharvest problem for marketing citrus in both domestic and export markets is decay caused by blue and green mould (caused by the fungus Penicillium) and sour rot (caused by Galactomyces citri-aurantii).

This decay is currently controlled using postharvest fungicides, but the development of technical resistance can lead to reduced efficacy and fruit breakdown. This can have significant consequences, particularly in export markets where transport and storage times can be long. Correct management of postharvest fungicides is critical to maintaining their efficacy, as well as ensuring that chemical residues are not a barrier to market access.

The research team assessed several approaches to postharvest decay control including fungicides, sanitisers and physical treatments. Each of these different and complimentary approaches were assessed against the major decay causing fungi, green and blue mould (Penicillium digitatum and P. italicum), finding:

• The results reinforced the need for good postharvest practices (e.g. fungicide applications within 24 hours of harvest), and the potential of new and improved postharvest chemistries to control green and blue mould
• Additionally, the assessment of potential postharvest fungicides to manage anthracnose damage showed potential for new postharvest treatments to reduce anthracnose symptoms in Imperial mandarins
• The improved use of current sanitisers and sanitising systems, such as the new electrolysed water system, were assessed and show promise as commercial treatments for the industry. Postharvest sanitisers and washing treatments can improve fruit appearance and remove pre-harvest fungicides /contamination from fruit surface
• Physical treatments for the control of postharvest decay gave mixed results, but there are opportunities to incorporate these to supplement control
• In assessing a range of current and new sanitation systems, the results identified some new water and cool room systems which may have application to the Australian citrus industry.

Managing fruit quality through the supply chain

As well as decay, there are a range of other fruit quality issues which impact the successful domestic and export marketing of citrus fruit:

• Managing fruit quality through the supply chain to ensure consumers have a good eating experience is essential, with the program team reviewing irradiation as a market access treatment for fruit quality as well as the opportunities to manage chilling injury
• Assessment of several pre and postharvest treatments showed some promise to minimise potential chilling injury, with the management of off-flavours in mandarins in export markets also assessed.

Postharvest treatments to reduce MRLs

The team conducted a review of the potential postharvest quality effects of low dose irradiation as an export market access treatment. It showed low dose irradiation has a place in the market access toolbox, where high value export markets allow this treatment:

• To minimise the risk of MRLs, a postharvest trial illustrated that any postharvest washing at treatment temperatures of 20 or 40°C reduced the preharvest orchard chemical residues in lemon fruit. This is a good result for industry and shows the benefits of postharvest washing and handling to reduce the potential of orchard residues. However, it is critical to minimise chemical use to ensure MRLs are not an issue.
• The potential to reduce dithiocarbamate residues in lemon fruit with postharvest washing treatments was highlighted, with commonly used sanitation treatments (chlorine or ‘Tsunami’). This result will give industry confidence to meet export MRLs.

Extension

The continued success of the Australian citrus industry relies on the delivery and adoption of relevant and timely information. Therefore, the development and extension of up-to-date postharvest resources (e.g. via regular articles in industry publications) and program updates at grower forums are essential to ensure industry are using best practice to meet the expectations of retailers and consumers.

ACT NOW

Further information on postharvest and outputs from this project can be found in the postharvest section of the Citrus Australia website. The site includes numerous articles, videos, newsletters and other resources regarding postharvest management.