Postharvest sanitation and fungicide resistance survey

Key points:

Postharvest fungicide resistance leads to a loss of fungicide efficacy
Managing postharvest fungicide resistance relies on good packing house hygiene, sanitation and fungicide rotation
Do not always use the same postharvest fungicide
A Postharvest Sanitation and Fungicide Resistance Service is available to the industry through the Citrus Australia website.

Postharvest fungicides control decay such as green and blue mould during storage. These fungicides are essential for marketing and storage, but their effectiveness can be reduced when resistance develops. Fungicide resistance occurs when fungicide-resistant decay spores multiply. Constantly using the same fungicide allows resistant spores to build-up in a packhouse, particularly if packhouse hygiene is poor. Over time, the decay fungus can grow on fruit following fungicide treatment, resulting in the decay growing and breakdown in the marketplace. Fungicide resistance is a serious and important postharvest problem that needs to be actively managed in the packinghouse to minimise any potential losses. It is therefore critical to measure and monitor both packinghouse sanitation and hygiene, as well as the levels of technical resistance to postharvest fungicides.

A Postharvest Sanitation and Fungicide Resistance Service is now available to provide timely information to packers on the levels of sanitation and technical resistance to postharvest fungicides in their packinghouses. This service looks for decay-causing fungi in the packinghouse and identifies if these fungi have any technical resistance to common postharvest fungicides. Agar Petri dishes containing different postharvest fungicides are exposed to the air in different areas of the packinghouse and coolroom to estimate the levels of fungicide resistance present. This service is run through Citrus Australia with technical support and reports by NSW Department of Primary Industries.

A sample of this season’s anonymous packinghouse results from this Postharvest Sanitation and Fungicide Resistance Service are summarised in Table 1. The results show there were large differences observed in the levels of sanitation and technical resistance to postharvest fungicides between the different packinghouses around Australia with different management methods.

Table 1. Summary table of some results from the 2020 Sanitation and Fungicide Resistance Packinghouse Service. Plates were assessed at the start of the line, the end of the line and in the cool room for each packinghouse.Scores were given coded colours according to the relative hygiene or levels of technical resistance in each area of the packinghouse: Bright green = very low spore levels (very good), pale green = low spore levels, yellow = moderate spores levels, orange = high spore levels, red = very high spore levels (very poor).

Overall there were high levels of general moulds in the packinghouse and coolroom (first column in Table 1). These are general moulds and bacteria that live in the environment and are generally not related to postharvest decay. Packinghouses with lower overall general moulds also often have fewer decay-causing fungi spores. Good general hygiene in the packinghouse reduces all general fungi and bacteria, including decay-causing fungi.

Looking at the number of decay-causing spores (second column in Table 1), there was considerable variability in the results between the different packinghouses. Some had excellent hygiene and sanitation with very few decay-causing fungi detected (for example Packinghouses B and H), while others had very high levels of decay-causing fungi both in the packinghouse and in the cool room (Packinghouses G and I). All packinghouses assessed were fully operational and commercial, which shows that it is possible to reduce the levels of decay-causing spores in the packinghouse with good management techniques.

In general, the highest levels of decay-causing fungi were detected at the start of the line. This is not unexpected as this is where the fruit is transferred from the orchard. However, it is important to improve hygiene and reduce the numbers of decay-causing fungi in all areas of the packinghouse and particular attention should be made in this area, as these spores can remain in the packinghouse and coolroom and be a risk for decay and resistance development.

The levels of technical resistance to the postharvest fungicide, thiabendazole (TBZ) in the sample packinghouses is presented in the third column in Table 1. Over half had some technical resistance to TBZ detected, with two packinghouses in particular (Packinghouse G and I) having very high levels of spores with technical resistance to TBZ. These resistance levels would have begun with low levels of resistant spores that were encouraged with poor sanitation and continued TBZ fungicide use. These high levels of technical resistance to TBZ are a concern and improvements in packinghouse hygiene and fungicide rotation are recommended. This is why it is important to know what levels of potential technical resistance in your packinghouse, so that it is possible to fine-tune and improve management practices during the packing season.

Other postharvest fungicides such as imazalil, pyrimethanil and fludioxonil with other modes of action against green and blue mould are widely used and are essential to help manage postharvest decay. However, in this sample of packinghouses from this season, there was one packinghouse with high levels of technical resistance to imazalil (fourth column in Table 1). Imazalil is a mainstay of citrus postharvest fungicides and its efficacy needs to be actively managed to maintain control of postharvest decay. Although this observation was uncommon among the different packinghouses, this is a big concern for this packer and needs to be eliminated. Fortunately, no technical resistance to fludioxonil was detected in these packinghouses at this time (fifth column Table 1).

Managing postharvest fungicide resistance requires a whole-of-system approach, starting from harvest through to packing and storage. Some of the key management factors in reducing the risk of fungicide resistance include:

Optimise fruit health: good postharvest practice to minimise physical damage to the fruit during harvest and handling.
Use the best hygiene practices: lowering the populations of decay-causing spores in the packinghouse, cool room and on the fruit are key to a successful management program. This includes removing rotten fruit from the packinghouse and cool rooms, regularly sanitising equipment, cool rooms and packing lines by washing (or using fogging technology).
Optimise fungicide use: understand the way each fungicide works to develop strategies to minimise resistance developing by using rotations and mixtures whenever possible and before resistance selection occurs.
Optimise fungicide efficacy: the correct fungicide concentration and coverage determines the efficacy of the treatment and minimises the chances of decay spores surviving following treatment.
Monitor fungicide resistance: detecting resistance early increases the chance that its development can be managed and stopped.

How to order a test kit and what happens

Orders for the Postharvest Sanitation and Fungicide Resistance Service are available through the Citrus Australia website. After the order is purchased, a set of test plates are sent to the packinghouse with instructions on where and how to put out the plates. After the test plates have been exposed to the air in different parts of the packinghouse and coolroom, they are returned to NSW Department of Primary Industries in an Australia Post Express Post bag for analysis. The results are then returned as a confidential report back to the packinghouse.

For more information, please see Citrus Australia website, or contact John Golding at NSW Department of Primary Industries. Phone 02 4348 1926, email john.golding@dpi.nsw.gov.au

Acknowledgements

We would like to thank the chemical companies, packinghouse managers and growers for allowing the use of their anonymised data for this article.