How to best manage postharvest fungicide resistance
6 October 2020
By John Golding and John Archer, NSW DPI
Resistance to postharvest fungicides leads to a loss of fungicide efficacy
Managing postharvest fungicide resistance relies on good packinghouse hygiene and sanitation with rotation of fungicides
Don’t always use the same postharvest fungicide
A Postharvest Sanitation and Fungicide Resistance Service to industry is available 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 with the development of resistance.
Fungicide resistance occurs from the formation of fungicide resistant decay spores which go on to multiply. Constantly using the same fungicide allows for the build-up of resistant spores within a packhouse, particularly if packhouse hygiene is poor.
Overtime this can lead to a situation where the decay fungus is able to grow on fruit following fungicide treatment resulting in the growth of the decay and breakdown in the marketplace.
Fungicide resistance is a serious and important postharvest problem which 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, and 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 the presence of 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.
Overall there were high levels of general moulds in the packinghouse and coolroom (first column in Table 1). These are general moulds, bacteria etc which live in the environment and are generally not related to postharvest decay. But 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 in the packinghouse (second column in Table 1), there was considerable variability in the results between the different packinghouses.
Some packinghouses had excellent hygiene and sanitation with very few decay causing fungi detected (for example Packinghouse B and H), while other packinghouses had very high levels of decay causing fungi both in the packinghouse and in the coolroom (Packinghouse G and I).
All packinghouses assessed with this Service were fully operational and commercial. This 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.
In this sample of packinghouses, over half of the packinghouses 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 very high levels of resistance to TBZ fungicide would have begun with low levels of spores with resistance and were encouraged with poor sanitation and continued use of TBZ fungicide. These high levels of technical resistance to TBZ are a concern and improvements in packinghouse hygiene and rotation of fungicides 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.
The use of 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 to control postharvest decay.
Although this observation was un-common 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).
The management of resistance to postharvest fungicides requires a whole-of-system approach, starting from harvest through to packing and storage. Some of key management factors in reducing the risk of fungicide resistance includes:
Optimise fruit health. Good postharvest practice to minimise physical damage to the fruit during harvest and handling.
Use best hygiene practices. Lowering the populations of decay-causing spores in the packinghouse, cool room and on the fruit are keys to a successful management program. This includes removal of rotten fruit from the packinghouse and coolrooms, the regular sanitation of equipment, coolrooms and packingline by washing (or using fogging technology).
Optimise fungicide use. Understand the way each fungicide works to develop strategies to minimise the development of resistance 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. The early detection of resistance increases the chance that its development can be managed and stopped.
How to order a test kit and what happens
The Postharvest Sanitation and Fungicide Resistance Service is coordinated by NSW DPI. Orders for test kits can be made through the Citrus Australia website here. 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. Third parties are also able to purchase the kit and place in your packingshed on your behalf.
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.