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Field validation of a novel tool to help growers manage the practical consequences of the development of resistance to apple scab fungicides

Project Code: PRR06-920

Project Lead

Vincent Philion - Institute of Research and Development in Agroenvironment (IRDA)


To validate a patent-pending method to determine the efficacy of fungicides against apple scab on potted apple trees

Summary of Results


Apple scab (caused by Venturia inaequalis) is one of the most serious diseases of apple. Symptoms include lesions on the leaves, which can lead to defoliation of the tree, and lesions on fruit, resulting in corky brown areas, and unmarketable product. Yield losses up to 100% are possible, and so producers have used an aggressive chemical spray regimen to protect their apple crops.

With the widespread and repeated use of fungicides comes the risk of the development of resistance to certain fungicide active ingredients within pathogen populations. This is a problem in the case of apple scab, where the development of resistance to systemic fungicides, including the sterol inhibitors and strobilurins is of great concern. The use of fungicides to which pathogens are resistant can result in waste of growers’ time and money, and detrimental pesticide loading in the environment. The Apple Scab Risk Reduction Strategy working group identified this as an important area for research to develop a practical approach which could be implemented by growers.

In 2006, this project was undertaken with the aim of validating a method to determine the efficacy of fungicides against apple scab on potted apple trees with the ultimate goal of providing growers with a tool to determine the potential for efficacy of fungicides in their orchards. In particular, the impact of declining efficacy on protection and post-infection fungicide efficacy was investigated under conditions of limited absorption so that growers can better use these products.


Work was undertaken in Quebec between 2006 and 2009 by the research team based at IRDA. In all experiments, potted trees of cultivar McIntosh, Royal Gala, or both were used.

The developed resistance monitoring method was validated in conditions of limited fungicide uptake, and in situations where populations of V. inaequalis of varying tolerance levels to fungicides were confirmed. Fungicide classes assessed included a sterol inhibitor, dodine, strobilurin, and anilinopyrimidine, and the factors impacting uptake included temperature, application timing relative to infection, dosage level, and sensitivity of the pathogen populations.


Challenges were faced in the generation of consistent results when conducting the trials with potted trees. Despite this, the project contributed to the development of knowledge related to fungicide uptake kinetics, and helped to finalize a protocol for fungicide resistance monitoring in orchards.

Results confirmed that in orchards with V. inaequalis populations partially tolerant to any given product, it is possible to maintain partial use of the product, provided that any applications are only made under optimal conditions. Results indicated that use in cold temperatures had a negative impact on the efficacy of most products tested.

The potential of this approach as a resistance monitoring service for growers was investigated following the conclusion of the work.

For more information please contact Vincent Philion, IRDA.

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