Reduced-Risk Strategy for Apple Scab Management

For inquiries please contact:
Pest Management Centre, Agriculture and Agri-Food Canada

March 2014


Pesticide risk reduction strategies are developed under the Pesticide Risk Reduction (PRR) of Agriculture and Agri-Food Canada's (AAFC) Pest Management Centre. To reduce undesirable impact of the use of pesticides in agriculture, PRR works with grower groups, industry, provinces, researchers and regulators to identify gaps in pest management and opportunities to develop and implement strategies to address these.

A pesticide risk reduction strategy is a detailed plan that aims to address grower needs for reduced-risk management tools and practices for specific pest issues. The strategies are developed through extensive consultation with stakeholders. The strategy document presented here summarizes the framework and activities supported by PRR. It is intended to provide an update on the progress in developing and implementing the strategy and new tools and practices made available through this process.

For more information, please visit the Pest Management Centre.


Pesticide Risk Reduction acknowledges all participating stakeholders for their contribution and collaboration in the development and implementation of this reduced-risk strategy for apple scab management.

1. Executive Summary

This document reports on the collaborative development of a reduced-risk strategy for control of apple scab management in orchards across Canada. The strategy is a national initiative developed with industry stakeholders and with the support of the federal government. The objective of the strategy is to minimize the need for using fungicides to control apple scab while promoting viable disease management and farm profitability.

The pesticide risk reduction strategy includes the identification of potential pesticide risks, knowledge gaps and barriers to the adoption of reduced-risk management practices, potential reduced risk solutions for these issues, goals and milestones, a strategic action plan and information on progress of the activities implemented to support the strategy.

Within the context of the strategy, pesticide risks and pest management issues were identified by an expert working group. One of the core issues raised by the group members was the development of resistance to commonly used fungicides among apple scab pathogen populations. This issue was addressed by identifying areas where new knowledge and practices were needed, initiating projects to generate the knowledge and to support the development of practices, and disseminate the information to growers.

Overall, the strategy addresses three main goals with identified targets, milestones, and activities:

  1. development and implementation of alternative strategies for apple scab management,
  2. facilitate the adoption of bio-fungicides, and
  3. improve adoption of resistance management strategies.

Since 2006, AAFC's Pest Management Centre has funded six risk reduction projects to support the implementation of apple scab strategy. One project facilitated the testing of a microbial control agent (Microsphaeropsis ochracea) that lowers the amount of overwintering inoculum of apple scab pathogen. Another has advanced the development of an in vitro method, which evaluates the efficacy of fungicides against pathogen ascospore samples in order to determine whether the pathogen population shows resistance to fungicides. A grower's guide including information on the biology of the apple scab pathogen and approaches for management of primary and secondary infections was developed and published online.

A national survey was also conducted to document the spread of pathogen resistance to sterol inhibitors and strobilurins fungicides. As an outcome, a commercial service is now offered to growers by the University of Guelph where leaf samples can be tested to determine the level of pathogen resistance to various fungicides in individual orchards.

It is anticipated that the use of the knowledge and tools developed through this strategy will help growers in adopting sustainable apple scab management in their orchards.

2. Introduction

Apples are produced across Canada on a total area of 17,000 hectares. The main regions are Ontario (37%), Quebec (29%), British Columbia (21%), and the Maritimes (12%).

In Canada, apple scab is the most serious disease of apple and is a significant economic threat in all growing regions. Caused by the pathogenic fungus Venturia inaequalis it is of particular importance in Eastern Canada, as infection and disease spread are favored by cold winters and wet growing conditions. In these areas the disease occurs every year and often causes significant economic losses, both directly from fruit infections (which affect yield, fruit quality and storability) or indirectly from defoliation (which affects tree vigor and winter survival). If not controlled, apple scab can result in 100% crop loss in any given year. In British Columbia (BC), where the weather is usually dryer, the disease is less serious although it could be significant in certain growing areas in the interior parts of the province.

3. Pest Management and Pesticide Risk Issues

Chemical control of apple scab by fungicides is the most common method of controlling this disease on commercial cultivars. Some fungicides have protectant activity, which inactivates and kills fungal spores and prevents infection. Some have eradicant activity which prevents or reduces further growth of infections that have already started. All pesticides used, sold, or imported into Canada, are regulated by Health Canada's PMRA.

Cultural practices are also used for apple scab management. These include scab-resistant cultivars (Belmac, Britegold, Macfree, Moira, Murray, Nova Easygro, Novamac, Novaspy, Primevere, Richelieu, Rouville and Trent), and sanitation by means of tree pruning and removal of leaf litter to physically reduce fungal inoculum in the orchard. As the pathogen overwinters in the dead leaves on the orchard floor, destruction of the leaf litter by burning or flail-mowing contributes to the reduction of primary inoculum in the next growing season. Urea applications between the growing seasons also contribute to the reduction of primary inoculum.

Several of the newer fungicides registered in Canada for apple scab pose medium to high risk for resistance development. If used repeatedly, these products are at risk of losing their efficacy over time. Resistance in V. inaequalis populations and lack of efficacy for some of the fungicides (for example strobilurins and sterol inhibitors) has been already observed over the years in some orchards.

Resistance development often results in increased fungicide use, as more frequent applications are required to compensate for the reduced efficacy, which increases production costs and environmental concerns.

Overall, successful management of apple scab must address various challenges due to the endemic nature of the disease, variable environmental conditions, potential removal of some pesticides following re-evaluation by PMRA, potential development of pathogen resistance to fungicides, and high apple quality standard demanded by consumers.

The apple scab strategy is thus centered on the development of reduced risk tools and practices to determine appropriate timing and need for fungicide applications, with the goal of minimizing unnecessary fungicide sprays and managing pathogen resistance to fungicides.

4. Reduced Risk Management Strategy for Apple Scab

Apple scab was identified as a priority issue for apple growers through consultations between AAFC's Pesticide Risk Reduction and the Canadian Horticultural Council's (CHC) Apple Working Group, a subgroup of the CHC's Apple and Fruit Committee. A new working group was established in 2006 under PRR that included researchers, growers' reps, provincial specialists, and other stakeholders and tasked with the development of the strategy. Recommendations generated through working group consultations were captured in the strategy's Action Plan and activities to address identified gaps were carried out through the support of various projects. Specific issues identified in the process of consultations are outlined below.

4.1 Issues and needs for Apple Scab Management

The apple scab working group identified the issues facing apple growers in coping with the disease. The following is a list of the most important issues brought forward through discussions with the working group. Some of these issues were addressed within the context of this strategy; others, in particular #6, are to be pursued through other channels available to the industry.

  1. Registration of new and effective low-risk products
  2. Knowledge and management of pathogen resistance to fungicides
  3. Develop new cultural control options
  4. Improve timing of fungicide applications
  5. Reduction of overwintering pathogen inoculum
  6. Replacement of susceptible apple cultivars with resistant ones

5. Apple Scab Pesticide Risk Reduction Action Plan

Based on the information collected through consultations with the apple scab working group, an action plan has been developed (Table 1). The plan outlines the goals of the strategy, the targets and the milestones, as well as the activities supported over the course of the strategy (2006-2014).

Table 1: Action plan for reduced risk apple scab management (status February 2014)

Strategy Goal #1 - Develop and implement alternative strategies for apple scab management
Target Milestones Completed Activities
Develop a management plan to reduce overwintering inoculum Assess efficacy of a new microbial agent X Work conducted through AAFC project BPI06-030 and focused on testing a novel biocontrol agent, Microsphaeropsis ochracea. Results indicate potential reduction in inoculum levels of apple scab in the field.
Develop a management plan to reduce overwintering inoculum Develop information on cultural practices for crop residue management X Work conducted through AAFC project PRR05-050. A guide on Apple scab: improving understanding for better management was published on AAFC's website and made available to growers. It demonstrates the use of various cultural techniques, such as flail-mowing and urea applications in the fall, to lower scab inoculum.
Use of practices that maximize sprayer efficacy Encourage an annual pruning to optimize spray coverage Outreach activities outside the scope of the strategy may be required (demonstrations, presentations, workshops, factsheets).
Develop and encourage the use of prediction tools Incorporate sequential sampling into IPM system X Work conducted through AAFC project BPI06-030. The use of a sequential sampling plan allowed a pesticide use reduction of 24% and 45% at two apple orchards in Quebec, respectively, as compared to the standard conventional practice. The Apple scab: improving understanding for better management guide which includes information about sequential sampling plan, was made available to growers in Quebec and Ontario.
Develop and encourage the use of prediction tools Promote the use of apple scab forecasting software Growers in some regions, primarily in Quebec, are using RIMpro forecasting software. The technology is posted on the web and is available to all growers.
Improve grower knowledge and adoption of newly introduced tools and methods Develop and disseminate a guide on apple scab management X As part of project PRR05-050, the Apple scab: improving understanding for better management guide can be downloaded from the PMC homepage and St-Jean-sur-Richelieu homepage. The guide was presented to growers and advisors at various grower meetings in Quebec and Ontario.
Strategy Goal #2 - Facilitate the adoption of new bio-fungicides
Target Milestones Done Activities
Timorex Gold and Regalia adoption Evaluation of Timorex Gold and Regalia for registration for use in apple scab management X Field trials to evaluate the efficacy of Timorex Gold and Regalia when used in an integrated management plan for apple scab were conducted in New Brunswick, Quebec and Ontario, through AAFC project BPI09-060. The use pattern of the two products was found to be ineffective for apple scab control under the conditions of the trials.
Timorex Gold and Regalia adoption Registration submission The biocontrol agent is licensed to the German company Prophyta, currently a unit of Bayer CropScience. Sufficient level of efficacy in the field, however, still needs to be demonstrated for a European registration. In Canada, further work towards the registration of M. ochracea for apple scab management is currently considered low priority by the working group.
Microsphaeropsis ochracea adoption Registration submission The biocontrol agent is licensed to the German company Prophyta, currently a unit of Bayer CropScience. Sufficient level of efficacy in the field, however, still needs to be demonstrated for a European registration. In Canada, further work towards the registration of M. ochracea for apple scab management is currently considered low priority by the working group.
Strategy Goal #3 - Improve adoption of resistance management strategies
Target Milestones Done Activities
Develop methods to reduce development of pathogen resistance in orchards Develop thresholds for summer sprays and refine the spray approach X Work conducted through AAFC project PRR06-910. A refined summer spray approach contributed to lowering pesticide residues on fruit by reducing the number of targeted fungicide applications and increasing their effectiveness.
Develop methods to reduce development of pathogen resistance in orchards Develop a tool to evaluate the level of resistance in fungal populations X Work conducted through AAFC project PRR06-920. An in vitro method was developed to evaluate the efficacy of fungicides against ascospore samples harvested from leaves of infested orchards. A delivery agent or mechanism is required to implement at the farm level.
Improve grower awareness of resistance management for apple scab Develop and publish a guide on resistance management X The guide Apple scab: improving understanding for better management developed under project PRR05-050, includes a section on resistance management.
Improve grower awareness of resistance management for apple scab Inform growers on apple scab resistance development X Work conducted through AAFC PRR06-920. Presentations at grower meetings served to promote fungicide resistance monitoring. Information on development of fungicide resistance was also made available through the Quebec agriculture warning network.
Enable growers to test for fungicide resistance in their orchards Conduct a national survey of resistance to fungicides; develop advanced testing protocols X Project PRR11-060 was initiated in 2011 to document the extent of apple scab resistance to systemic fungicides, including sterol inhibitors and strobilurins, in apple orchards. Led by Ontario Apple Growers, the survey involved all apple growing regions in Canada. The survey confirmed the presence – and revealed new trends – of resistance development to these fungicide groups. As an additional outcome, a new established testing protocol is now offered to growers as a commercial service by the University of Guelph's Pest Diagnostic Laboratory (PDL) where leaf samples sent to the lab can be tested to determine the level of pathogen resistance to various fungicides in individual orchards. Provincial results of the 2011 field season of the apple scab resistance survey in Ontario were published by Ontario Ministry of Agriculture and Food. A new factsheet Resistance of apple scab pathogen to fungicides in Canadian Orchards: Knowing and managing the risk has been developed to disseminate results from the national resistance survey to stakeholders.

6. Strategy progress

The goals outlined below have been actively pursued since 2006.

6.1 Goal #1: Development and implementation of alternative control strategies

Pesticide Risk Reduction supported the development and publication of a manual Apple scab: improving understanding for better management, written by Dr. Odile Carisse. This document contains information on the biology of the apple scab pathogen and recommendations on integrated management of the primary and the secondary infections. It provides information on cultural practices and preventive approaches, such as using resistant cultivars. The guide suggests the use of fall sampling plans, which can help forecasting disease pressure for the following year and reduce the number of fungicide sprays in cases of low disease pressure. Other parts of the manual are dealing with fungicide resistance management.

Reduced risk alternatives, such as biocontrol agents, were identified as potential tools to help in resistance management and to be part of the control toolkit in case older fungicides would be lost due to re-evaluation. Dr. Odile Carisse (Agriculture and Agri-Food Canada, St. Jean-sur-Richelieu, Quebec) and her team had discovered and studied a microbial pest control agent (Microsphaeropsis ochracea) which could lower the level of overwintering inoculum in the orchards. With the reduction in pathogen inoculum, it was demonstrated that the first chemical fungicide application may be delayed in the spring and potentially reduce the overall number of fungicide applications required per season. Workshops were held in Ontario and Quebec to inform growers about the efficacy of M. ochracea and the potential of integrating it into an Integrated Pest Management (IPM) system in the event that it becomes available commercially.

6.2 Goal #2: Facilitate the adoption of bio-fungicides

Pesticide Risk Reduction supported field tests with Timorex Gold, a reduced-risk, broad spectrum fungicide derived from tea tree oil, to evaluate its efficacy in controlling the disease. PRR also supported field tests with Regalia, a biofungicide designed to activate the plant's natural defenses to protect against a variety of fungal and bacterial diseases. These trials were conducted in apple growing areas in three provinces (Ontario, Quebec, New Brunswick). While results indicated inefficacy for apple scab control, Timorex Gold has been registered in Canada to control/suppress other diseases on other crops.

6.3 Goal #3: Improve adoption of fungicide resistance management strategies

Vincent Philion, a researcher from the "Institut de Recherche et de Développement en Agroenvironnement" in Quebec, developed an in vitro method to evaluate the efficacy of fungicides on samples of the pathogen's ascospore collected from leaves in infected orchards. This tool could help growers determine if pathogen populations in their orchard show resistance to various control products. This, in turn, could allow for improved selection of fungicides for individual orchards.

The same team has also developed a fungicide spray action threshold to help minimize the use of captan for summer management of scab. A refine summer spray approach could contribute to lower the risk of resistance development, as the fungicide applications are expected to be fewer and more effective.

The guide by Dr. Carisse contains a section on resistance management for apple scab and makes references to the state of resistance in Quebec orchards.

A 2-year national apple scab fungicide resistance testing project launched in 2011 elucidated the spread of pathogen resistance in apple-producing regional of Canada. The project also led to the establishment of a commercial standard protocol for apple scab fungicide resistance test which is being offered to growers as Plant Testing Services since 2013 for a fee. The service enables growers to send leaf samples and obtain an indication about the status of pathogen's resistance to a range of fungicides in their orchards. This new tool allows for better decisions regarding the use of fungicides, including tank mixes, timing, and number of applications.