Reduced-Risk Strategy for Downy Mildew Management in Cucumber

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

January 2018


Pesticide risk reduction strategies are developed under 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 and provides an update on the progress in developing and implementing the new tools and practices made available through this strategy.

For more information, please visit the Pest Management Centre.


Pesticide Risk Reduction acknowledges all participating stakeholders, including members of the Cucumber Downy Mildew Working Group, for their contribution and continued collaboration in the development and implementation of a reduced-risk strategy for downy mildew management in cucumber.


Downy mildew, a disease impacting numerous crops, was identified as a high priority issue for strategy development through stakeholder consultation and a systematic assessment of pesticide risk reduction potential. As of 2011, downy mildew threats were being managed mainly via repetitive calendar sprays of fungicide products, some of which were under regulatory re-evaluation. At the same time, the economic significance of the problem was very high due to the high value (greater than $1 billion) of the crops involved. Thus the issue was deemed to offer good potential both for risk reduction and for benefit to the sector through the development and implementation of a reduced risk management strategy.

Because downy mildew is caused by different pathogens on different crops, the decision was made to target a single crop species for the development of a reduced risk downy mildew management strategy. In consultation with the PRR Technical Working Group, cucumbers were selected as the target crop.

Cucumbers in Canada are grown both in the field (mainly Ontario and Quebec), and in the greenhouse (mainly Ontario and British Columbia).

Downy mildew is an economically important disease of both field and greenhouse grown cucumber due to its ability to spread quickly and significantly reduce yield and fruit quality. In the field, epidemics are influenced by the geographical location and local climatic conditions. In British Columbia, for instance, downy mildew in field cucumber has not been recorded, whereas in Ontario it is considered a very serious disease.

In 2012, a Working Group was established with representation from researchers (AAFC and academia), grower organisations (Ontario Processing Vegetable Growers, Ontario Greenhouse Vegetable Growers, Fédération québécoise des producteurs de légumes de transformation), provincial specialists, and other stakeholders, to help with the development of a reduced-risk strategy for downy mildew management. The strategy is aimed at identifying, developing, and implementing pest management approaches that may serve as alternatives to conventional pesticides, or be used in combination with the existing tools for integrated disease management systems, primarily in field cucumber while also considering implications to greenhouse cucumber.

Pest management and pesticide risk reduction issues

Downy mildew in cucumber is caused by the Oomycete Pseudoperonospora cubensis. The pathogen spreads through airborne spores and may affect the cucumber at any developmental stage, from seedlings to mature plants. As an obligatory parasite it requires a living host; therefore, it is believed that in Canada it does not survive the winter in outdoor conditions where no hosts are available. Infestations are typically initiated by spores moving in from warmer production regions in the south. The disease may also be introduced to fields through infested seedlings, from infestations in nearby fields, or from greenhouses where cucumber crops are grown concurrently. Once established, the disease quickly spreads throughout the crop and may cause severe defoliation within days. Subsequently, heavy financial losses result from reduced yield and fruit quality.

Current recommendations for managing the disease in the field are based on calendar applications of fungicides, in combination with cultural practices. Preventative fungicides are to be applied shortly after transplanting, followed by frequent reapplications. Intervals as short as 5-7 days may be required under conditions that favour the spread of the disease (that is, cool, wet and humid). This may result in as many as 6 or 7 applications per season with certain products.

Apart from representing a high environmental pesticide load, repeated use of these products increases the probability that the pathogen will develop resistance to any one of the chemistries currently in use, as has already been documented with QoI fungicides (quinone outside inhibitors). Other single-site fungicides that are of particular concern for resistance development are fluopicolide, propamocarb, cyazofamid, and ametoctradin.

The development and adoption of reduced-risk pest management approaches was identified as a need to help to diversify the toolbox for downy mildew and reduce the use of fungicides while maintaining effective disease control.


Working group consultations

Beginning in 2012, consultations with the Cucumber Downy Mildew Working Group took place to identify the priority issues and gaps related to reduced-risk management of downy mildew in cucumber. Input from members of the Working Group was collected via email correspondence, phone calls, teleconferences, and face-to-face meetings. During the course of these discussions, the Working Group identified key goals and priority solutions for a reduced risk strategy for downy mildew management in cucumber. The information led to the development of an action plan as detailed below.

Priority issues and gaps

The consultations raised the possibility of a gap between the knowledge currently held by the cucumber industry and new information and approaches under development. A need for review of available knowledge was therefore considered a priority.

Another identified gap was knowledge of other, currently unknown hosts of the cucurbit downy mildew pathogen (only hop is known to serve as an alternative, although less preferred, host outside the cucurbit group). Such hosts could theoretically have a role in the epidemiology of the disease as it spreads from one growing area to another. Identifying these alternative hosts was therefore considered a priority.

Lack of verified information regarding other sources of inoculum was also identified as a gap. These potential sources could include seeds and other propagating material, plant residues from the previous season, as well as the possibility of cross-contamination between the field and the greenhouse.

The working group also identified a need to enhance disease forcasting capabilities by establishing an early-warning system based on trapping and molecular identification of airborne spores.

In response to these identified priority issues and gaps, the following strategy goals were established:

Goal #1: Establish a knowledge base on downy mildew epidemiology and management

Goal #2: Develop recommendations for best practices to reduce disease incidence

Goal #3: Facilitate adoption of resistant or tolerant cucumber cultivars

Goal #4: Improve disease monitoring and forcasting

Goal #5: Communicate results

Strategy Action Plan

The five strategy goals and potential solutions prioritized through consultations with the working group and with other stakeholders led to the development of an action plan for the reduced risk management of downy mildew in cucumber. The plan is centered on obtaining lower risk control tools and practices that reduce reliance on conventional fungicides while offering effective, economically-feasible pest management options. The tables below outline the goals, the milestones, and the implementation activities completed or underway in support of this strategy.

Action plan tables including goals, milestones and activities for implementing the reduced-risk strategy for downy mildew management in cucumber in Canada

Table 1: First Goal - Establish a knowledge base on downy mildew epidemiology and management
Milestones Status Implementation Activities Completion Date
Literature review to document existing information on downy mildew management approaches from around the world Complete AAFC's project PRR14-030: Literature review on the background, the control practices, and Integrated Pest Management strategies available for sustainable management of downy mildew in cucumber production. Information obtained through published sources and discussions with crop specialists and researchers. It was concluded that no single available method provides full control of the pest, therefore the development of several management tools is required as part of an integrated approach to reduce disease occurrence and to limit the use of conventional fungicides in the field and in the greenhouse. August 2015
Identify relationship between field and greenhouse epidemics Ongoing Part of the literature review of project PRR14-030 dealt with the relationship between field and greenhouse (GH) epidemics. It is speculated that production of cucumber in year-round greenhouses in the Great Lakes region could be potential sources of inoculum during the season, particularly when disease control measures are ineffective. Investigation of potential relationships between field and GH epidemics is underway (PRR16-050) and new information is anticipated at the conclusion of this three year project. Not applicable
Establish the host range of cucumber downy mildew and determine potential alternative hosts as a factor in disease transmission Ongoing Work is ongoing under AAFC's project PRR16-050: Infection pathways in cucumber downy mildew. Focusing on inoculum sources of P. cubensis in commercial cucumber production in Canada, the project is identifying potential pathways that may include seeds, cucurbits grown in nearby fields and greenhouses, wild cucurbits, and crop residue of cultivated cucurbits from previous growing seasons. Material collected is being tested for presence of the pathogen. Not applicable
Develop molecular tools for quantitative detection and identification of downy mildew pathogen Future Not applicable Not applicable
Table 2: Second Goal - Develop recommendations for best practices to reduce disease incidence
Milestones Status Implementation Activities Completion Date
Identify areas where sanitation may be insufficient in field and greenhouse production Ongoing Project PRR16-050 will provide initial indications regarding sources of inoculum in commercial cucumber production. Not applicable
Research to improve existing sanitation practices (for example: timing) and develop/evaluate novel ones Future

Not applicable

Not applicable
Evaluate pathogen resistance to fungicides Ongoing Work is ongoing under AAFC's Project PRR17-030: Survey of resistance to fungicides in cucumber downy mildew. Pathogen isolated from infected plant material collected in cucumber production sites in Ontario and Quebec is tested for resistance to commercially-available, single-site fungicides. The results from this two year project will be communicated to stakeholders and may be helpful in making informed disease management decisions while taking into account pathogen resistance profiles. Not applicable
Table 3: Third Goal - Facilitate adoption of resistant or tolerant cucumber cultivars
Milestones Status Implementation Activities Completion Date
Assess plant response to the pathogen under Canadian production systems when new cultivars become available Ongoing Work is ongoing under AAFC's project PRR16-040:Best Management Practices for Downy Mildew in Cucumber. The project is evaluating the potential for adoption of new cucumber hybrids tolerant to cucurbit downy mildew. Field trials in Simcoe and Ridgetown, Ontario, are assessing disease management under current Ontario Ministry of Agriculture, Food and Rural Affairs recommendations, while comparing with new regimes of extended intervals between fungicide applications in two tolerant hybrids and one standard susceptible hybrid. Not applicable
Table 4: Fourth Goal - Improve Disease Monitoring and Forecasting
Milestones Status Implementation Activities Completion Date
Support the development of an early-warning system based on spore trapping and identification Future Not applicable Not applicable
Table 5: Fifth Goal - Communicate results
Milestones Status Implementation Activities Completion Date
Use appropriate communication platforms to bring results to the attention of stakeholders Ongoing Annual activities are reported to stakeholders via emails and conference calls. Abstracts of new projects and summaries of complete projects are posted on AAFC's website. Additional communication material will be developed as results become available. Not applicable

As the strategy progresses the Action Plan will be refined. Goals and specific activities may be added or modified. This is a living document that will be kept updated on a regular basis.