Literature review on the background, the control practices, and Integrated Pest Management strategies available for sustainable management of downy mildew in cucumber production

Project Code PRR14-030

Project Lead

Odile Carisse  Agriculture and Agri-Food Canada

Objective

To conduct a comprehensive review, through searching in published scientific and extension literature, and by collecting input from experts and stakeholders, to compile world-wide information on Integrated Pest Management (IPM) strategies for managing downy mildew in cucumber; provide recommendations on best management practices which could be adopted in Canadian field cucumber production, and on approaches which merit further research and/or testing under Canadian conditions

Summary of Results

Background

Cucumber downy mildew, caused by the pathogen Pseudoperonospora cubensis, is a threat to the industry and was identified as a priority pest issue by the Pesticide Risk Reduction Program. The pathogen spreads through airborne spores. Under ideal conditions for disease development, foliage of infected crop may become completely blighted, leading to significant yield losses within days. Current pest management measures involve a preventative spray program where multiple fungicide applications may be required on a weekly basis.

As the first under the Reduced-Risk Strategy for Downy Mildew Management in Cucumber, this project aimed to establish a consolidated knowledge base on the biology, current practices, and potential new tools for the management of cucurbit downy mildew. The intent was to enable the drafting of recommendations on best management practices and IPM systems which could be adopted by Canadian growers, as well as identifying new areas in need for further research.

Approaches

The objective was accomplished by reviewing existing information and pest management approaches, and recommending those that could be relevant for adoption by Canadian cucumber growers. Information was compiled through a literature survey of Canadian and foreign publications (scientific and extension), and through consultations with researchers, crop specialists, grower representatives, and other stakeholders.

Results

As an obligatory parasite, Ps. cubensis only affects certain species, but not all, in the Cucurbitaceae family. Its hosts include cultivated, semi-cultivated, weedy, and wild species. Laboratory inoculations and field observations indicated that at least 60 species could potentially be hosts of this pathogen.

Potential sources of inoculum of Ps. cubensis are not well documented. One probable source is sporangia blown in from infected crops in areas with mild winters, primarily in the south. It is also possible that the pathogen survives on wild cucurbits, on other hosts, or that cucumber production in greenhouses may contribute to the availability of inoculum in cucumber fields in the spring. Considering the importance of the initial inoculum, more research is needed to determine the complete host range of Ps. Cubensis, to clarify its strategies of reproduction and overwintering, and to clearly identify the various sources of inoculum that are significant in disease initiation.

A closely-related species, Ps. humuli, is the causal agent of downy mildew on hop. Some researchers consider Ps. Cubensis and Ps. Humuli to be the same species whereas others see them as distinct species. More work is required in this area of species identification due to its implications for both disease management and species-specific plant quarantine regulations.

Some varieties and lines within the same Cucurbitaceae species show resistance to downy mildew. For many years, using resistant cultivars was the basis of disease management in commercial cucumber production. The pathogen, however, was able to adapt to its host by developing physiologically specialized pathotypes and races capable of overcoming the resistance of these cultivars. This has given rise to the need to develop new resistant lines as a way of reducing the reliance on conventional fungicides. Progress in this area will require identification of effective sources of resistance, and a greater understanding of the complex interactions between Ps. cubensis and its cucurbit hosts.

Weather-based forecasting of disease risks enables better decision-making regarding the timing of fungicide applications. The system most often used, the Cucurbit Downy Mildew Forecasting Service offered by North Carolina State University, combines knowledge of long distance dispersal of airborne inoculum, influence of weather on disease risk, and observed disease outbreaks. This system is considered reliable and has greatly improved cucurbit downy mildew management in areas where it has been implemented. It could be further improved by adaptation to the Canadian conditions, and the addition of information on airborne sporangia coming from potential local sources of inoculum.

The concentration of airborne sporangia, which plays a key role in initiating the disease, varies depending on distance from the source, environmental conditions, and other factors. Early warning can therefore be based on monitoring of airborne inoculum. In Michigan, traps are used to alert growers to any influx of spores coming into the state’s production regions. However, the number of spore traps currently installed is insufficient to obtain a full coverage of the areas at risk. A network of inoculum monitoring could therefore be very useful.

Determining the genetic diversity within the airborne spore populations of Ps. cubensis, especially with regards to virulence and fungicide resistance, is also essential for proper monitoring and forecasting. It could help in assessing the risk of disease outbreaks, and in selecting the right fungicides both for managing the disease and for reducing the risk of the pathogen developing resistance to these fungicides.

The results of this project are being used to further working group discussions and development of the Action Plan associated with the Reduced Risk Strategy for Downy Mildew Management in Cucumber.

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