Survey of resistance to fungicides in cucumber downy mildew pathogen

Project Code: PRR17-030

Project Lead

Geneviève Marchand - Agriculture and Agri-Food Canada

Objective

To assess populations of cucumber downy mildew pathogen isolated from field and greenhouse production sites in Ontario and Quebec for resistance to single-site fungicides

Background

Cucurbit downy mildew is an important disease of cultivated cucumbers in the field and in the greenhouse. The disease is caused by the oomycete pathogen Pseudoperonospera cubensis and is spread primarily by airborne spores. Severe infections can result in rapid crop failure, with foliage completely blighted within ten days from first symptoms. Conventional fungicides are commonly used to control the disease through a preventative spray program where applications may be required on weekly intervals over a prolonged period. Single-site systemic fungicides are more effective than multi-site ones; however, the pathogen is known for its ability to rapidly develop resistance to single-site fungicides. Fungicide resistance was identified as a priority issue for cucumber downy mildew management through stakeholder consultations led by the Agriculture and Agri-Food Canada’s (AAFC) Pesticide Risk Reduction Team. This two year project, supported through the Reduced-risk Strategy for Downy Mildew Management in Cucumber, evaluated resistance to the commonly-used single-site fungicides in pathogen populations in commercial cucumber production in Ontario and Quebec.

Approaches

The project was conducted at both AAFC’s Research and Development Centers in Harrow, Ontario and St. Jean-sur-Richelieu, Quebec. To determine the fungicide resistance profiles of Ps. cubensis isolates from commercial cucumber sites, in vivo assays were used. Downy mildew infected leaves were collected from field cucumbers in Quebec and from greenhouse cucumbers in Ontario, and were used to infect mature plants. Efficacy of the following active ingredients were evaluated on these isolates: pyraclostrobin (Cabrio EG), fenamidone (Reason 500 SC), fluopicolide (Presidio), propamocarb (Previcur N in Ontario and Tattoo in Quebec), cyazofamid (Torrent 400 SC), dimethomorph (Acrobat 50WP in Ontario and Forum in Quebec), co-formulation of ametoctradin and dimethomorph (Zampro), Oxathiapiprolin (Orondis Ultra B, Canadian formulation), and mandipropamid (Orondis Ultra A, Canadian formulation in Ontario and Revus in Quebec).

Commercial field and greenhouse cucumber production sites in Quebec and Ontario were surveyed through scouting crops for the presence of downy mildew to gain a better understanding of disease distribution and prevalence throughout these regions. This was completed in collaboration with respective provincial extension specialists (Ontario Ministry of Agriculture, Food and Rural Affairs and Ministère de l'Agriculture, des Pêcheries et de l'Alimentation) and researchers from the University of Guelph and Agriculture and Agri-Food Canada.

Results

All tested isolates showed evidence of lower efficacy to both of the strobilurin group of active ingredients (fenamidone and pyraclostrobin). These findings of low efficacy for the strobilurins confirm similar reports from field extension specialists. Variation was observed between isolates for the efficacy of dimethomorph, mandipropamid, and fluopicolide. The other active ingredients that were evaluated (cyazofamid, oxathiapiprolin, propamocarb, and co-formulation of dimethomorph and amectoctradin) maintained higher levels of efficacy against downy mildew.

Findings from scouting commercial cucumber sites revealed that in 2017, cucurbit downy mildew was first observed in southwestern Ontario at the end of June, and in 2018 at the end of August. In Quebec in 2017 and 2018, downy mildew was not observed in field cucumbers until mid-August, near the end of the cropping season for pickling cucumber. Throughout the duration of this project, prevalence of downy mildew in greenhouse cucumber in Ontario, and field cucumber in Quebec, generally remained low in most areas.

Conclusion

The project was set out to assess the resistance profile of a number of sampled isolates to a wide range of fungicides. However, due to the nature of downy mildew, working with the pathogen proved challenging both in the field and in laboratory assays. Despite the challenges, conclusive evidence was obtained regarding the level of performance for a number of fungicides against Ps. cubensis. These results contribute to the existing body of literature on Ps. cubensis resistance showing that cucumber growers have increasingly fewer reliable options for the control of downy mildew. These findings were presented to growers at the 2019 Canadian Greenhouse Conference, the 2019 Ontario Fruit and Vegetable Growers Association Meeting, as well as at the annual meetings of the Canadian Phytopathological Society in 2018 and 2019. Providing information about which active ingredients are still effective against this pathogen allows growers to make informed decisions about managing cucumber downy mildew in their crops.

The results from this project can be further used to develop a molecular method for early detection of known mutations that result in resistance to fungicides.