Project Code: PRR16-040
Project Lead
Cheryl Trueman - University of Guelph
Objective
To assess the potential of incorporating cucumber hybrids tolerant to cucurbit downy mildew into the commercial production of cucumber in Canada
Summary of results
Background
Cucurbit downy mildew, caused by the pathogen Pseudoperonospora cubensis, continues to be a serious threat to the Canadian cucumber industry. It was identified as a priority pest issue through stakeholder consultations and a systematic assessment of pesticide risk reduction potential led by the Agriculture and Agri-Food Canada’s Pesticide Risk Reduction team. 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. Typical disease control measures relied on a preventative spray program where fungicide applications were often required on a weekly interval basis over a prolonged period. This 3-year project supported through the Reduced-risk Strategy for Downy Mildew Management in Cucumber aimed at developing best management practices, such as the integration of new disease partially resistant cucumber varieties as part of the crop protection program to enable a sustainable management of downy mildew in cucumber production.
Approach
Starting in summer 2016, nine field trials were conducted over three years at the University of Guelph’s Simcoe and Ridgetown research stations in Ontario. The trials were conducted following Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) spray program recommendations, and compared new regimes of extended intervals between fungicide applications in two partially resistant hybrids and one standard susceptible hybrid. A total of 12 treatments, including a control and fungicide sprays at 7, 10, and 14 days intervals were evaluated for each of three cucumber varieties. The three cucumber varieties evaluated included the new partially resistant hybrids Citadel and Peacemaker and the standard hybrid Vlaspik. Two preventative fungicide programs were tested: one consisted of applying one type of fungicide (Bravo) at 7, 10, or 14 days intervals when conditions were ‘low’ risk for disease development; the other program consisted of applying multiple fungicides in rotation (Orondis Ultra, Torrent, and Zampro) also at 7, 10, or 14 day intervals when conditions were ‘high’ risk for disease development. The control treatment consisted of the three cucumber varieties, untreated.
The new hybrids were tested under both conditions of low and high disease pressure to see if a commercially acceptable level of disease control could be achieved while reducing the number of fungicide sprays by extending the standard application interval of 7 days to 10 or 14 days. Each treatment was evaluated for downy mildew incidence, severity and yield. Disease incidence was evaluated by determining the percentage of leaves affected by downy mildew in each plot. The Area Under the Disease Progress Curve (AUDPC) was determined and the disease severity was determined by evaluating the percentage of each leaf which showed downy mildew symptoms. Plot severity was determined by estimating the total percentage of leaf area affected with downy mildew in each plot.
Results
Due to very low or no disease pressure in five of the trials, the results here include only the data generated from three 2017 trials where moderate to high disease pressure was observed. In the five trials with little to no disease pressure, the control treatments for each variety had a disease incidence that ranged from 0 to 44% while the plot severity ranged between 0 and 39%. In the trials with moderate to high disease pressure the disease incidence ranged from 37% when the plot began to show symptoms of downy mildew to 97% by the final assessment date, and plot severity ranged from 20% when the plot began to show symptoms of downy mildew to 90% by the final assessment date. The standard 7 day interval generally provided the best control of the disease for all hybrids tested especially at high disease pressure as indicated by a lower AUDPC. The longer spray intervals at 10 and 14 days were also often sufficient to manage disease and limit yield losses in the two downy mildew partially resistant hybrids. There was also evidence of higher total disease in the susceptible hybrid compared to the two partially resistant hybrids. The results indicate potential use of the partially resistant hybrids as a best management practice to reduce fungicide sprays for downy mildew on cucumber. The decision to reduce the number of sprays by using partially resistant hybrids can result in economic savings on pesticide product costs and a reduction in the pesticide load in the environment. However, higher disease levels were observed with the longer intervals resulting in higher inoculum load in the crop. The higher inoculum load should be taken into consideration when using the longer intervals. High inoculum load can be problematic for late season management and pose high risk for other neighbouring cucurbit crops with no host resistance.
Yield varied between trials for the number of fruit and weight of fruit for each of the hybrids. There was evidence, however, the 7 and 10 day interval had an increase in fruit weight for the downy mildew susceptible hybrid as well as the partially resistant hybrids when compared to the control treatments.
In some trials, risk of downy mildew development was considered high but still took several weeks to appear in the research plots and commercial fields near the research locations. In the Great Lakes region, the determination of risk (low or high risk) of cucumber downy mildew is based on weather and reports of cucumber downy mildew detection in the area such as in the Cucurbit Downy Mildew IPM Pipe forecasting tool. There is an opportunity for research to find alternative and more accurate ways to determine when crops are at high risk of infection and when growers should spray their crops for downy mildew.
The results were made available to the grower communities through various technology transfer activities such as industry conferences, public press articles, and research station field tours. Researchers, crop specialists and growers will benefit from the information derived from this project given that new possible areas of research have been identified and the use of partially resistant hybrids has been identified as a possible new best management practice for growers looking to minimize the use of fungicides.