Project Code: PRR16-050
Dr. Katerina Jordan - University of Guelph
Assessment of propagative material, alternative downy mildew host plants, and plant residue as inoculum sources for the downy mildew disease in cucumber production
Cucumber downy mildew, caused by the pathogen Pseudoperonospora cubensis, is a threat to the industry and was identified as a priority pest issue to be addressed by Agriculture and Agri-Food Canada's Pesticide Risk Reduction. Under ideal conditions for disease development, foliage of the infected crop may become completely blighted, leading to significant yield losses. Current disease management measures involve a preventative spray program where fungicide applications may be required on a weekly interval over a prolonged period. As the pathogen spreads mainly through long-distance movement of airborne spores, disease risk forecasts currently available are based on weather patterns across large geographic regions. However, the role of local sources of inoculum in spreading the disease was not well understood. This three year project supported through the Reduced-risk Strategy for Downy Mildew Management in Cucumber addressed this identified knowledge gap through assessing different potential transmission pathways and inoculum sources for downy mildew in the largest commercial cucumber production in the southern Ontario region.
Fruits and seeds of various cucumber and squash species from different sources were analysed in controlled lab and greenhouse trials to determine the potential transmission pathways for downy mildew. From 2016-2018, cucumber fruit were collected from the University of Guelph's Simcoe Research station fields in Ontario as well as cucumber and squash fruit from a local grocery store (2016 only). The plant materials were processed and analysed for the presence of Ps. cubensis using microscopic and molecular tools. Seeds collected from the fruit of the field infected plants (self-harvested seeds) as well as samples of commercial seeds were also planted in a greenhouse into pots. The plants growing from these seeds were observed for downy mildew symptoms. Polymerase Chain Reaction (PCR) analysis was conducted on deoxyribonucleic acid (DNA) extracted from the collected fruit, seeds of commercial and field infected plants and leaf and stem tissues of the plants that showed symptoms of downy mildew in greenhouse experiments.
To assess the potential for alternative hosts as inoculum sources, four related cucurbit species were examined, including wild cucumber (Echinocystis lobata) (from Simcoe Research Station fields), bur cucumber (Sicyos angulatus) (from Ridgetown campus fields), and greenhouse grown balsam pear (Momordica charantia) and golden creeper plants (Thladiantha dubia). The leaves of each of the above plants were exposed to downy mildew sporangia and monitored for lesions and the development of fresh sporangia. In addition, over the period of two years, leaves showing symptoms of downy mildew were collected from wild cucumber, bur cucumber, and golden creeper from locations across Ontario and analysed. Golden creeper rhizomes collected from Ridgetown were also planted in a greenhouse before the 2017 field season then transplanted to a downy mildew infected cucumber field in July and observed for disease symptoms. DNA was also extracted for PCR analysis from bur and wild cucumber seeds, as well as from bur and wild cucumber and golden creeper rhizomes.
Downy mildew infected cucumber leaves collected from field and greenhouse crops were examined to assess leaves for the presence of oospores. Similarly, soil samples containing crop residues were collected from fields that had been plowed at numerous locations and examined for oospores presence. To further assess the potential for oospores overwintering in crop residues, leaves infected with downy mildew were chopped and mixed with field soil and exposed to the outdoors during the winter months. Zoosporangia were monitored prior to and after placing the samples outdoors.
In Ontario, two commercial greenhouses in 2017, and one commercial greenhouse in 2018 were monitored for the presence of downy mildew in cucumber. Incidence was recorded during the spring, summer, fall and winter and the results were compared with field outbreaks of downy mildew during each year.
The collected cucumber fruit did not show downy mildew sporangia or sporangiophore in the microscopic analysis. Also, plants grown from commercial and self-harvested seeds from infected plants did not result in many plants showing downy mildew symptoms. Although the traditional identification methods did not show much downy mildew, the PCR analysis indicated the presence of Ps. cubensis in cucumber fruit harvested from downy mildew infected plants and in commercial and self-harvested seeds. The PCR analysis also confirmed Ps. cubensis presence in few of the symptomatic plants grown from self harvested and commercial seeds.
All wild cucurbit (golden creeper, balsam pear, wild cucumber, and bur cucumber) developed downy mildew symptoms and fresh sporangia on their detached leaves when inoculated with Ps. cubensis sporangia. There was also some indication that leaves collected from the wild cucurbits in the field contained some Ps. cubensis sporangia and sporangiophore. When greenhouse grown Golden creeper plants transplanted to a downy mildew infected field, some of the plants developed downy mildew symptoms. The results indicate that Ps. cubensis has the potential to infect wild cucurbits and that downy mildew could occur naturally in wild cucurbits. The PCR analysis also confirmed presence of Ps. cubensis in the seed of bur and wild cucumber, and the rhizomes of golden creeper, bur and wild cucumber. Further research could be conducted to determine whether the pathotype that infects wild cucurbit is the same as the one causing downy mildew in cultivated cucumber in the field.
There was no evidence of oospore production or overwintering observed in the studies. Typically, zoosporangia have limited survivability and previously reported to survive 3-4 months at −18 °Celsius. Given the extreme Ontario weather, zoosporangia are likely not a viable source of inoculum. Also, when heavily infected leaves were frozen at −20 °Celsius for 6 months and then used to infect other detached cucumber leaves, none of the leaves showed presence of lesions or fresh sporangia. Transmission of airborne inoculum from greenhouses to the field was thought to be unlikely as the field outbreaks in the Ontario region were consistent with reports of field outbreaks from Michigan, United States as opposed to reports of downy mildew in the greenhouses.
The results of these studies were made available to the grower and scientific communities through annual presentations at local grower meetings (for example Ontario Fruit and Vegetable Convention, the Canadian Phytopathological Society meetings and the Ontario Pest Management Conference). The information presented in this study is useful for growers and crop specialists as it reverses the previous belief that field outbreaks of downy mildew originate from greenhouses, while suggesting the need to identify the pathotype that resides in Ontario greenhouses and in wild cucurbits. Based on this study, air borne tracking of spores is a great management strategy for Ontario growers to manage the disease in their field. Due to seed and fruit transmission of downy mildew as shown in this study, it is an important management strategy to produce disease free commercial seeds. Export and import of cucumber seed and fruit may require testing in future as more evidence becomes available on the role of these sources of inoculum. Both growers and crop specialists will benefit from the new knowledge on possible cucumber downy mildew transmission pathways derived from this project.