Basil downy mildew has been identified in both Norfolk and Wellington Counties on July 19. Infections probably began sometime last week. It is likely the disease is now widespread in southern Ontario and basil growers need to protect the crop now with a rotation of products registered for “control”. There are 5 fungicides registered for control of basil downy mildew in Canada: Reason 500 SC, Torrent 400 SC, Revus, Orondis Ultra, and Presidio. Orondis Ultra contains mandipropamid (Revus), so do not rotate Orondis with Revus. Presidio must be tank-mixed with another registered product. Confine Extra is also registered for suppression, but suppression products are not a good option once the disease is confirmed in the field.
The products registered for this disease are listed in the “Herbs” section of OMAFRA Publication 838 Vegetable Crop Protection Guide (Page 140 of the 2021 Guide). Also consult the label for additional details and use precautions.
There are no fungicides available for organic growers. Once the disease is identified in the field, it may be possible to conduct one last harvest. Keep in mind though that the disease can continue to progress postharvest, since basil must be stored at higher temperatures to avoid chilling injury. New resistant cultivars are available and these would be the best method of controlling the disease organically in the future. Otherwise, promoting good airflow through the canopy may help to delay the onset of disease.