Basil downy mildew was first identified in Ontario in 2010 and caused devastating losses in the first few years after its introduction (Figure 1). The disease often shows up in Ontario in late July or early August, and once in a field, it can cause complete defoliation and 100% yield loss within a few weeks. After several years, new conventional fungicides were registered that provided good control, but there were few options for organic producers. Since then, new resistant or tolerant cultivars of basil have been released, and these have provided the best option for managing the disease for both conventional and organic producers.

Figure 1. Basil downy mildew begins as a yellowing of sections of the leaf with purplish spores produced on the underside of the leaves in the morning. Defoliation occurs within a few weeks in susceptible cultivars without the use of fungicides.

Over the past few years, the pathogen (Peronospora behlbarii) has been able to overcome the resistance of some of the earliest released resistant cultivars. New strains have emerged that may overcome resistance in one cultivar, but may not affect another, depending on what strains are in a particular field and the basis of the genetic resistance of the cultivar. The following article in Growing Produce magazine provides a good overview of the types of resistance, the breakdown of the resistance of some cultivars, and how best to manage the disease going forward: Research Finding Better Ways To Deal With Basil Downy Mildew

Key lessons for Ontario basil growers include:

1. A cultivar that was resistant in the past may not be resistant anymore. You may need to grow the newest released cultivars each year to stay ahead of the pathogen’s ability to overcome resistance. Note: Some newly released cultivars may not be resistant to all strains of the fungus.
2. Learn more about the resistant cultivars you intend to purchase by contacting the seed company to find out about the types of resistance in their cultivars and any reports that the resistance is breaking down.
3. Do not rely solely on the use of resistance cultivars. The fungus can overcome resistance over time. Use other measures to reduce disease pressures, including the use of fungicides, wider plant spacing, avoiding overhead irrigation, and managing crop debris.

For fungicides registered for control of basil downy mildew in Ontario, visit the Ontario Crop Protection Hub