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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.