Basil downy mildew has been identified in Norfolk and Prince Edward Counties in Ontario and is likely to be present elsewhere over the next week. Basil downy mildew is a highly destructive disease that can completely defoliate a plant within a few weeks if not protected by fungicides. Resistant cultivars of basil are starting to become available but have not yet been widely grown by commercial growers.
Basil downy mildew is caused by the oomycete (water mould) pathogen Peronospora belbahrii. The first symptoms of disease appear as a mild chlorosis (yellowing) of sections of the leaf often confined by the leaf veins (Figure 1). If you flip over the leaves, especially in the morning, you will find a patch of purplish-grey spores (sporangia) under the affected areas (Figure 2). These spores are spread on the wind and once present in a field will infect all the plants in the field within a week or two. Eventually the whole leaf will turn yellow and then fall off the plant. Infection by this pathogen does not require extended periods of leaf wetness like some other downy mildews and can infect in the normal dew periods at this time of year.
Figure 1. Yellowing of basil leaves collected on July 31 due to basil downy mildew. The brown lesions are atypical and are likely due to secondary infection of the damaged tissues.
Figure 2. Purplish/grey fuzzy spores on the underside of the leaves.
Conventional growers should apply targeted downy mildew fungicides immediately if not already applied. The registered fungicides for control of downy mildew on field-grown basil include Orondis Ultra, Torrent, Revus, Reason, and Presidio. A rotation of these products should provide effective control of the disease while ensuring resistance does not develop in the pathogen. Note that the active ingredient in Revus is present in Orondis Ultra, so Revus should not be rotated with this fungicide. Confine Extra is also registered for suppression of downy mildew in the field. While this product can be an effective preventative fungicide in rotation with other more targeted fungicides, suppression of the disease is not acceptable once disease risk is high.
There are no organic fungicides that provide effective control of this disease. Growers will need to continually scout the crop and consider harvesting any remaining basil as soon as the first symptoms appear in the field. However, note that the symptoms can continue to develop postharvest if already infected. Ensuring good airflow through the plants can delay the disease somewhat. Once resistant cultivars become more available, use of these cultivars will be essential for organic growers.