Basil downy mildew has been reported from a basil field in southwestern Ontario. Initial infection probably occurred within the last 2 weeks. Over the past five years, the average first report has been around August 1. Conventional basil growers should be applying preventative fungicides on a regular basis. Torrent, Revus and Confine Extra are registered for control of basil downy mildew. These should be applied in a rotation to prevent the development of resistance. Consult to product label for detailed application instructions. Products registered for herbs are listed in the Vegetable Crop Protection Guide – OMAFRA Publication 838.
There are no effective organic products registered to control basil downy mildew. Organic growers should encourage good air movement around the plants and scout regularly for the disease. When the disease first appears, affected leaves should be removed and destroyed. This will only delay the inevitable development of widespread disease. Consider harvesting the crop early to salvage it before widespread damage.
All basil growers should keep in mind that the disease can continue to develop postharvest. If the leaves have already been infected, they can begin to discolour and sporulate in the moist and mild conditions of basil storage. This is not an issue if the basil is to be dried, as long as the leaves are dried immediately after harvest.
Basil downy mildew usually first appears within the canopy on older leaves. Sections of the affected leaves turn a lighter green/yellowish colour (Figure 1). When the leaves are turned over, especially in the morning, purplish/grey fuzzy spores are often present (Figure 2). Eventually the whole leaf turns yellow and falls off of the plant. The leaves may also develop brown spots as secondary weak fungi move in and cause additional damage.
Figure 1. Symptoms of basil downy mildew begin as yellowing of sections of the leaf.
Figure 2. The underside of infected leaves will develop purplish fuzzy spores.