Basil downy mildew was detected in an herb field in Norfolk County, Ontario this week. Symptoms were observed on only a few leaves of isolated plants. The dry conditions we have been experiencing in much of Ontario lately are not conducive to the spread of this disease, however foliar leaf diseases often become more prevalent during periods of longer night-time leaf wetness, as is typically seen in mid to late summer. Ontario basil growers should be scouting their fields for symptoms of downy mildew in basil.
Remember that the initial signs of basil downy mildew, yellowing of the upper leaf surface, can superficially resemble a nutritional problem. Plants infected with the disease can be distinguished by the purplish spores which typically develop on the lower leaf in areas corresponding to the yellow lesions on the upper surface. Additionally, yellowing due to downy mildew is usually restricted to sections of the leaf bound by the veins, because the fungus cannot grow past the main leaf veins.
As mentioned in a previous post, conventional herb growers in Ontario and Alberta can apply Revus (mandipropamid) and Confine (potassium salts of phosphorus acid) fungicides preventatively for the control of basil downy mildew during the 2011 field season, thanks to an Emergency Use registration from Canada’s Pest Management Regulatory Agency. This registration is effective until December 31st, 2011 only.
Unfortunately, no organically acceptable products are currently registered for the control of basil downy mildew in Canada. Cultural practices that promote air flow and reduce leaf wetness may help organic growers control this disease. These include locating plants away from sheltered areas, planting rows parallel to prevailing wind directions, reducing planting densities and using drip irrigation if possible.
For more information and to view photos of disease symptoms, refer to the following articles: