Basil downy mildew has been confirmed in the field in Norfolk County. The affected basil was infected in the field and was not directly exposed to previously infected potted basil plants. This suggests that airborne spores were widespread over the past week and the disease is likely to begin showing up throughout southern Ontario fields. Growers need to apply fungicides preventatively before symptoms are widespread in the field. Growers also need to scout fields regularly to identify symptoms before they become widespread.

In the 5 years this disease has been confirmed in Ontario, this is the earliest known spread of the disease in the field. In other years, potted basil in home gardens had been identified as early as May but the disease did not spread to field-grown basil until early August. Early infection could lead to loss of the crop before significant product can be harvested from the field.

The following preventative fungicides are registered for management of this disease.  Note that the information does not replace the product label.  Before application, consult the product label.

• Ranman and Torrent (cyazofamid), at a rate of 0.20-0.22 L/ha is registered for control of downy mildew in field and greenhouse basil.  There is a 12-hr re-entry (REI), 0 day preharvest interval (PHI) and a maximum of 4 applications of Ranman per year.  Do not exceed more than one application of Ranman/Torrent or other Group 21 fungicide out of every three fungicide applications.
• Revus (mandipropamid) is also registered for control of downy mildew in field and greenhouse basil at a rate of 583 ml/ha, with a 12-hr REI, 1 day PHI and a maximum 4 applications of Revus per year.  Rotate with fungicides that are not in Group 40.
• Confine Extra (phosphorous acid) is registered for suppression of downy mildew on field basil at a rate of 3-5 L/ha, with a 1 day PHI.  Do not make more than 6 applications of Confine Extra per season, and rotate with fungicides that are not in Group 33.

There are no organically acceptable products 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.