Basil Update – Downy Mildew Now Widespread


Over the last few weeks, we have had numerous calls from homeowners and growers wondering why their basil is turning yellow and defoliating.  The reason in most cases is downy mildew.  Basil downy mildew seems to be particularly common this summer, likely due to the rainy weather and the fact that the disease first appeared in the field in mid-July rather than August which has been more typical.

In Ontario, the fungicides cyazofamid (Ranman and Torrent), mandipropamid (Revus) and phosphorous acid (Confine) are registered for control of downy mildew in commercial field basil.   All of these products are preventative, and will have limited effect on the disease once symptoms are widespread in the field.  It is important for growers to also be aware that once leaves are infected with downy mildew, it takes at least a week for symptoms to develop. Consequently, seemingly healthy leaves at harvest may develop symptoms…

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