M. McGrath, Cornell University, Bugwood.org
Photo 1: Angular leaf yellowing caused by downy mildew on basil
Michelle Grabowski, University of Minnesota Extension Educator
Downy mildew of basil has been officially reported in Ramsey and Washington counties in Minnesota. This new disease was first reported in Florida in 2007. Since then it has spread to many more states.
Basil downy mildew is caused by the pathogen Peronospora belbahrii. This pathogen thrives in warm, humid conditions. It can move into the garden in infected seed or transplants or as airborne spores.
Plants infected with basil downy mildew first display yellowing of lower leaves. Upon close examination, gardeners will notice that the yellowing appears to occur in sections restricted by major veins. This causes a blocky or angular yellow sections on the leaf. If the lower surface of the leaf is examined, dark colored spores can be seen as a dirty looking fuzz that grows directly below the yellow sections of the leaf. A small magnifying glass may be useful in viewing these spores. As the disease progresses, infected sections of the leaf turn dark brown to black and leaves may fall off.Cornell University webpage.
Symptoms of basil downy mildew can easily be confused with several disorders including nutrient deficiency and sunscald. It is therefore important for the disease to be confirmed by a laboratory examination. The Minnesota Department of Agriculture requests that suspected cases of basil downy mildew be reported to the Arrest a Pest Hotline at onlinne at Arrest.the.Pest@state.mn.us or by phone at 1-888-545-6684.