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Basil Downy Mildew found in Minnesota

M. McGrath, Cornell University,

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.

M. Grabowski, UMN Extension

Photo 2: severe damage on basil caused by downy mildew

Basil downy mildew appears to only infect basil (Ocimum sp.). Sweet basil, Ocimum basilicum, is highly susceptible to the disease. Ornamental or exotic basil (Ocimum citriodorum and Ocimum americanum) can be infected by basil downy mildew but symptoms are less severe than in sweet basil. More information about susceptibility of common basil varieties can be found at the 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 online at or by phone at 1-888-545-6684.

M. McGrath, Cornell University,

Photo 3: Gray sporulation of downy mildew on the loser surface of infected basil leaves

Gardeners do not have many management options available this late in the growing season. Healthy leaves can be harvested and eaten. Infected plants should not be brought indoors to overwinter. Bury or burn infected plant debris. In 2013, gardeners should inspect all basil transplants for symptoms prior to purchase. Grow a less susceptible variety of basil like Lemon, Blue Spice or Thai basil.

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