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Basil downy mildew is taking down MN basil

Sweet basil plant with yellowing leaves with brown blotches on leaves
Sweet basil plant infected with basil downy mildew
M. Grabowski, UMN Extension 

Warm summer days are typically a great time for making pesto and caprese salad. This year many gardeners, disappointed by basil plants with yellowing leaves with dark blotches, are wondering what went wrong. The answer is a plant disease called basil downy mildew.

First found in Minnesota in 2012, downy mildew is a devastating disease of basil, resulting in infection and complete loss of the plant. Basil downy mildew is caused by Peronospora belbahrii, a water mold that thrives in warm humid conditions. The pathogen cannot survive Minnesota’s harsh winter weather, but can be brought into the state on infected seeds, transplants, or as spores on warm moist air currents. Weather conditions have been ideal for basil downy mildew in Minnesota this summer.

How do I know if my basil has downy mildew?

  • Areas of the leaf between major veins turn yellow
  • Dark brown to black blotches form on leaves
  • Infected leaves fall off
  • On the lower leaf surface, gray powdery spores form, making the leaf look dirty
close up of basil leaves with yellow and brown discoloration between leaf veins
Yellowing of basil leaves infected with downy mildew
M. Grabowski, UMN Extension 

lower leaf surface of a basil plant covered with gray powdery spores
Downy mildew spores on the lower leaf surface of  basil
M. Grabowski, UMN Extension 

Management options

If the basil plants are not showing symptoms of basil downy mildew, gardeners should work to keep the leaves dry. Selectively harvest to open up airflow through the center of the plant and between neighboring plants. Remove any weeds that might be crowding the plant. When water the plants, use a soaker hose, drip irrigation, or direct water at the soil below the plant to prevent splashing the leaves.

Once a plant is infected with basil downy mildew, there is nothing that can be done to save the plant. Gardeners should harvest any remaining healthy leaves as soon as possible. The infected plant should then be buried under garden soil or in the compost pile to prevent spores from blowing to nearby plants. The basil downy mildew pathogen will not survive the winter in soil or compost.

M. Grabowski, UMN Extension educator

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