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Basil downy mildew

Quick facts

  • Basil downy mildew can spread rapidly and result in complete yield loss.
  • Infection starts on lower leaves and moves up the plant. The disease causes pale yellow areas on the upper side of the leaf with velvety grey fuzz on the underside.
  • The pathogen that causes basil downy mildew can be transmitted on seed, transplants or fresh leaves.
  • Monitor all seedlings and transplants closely for yellowing leaves and gray downy growth on the lower surface of the leaf.
  • Resistant varieties are the best way to prevent basil downy mildew.
Basil leaves with holes from basil downy mildew

How to tell downy mildew from other basil issues

  • Infection starts on lower leaves and moves up the plant.
  • Infected leaves first turn yellow in areas restricted by major veins. With time, the entire leaf turns yellow.
  • Irregular black spots appear on infected leaves as they age.
  • Fluffy gray spores grow on the underside of infected leaves.

Biology

Closeup of a basil plant showing back of leaves infected with downy mildew spores.
Leaves infected with basil downy mildew
  • Basil downy mildew is caused by Peronospora belbahrii, which is a water mold as opposed to a true fungus.
  • Basil downy mildew can be transferred by seed, transplants or fresh leaves. Infected transplants and leaves may not show symptoms if maintained in cool, dry conditions. Spores of P. belbahrii can also travel long distances on moist air currents.
  • Peronospora belbahrii tolerates cool weather and can infect and produce spores in temperatures as low as 59°F. But it thrives in warm, humid conditions. As a result, the most devastating damage is often in late summer.

Peronospora belbahrii needs two different mating types to produce tough resting spores known as oospores. Only one mating type is present in the USA. As a result, no oospores form and the pathogen will not be able to survive Minnesota's harsh winters. This may change if the second mating type comes to the country.

Managing downy mildew in the home garden

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Managing downy mildew on the farm

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Authors: Marissa Schuh, horticulture IPM extension educator, and Michelle Grabowski

Reviewed by Natalie Hoidal, Extension educator

Reviewed in 2022

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