Onions, Garlic: Downy mildew
Photo by: R.S. Byther
Biology
Downy mildew is a fungal disease affecting onions and related species, including weedy species. Leaves initially develop pale, elongate patches. The spots may be water-soaked at first, then later appear somewhat purplish. The spots become covered with a downy, grayish or white fungal growth. Affected leaves often die back. Bulbs of affected plants are smaller than normal and of poor quality. Infected bulbs may become shriveled and discolored in storage, or may sprout prematurely. Cool, damp weather favors spread of the disease.
Management Options
See "Using Integrated Pest Management (IPM) in the Landscape" for successful problem management techniques.
Non-chemical Management
Select non-chemical management options as your first choice!
- Plant disease-free materials in well-drained soils.
- Rotate crops. Do not plant onions and related crops into the same locations each year. Allow at least three years before replanting susceptible crops in a location which was previously diseased.
- Space plantings to provide good air circulation.
- Avoid overhead watering.
- Remove and destroy diseased plants and debris. Destroy or discard (do not compost) diseased materials.
- Control weeds in and around the garden.
Chemical Management
IMPORTANT: Always read and follow label directions when working with pesticides.
For more information on using pesticides visit :
- Apply when plants are 4 to 6 inches tall and repeat at 7- to 10-day intervals.
- Copper products can damage foliage.
Approved Pesticides
If you choose to use a pesticide, some examples of products that are legal in Washington are listed below. Some products are labeled for just INDOOR or just OUTDOOR use, or may allow both uses. Be sure to choose a product appropriate for your situation. Always read and follow all label directions.
WSU Extension programs and employment are available to all without discrimination.
Evidence of noncompliance may be reported through your local Extension office.