Onion Downy Mildew

Onion Downy mildew is a water mold or oomycete disease of alliums caused by Peronospora destructor. It causes irregular foliar lesions that begin as pale-green, then progress to yellow or brown necrotic tissue. Eventually, lesions coalesce and lead to the collapse of the leaf. During periods of high moisture, fuzzy, gray-to-violet sporangia appear on leaf surfaces. These symptoms can also be seen on seed stalks and flowers. Bulbs can be stunted and sponge-like during systemic plant infection and, when the disease infects the bulb itself, can become watery.

Primary Source: Infested soil, infected plant debris, and perennial alternate host plants
Spread: Airborne spores, wind currents, splashing water
Favorable Conditions: High humidity, moderate temperatures with an optimal temperature of 55ºF, high moisture

Infection & Disease Cycle

Peronospora destructor mainly overwinters in volunteer infected onions and those in cull piles as mycelium. It is also known to overwinter in perennial onion varieties, as well as soil-bound spores and plant debris. On nights with moderate temperatures and high humidity, overwintered mycelia produce spores (sporangia) that spread throughout the day via wind currents. These spores require free water, whether from rain, irrigation, or heavy dew to germinate and infect the plant, often beginning infection at the tops of leaves. Resulting lesions produce new sporangia and zoospores (swimming water spore borne out of the sporangia), which travel both down the leaves of individual plants and to new nearby plants via wind currents or splashing water.

Cultural Control

Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:

  • Plant resistant varieties when possible
  • Rotate away from susceptible crops ( 3+ years)
  • Plant in areas with good airflow
  • Maintain proper spacing between plants
  • Avoid overhead irrigation
  • Maintain proper soil moisture
  • Destroy plant debris, cull piles, volunteers

Chemical Control

For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.


  • Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
  • UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.



Written by Ariana Abbrescia, Amanda Gevens, Russell Groves, and Ben Bradford. Last updated Nov 2023

Photo Credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Caption: Early foliar symptoms of downy mildew on garden onion. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org
Caption: Progressed foliar decline caused by downy mildew on garden onion. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org