Downy Mildew

At the field level, symptoms of downy mildew are first noticed as circular clumps of yellowed plants that are a few to many feet in diameter. As the disease progresses, the yellowing patterns often enlarge in the direction of prevailing winds.

On individual plants, leaf tissue becomes pale green, then tan to brown or yellow, and finally collapses. Lesions may exhibit a violet-to-purple color. When leaves are wet, or when humidity is very high, signs of Peronospora destructor are visible on the surface of older leaves as fine, furry, grayish-white growths. These growths may later turn purple or brown as a result of a secondary infection of the lesion by other fungi, such as those that cause PURPLE BLOTCH AND STEMPYLIUM LEAF BLIGHT, as these fungi can produce purple pigmentation and dark spores.

In addition to leaves, the downy mildew pathogen can infect seed stalks and flower parts and can infect bulbs systemically. After storage, systemically infected bulbs may become soft and watery. Symptoms of this disease are typically observed in the winter in California.

Peronospora destructor only infects species in the genus Allium, including garlic, onion, shallot, chives, and leek. The pathogen only grows and produces spores in live hosts but may survive in plant debris or soil. Initial sources of the pathogen can include infected bulbs, sets, seeds, and plant debris.

Downy mildew has complex environmental requirements, needing both cool temperatures and high humidity. Based on studies in Canada and Europe, spore production occurs at or above a relative humidity of 95% in the canopy. Spore production declines at temperatures above 75°F and may be suppressed completely if temperatures are sustained above 82°F for more than four hours, or above 84°F for more than two hours. Nightly rainfall can also suppress spore production.

Spores are airborne. After landing on healthy plants, they require leaf wetness for infection to occur. The length of leaf wetness required is directly proportional to air temperature. The research mentioned above assumes that for air temperatures of 43° to 61°F, only 2 to 3 hours of leaf wetness is necessary for infection, whereas infection requires 5 hours of leaf wetness at 61° to 68°F. The accuracy of these models for describing downy mildew in California, where it typically occurs during the winter, is unknown.

The time between infection and sporulation can range from 8 to 16 days, but spores produced during a given night can infect new plants the following morning, and up to 3 days later. Therefore, downy mildew can develop into a damaging epidemic very quickly under favorable conditions.

To reduce the incidence and severity of downy mildew:

• Minimize canopy leaf wetness:
  • Avoid sprinkler irrigation, especially when the canopy begins to fill.
  • If avoiding sprinkler irrigation is not feasible, adjust the irrigation schedule to maximize leaf drying and prevent extended leaf wetness between irrigation events.
  • Plant in fields where there is good air movement.
  • Select fields that are well drained.
  • Align rows with the prevailing winds.
  • Change planting date to avoid having a full canopy during conditions favorable for downy mildew.
• Use a 3-year rotation away from Allium crops in fields where the disease has occurred.
• Use disease-free bulbs, sets, and seed.
• Remove and destroy material of any Allium plants, including residue from the previous crop, volunteer plants, and culls from storage.
• Avoid entering fields when leaves are wet.
• Avoid injuring the crop with herbicides and other materials.

There are also currently a few red onion cultivars (for example, Calred) that are resistant to downy mildew, but this resistance is active only in the flower stalks and not the leaves.

Spray at the first sign of the disease. After the first spray, scout fields and make subsequent applications when weather conditions are favorable for the disease. Rotate with fungicides from different mode-of-action groups to reduce the risk of fungicide resistance development. Fungicides may be applied on a 7-day schedule (only as consistent with the label), if necessary. For all fungicides, thorough coverage of foliage is important in the control of downy mildew.