Symptoms and Signs
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.
Comments on the Disease
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Zampro)||14 fl oz||12||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone-outside inhibitor, stigmatellin binding type (45)/ carboxylic acid amide (40)|
|COMMENTS: For resistance management, no more than three applications per year or two sequential applications.|
|(Orondis Ultra)||5.5–8.0 fl oz||4||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Oxysterol binding protein analogue inhibition (49)/carboxylic acid amide (40)|
|COMMENTS: See label for resistance management.|
|(Reason 500 SC)||5.5 fl oz||12||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)|
|COMMENTS: Do not make more than one application before alternating with a fungicide that has a different mode-of-action group number.|
|(Ridomil Gold MZ WG)||2.5 lb||48||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) and multi-site contact (M3)|
|COMMENTS: Registered for use on garlic, bulb onions, and shallots (dry bulb). Do not apply to exposed bulb.|
|(Ridomil Gold Bravo SC)||2.5 pt||48||Bulb onion, garlic: 7
Green onion, leek, shallot: 14
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) and multi-site contact (M5)|
|COMMENTS: Registered for use on garlic, bulb onions, leeks, green onions, and shallots (dry bulb).|
|(Forum)||6 fl oz||12||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxylic acid amides (40)|
|COMMENTS: If spraying on the day of harvest, wait until spray dries to harvest the crop.|
|(Dithane F-45 Rainshield)||2.4 qt||24||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)|
|COMMENTS: Registered for use on dry bulb onion, garlic, and shallot. Do not apply to exposed bulb.|
|(Omega 500F)||1 pt||Hand weeding: 24||7|
|Other activities: 12|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Uncouplers of oxidative phosphorylation (29)|
|COMMENTS: Registered for use on bulb onion, garlic, and bulb shallot.|
|(Bravo Weatherstik, Echo 720)||Label rates||12||See label|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)|
|COMMENTS: For suppression. See label regarding special instructions related to the 12-hour REI.|
|(Basic Copper 53)#||1.9 lb||48||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)|
|COMMENTS: Not as effective as other pesticides, but some products are acceptable for use in an organically certified crop. Can cause phytotoxicity to leaves. Basic Copper 53 is allowed with restrictions in organically certified crop. OMRI certification expires December 31, 2019; after this date, check organic status before applying in organic onion or garlic production.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.|