Agriculture: Onion and Garlic Pest Management Guidelines

Downy Mildew

Symptoms and Signs

Downy mildew can infect both onions and garlic. The first evidence of disease is a fine, furry, grayish white to purple growth on the surface of older leaves. Leaf tissue under the growth becomes pale green, then yellow, and finally collapses. Large, yellowish, circular clumps of infected plants, a few to many feet in diameter, may be the first symptom noticed in the field. The yellowing patterns often enlarge in the direction of prevailing winds.

Comments on the Disease

Downy mildew can develop from an initial infection by airborne spores into an epidemic very quickly if humidity and temperature conditions (1.5 to 7 hours of leaf wetness and 43° to 80°F) are favorable. Spores can travel long distances in moist air, but are quickly killed by dry conditions. Initial sources of disease can be infected bulbs, sets, seeds, and plant debris.

Management

Cultural Control

Use disease-free bulbs, sets, and seed. Use a 3-year rotation away from Allium crops in fields where the disease has occurred. Destroy volunteer Allium plants in and around the field and buildings. Locate onion fields where there is good air movement to promote rapid drying of foliage. Currently there are a few red onion cultivars (e.g., Calred) that are resistant to downy mildew.

Chemical Control

Spray at the first sign of disease; fungicides may be applied on a 7-day schedule, 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 to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
A. FENAMIDONE
(Reason 500 SC) 5.5 fl oz 12 7
MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
COMMENTS: Do not apply more than 22 fl oz/acre per season or make more than one application before alternating with a fungicide that has a different mode-of-action group number.
B. MANCOZEB/MEFENOXAM
(Ridomil Gold MZ WG) 2.5 lb 48 7
MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) and Phenylamides (4)1 dithiocarbamate and Phenylamide (4)
COMMENTS: Registered for use on garlic, bulb onions, and shallots (dry bulb). Do not apply to exposed bulb.
C. MEFENOXAM/CHLOROTHALONIL
(Ridomil Gold Bravo SC) 2.5 lb 48 Bulb onions: 7
Green onions: 21
MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) and Multi-site contact (M5)
COMMENTS: Registered for use on garlic, bulb onions, green onions, and shallots (dry bulb).
D. DIMETHOMORPH
(Forum) 6 fl oz 12 0
MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxylic acid amides (40)
E. CHLOROTHALONIL
(Bravo Weatherstik, Echo 720) See label 12 Garlic, bulb onions: 7
Green onions, leeks, shallots: 14
MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
COMMENTS: See label regarding special instructions related to the 12 hour REI
F. MANCOZEB
(Penncozeb 75DF) 2–3 lb 24 7
(Dithane M45) 3 lb 24 7
(Dithane F-45 37%F) 2.4 qt 24 7
MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
COMMENTS: Registered for use on garlic, shallots, and bulb onions only. Do not apply to exposed bulb.
G. COPPER#
Label rates 48 See label
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.
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 actions. 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, 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.
Text Updated: 06/08
Treatment Table Updated: 06/16