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UC Pest Management Guidelines


Downy mildew sporulation on onion leaf.

Onion and Garlic

Downy Mildew

Pathogen: Peronospora destructor

(Reviewed 1/07, updated 6/08)

In this Guideline:


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/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of 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/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 1.5–2 lb 48 7
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) and A multi-site (Group M3)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) 1.5–2 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. CHLOROTHALONIL
  (Bravo Weatherstik, Applause, Echo) 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 R.E.I.
 
E. MANCOZEB
  (Penncozeb) 75DF 2–3 lb 24 7
  (Dithane M45) 80W 2–3 lb 24 7
  (Dithane F45) 37%F 1.7–2.4 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.
 
F. MANEB 80W 2–3 lb 24 7
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Registered for use on garlic and bulb and green onions only. Do not apply to exposed bulb.
 
G. COPPER# Label rates 24 check label
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Not as effective as other materials, but some products are acceptable for use in an organically certified crop.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 (for more information, see http://www.frac.info/). 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Diseases
R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara/San Luis Obispo Counties
R. E. Voss, Vegetable Crops, UC Davis

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