UC IPM Online UC ANR home page UC IPM home page

UC IPM Home

Search

SKIP navigation
How to Manage Pests

Home & garden
Agriculture
Natural environments
Exotic & invasive

Weather data & products
Degree-days
Interactive tools & models

Educational Resources

Publications & more
Workshops and events
PCA exam helper
Pesticide information

Research and IPM

Grants programs
Funded-project results

 
  Printer-friendly version

How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Purple blotch lesion on onion leaf.

Onion and Garlic

Purple Blotch and Stemphylium Leaf Blight

Pathogens: Purple blotch: Alternaria porri
Stemphylium leaf blight: Stemphylium vesicarium

(Reviewed 1/07, updated 1/07)

In this Guideline:

SYMPTOMS

Purple blotch and Stemphylium leaf blight occur primarily on onions as oval-shaped tan and deep purple lesions on leaf blades. Yellow streaks, which turn brown, extend along the blade in both directions from the lesion. In advanced stages lesions may girdle and kill leaves and seed stems. Concentric zones may develop within the lesions.

COMMENTS ON THE DISEASES

Stemphylium leaf blight is more common than purple blotch in California, but symptoms are identical for both diseases and they are managed in the same manner. These diseases are favored by heavy dew in desert areas and by foggy and rainy weather in other regions; optimum temperature for disease development is in the mid-70s (°F). The spores are airborne. In California, these diseases are often associated with downy mildew lesions on onions; they occur less commonly on garlic. Infection of seed stalks can reduce seed yield and quality.

MANAGEMENT

Chemical Control
Treatment for downy mildew also controls purple blotch and Stemphylium leaf blight. These diseases are usually not a problem after the end of the rainy season except in the Imperial Valley where they can cause damage up to harvest under conditions of high humidity and heavy nighttime dew.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider information relating to environmental impact.
 
A. FENAMIDONE
  (Reason) 500 SC 5.5 fl oz 12 7
  MODE OF ACTION: A QoI (Group 11)1 fungicide.
  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. PYRACLOSTROBIN
  (Cabrio) EC 8–12 oz 12 7
  MODE OF ACTION: A QoI (Group 11)1 fungicide.
  COMMENTS: Do not exceed 72 oz/acre/season or make more than 2 sequential applications before rotating to a fungicide with a different mode of action Group number.
 
C. CHLOROTHALONIL
  (Bravo Weatherstik, Applause) 1.5–3 pt 12 Garlic, bulb onions: 7
  (Echo) 1–2 pt 12 Green onions, leeks, shallots: 14
  MODE OF ACTION: A multi-site contact (Group M5)1 chloronitrile fungicide.
  COMMENTS: See label regarding special instructions related to the 12 hour R.E.I.
 
D. MANCOZEB
  (Penncozeb) 75DF 3 lb 24 7
  (Dithane M45) 80W 3 lb 24 7
  MODE OF ACTION: A multi-site contact (Group M3)1 dithiocarbamate fungicide.
  COMMENTS: Do not apply more than 30 lb/acre/crop. For use on bulb onions, garlic, and shallots only; do not apply to exposed bulb.
 
E. MANEB 80W 2–3 lb 24 7
  MODE OF ACTION: A multi-site contact (Group M3)1 dithiocarbamate fungicide.
  COMMENTS: For use on garlic and bulb and green onions only; do not apply to exposed bulb.
 
 
+  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 until the harvest may take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
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. For more information, see http://www.frac.info/.

[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 Co.
Acknowledgment for contributions to the diseases section:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara/San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

Top of page

Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2007 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See How to manage pests, or in the U.S., contact your local Cooperative Extension office for assistance. /PMG/r584100211.html revised: January 31, 2007. Contact webmaster.