How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

Purple Blotch and Stemphylium Leaf Blight

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

(Reviewed 1/07, updated 6/08, pesticides updated 6/16)

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 per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

UPDATED: 6/16
Calculate impact of pesticide on air quality
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. 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. PYRACLOSTROBIN
  (Cabrio EG) 8–12 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not exceed 72 oz/acre per season or make more than two sequential applications before rotating to a fungicide with a different mode-of-action group number.
 
C. CHLOROTHALONIL
  (Bravo Weatherstik, Echo 720) Label rates 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
 
D. MANCOZEB
  (Penncozeb 75DF) 2–3 lb 24 7
  (Dithane M45) 3 lb 24 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Do not apply more than 30 lb/acre per crop. For use on bulb onions, garlic, and shallots only; do not apply to exposed bulb.
 
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.
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.

[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
T. Turini, UC Cooperative Extension, Fresno County
A. Ferry-Abee, UC Cooperative Extension, Tulare and Kings counties

Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara and San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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