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UC Pest Management 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 6/08)

In this Guideline:


SYMPTOMS AND SIGNS

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)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider infomation relating to environmental impact.
 
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. 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/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 GROUP NAME (NUMBER1): Multi-site contact (M5)
  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 GROUP NAME (NUMBER1): Multi-site contact (M3)
  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 GROUP NAME (NUMBER1): Multi-site contact (M3)
  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 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 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[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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