How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

Botrytis Leafspot

Pathogen: Botrytis cinerea

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

In this Guideline:


SYMPTOMS

Botrytis leafspot occurs on onions. White sunken spots on leaves are usually the first sign of infection; spots are small—0.06 inch (0.5 mm) up to 0.25 inch (6 mm) long—and tend to be oval. They sometimes have a light green halo and may appear water soaked. The epidermis around the spots may be silvery. When numerous spots are present, leaf tips die back and whole leaves may be killed.

COMMENTS ON THE DISEASE

Botrytis cinerea spores land on leaf surfaces and, in the presence of moisture, germinate and produce enzymes that kill leaf tissue. The fungus damages the leaf by causing leaf spotting. Leaf surfaces must be wet by dew or rain for long periods (20 or more hours) for leafspot to develop. Optimum temperature for germination of spores is 59°F; optimum temperature for mycelial growth is in the mid-70s (°F).

MANAGEMENT

Cultural Control

Isolate seed fields from fresh market or processing onion fields as bulbs are a major source of spore inoculum. To reduce the level of inoculum in the soil, use a 3-year rotation scheme away from Allium crops and destroy volunteer and cull onions during this period.

Chemical Control

Monitor fields and apply a treatment at the first evidence of leaf spotting.

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

UPDATED: 6/16
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Bee precaution pesticide ratings
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. CHLOROTHALONIL
  (Bravo Weatherstik, Echo 720) Label rates 12 Onion (Dry bulb) and Garlic: 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
 
B. MANCOZEB
  (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 more than 24 lb a.i./acre per 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 until the harvest may take place. In some cases the REI exceeds the PHI 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. 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 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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