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UC Pest Management Guidelines


Leaf spotting and dieback of leaf tips caused by Botrytis.

Onion and Garlic

Botrytis Leafspot

Pathogen: Botrytis cinerea

(Reviewed 1/07, updated 6/08)

In this Guideline:


SYMPTOMS AND SIGNS

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

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider general properties of the fungicide as well as information relating to environmental impact.
 
A. CHLOROTHALONIL
  (Bravo Weatherstik, Applause 720) 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.
 
B. MANCOZEB
  (Dithane F45) 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/crop.
 
C. 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.
 
+ 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 (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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