How to Manage Pests

UC Pest Management Guidelines

Carrot

Cercospora Leaf Blight

Pathogen: Cercospora carotae

(Reviewed 1/09, updated 1/09)

In this Guideline:


SYMPTOMS AND SIGNS

Cercospora blight can occur on any above ground carrot tissue. However, symptoms are usually most severe and obvious along leaflet margins. Infection sites initially appear as a brown spot with a dark brown margin. A chlorotic halo often develops around these spots. As the disease progresses, the leaflets become yellowed and curl at the margins. Lesions on leaf petioles, stems, and flower parts are usually elongated and dark brown. Under heavy disease pressure, severe loss of foliage may occur.

COMMENTS ON THE DISEASE

Cercospora blight is usually of minor importance in California but often occurs in coastal carrot-growing regions. Cercospora carotae can be seedborne, but also survives between crops on plant debris or in the soil. Infection occurs over a wide range of temperatures with an optimum at 82°F.

MANAGEMENT

Cultural Control

Use disease-free seed or treat seed with fungicides before planting. Destroy diseased crop refuse by plowing it under to insure decomposition and practice 2- to 3-year crop rotations.

Organically Acceptable Methods

Cultural controls are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

In fields with a history of Cercospora leaf blight, start fungicide applications when conditions are conducive to disease development or when disease is observed.

Common name Amount per acre R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  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.
 
AFTER PLANTING
 
A. AZOXYSTROBIN
  (Quadris) 9.2–15.5 fl oz 4 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not make more than one application before rotating to a fungicide with a different mode of action. Do not make more than four foliar applications of strobilurin fungicides per crop.
 
B. CHLOROTHALONIL
  (Bravo Weatherstik) 1.5-2 pt 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  COMMENTS: Apply in sufficient water to obtain good coverage. Repeat applications at 7- to 10-day intervals as needed to maintain control. May be applied by sprinklers. Check the product label for specific application requirements.
 
C. COPPER HYDROXIDE Label rates 24 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Repeat applications at 7- to 14-day intervals, depending on disease severity.
 
D. PYRACLOSTROBIN
  (Cabrio) EG 8–12 oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not make more than two sequential applications before rotating to a fungicide with a different mode of action. Do not make more than three applications of strobilurin fungicides per crop.
 
E. TRIFLOXYSTROBIN
  (Flint) 2–3 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: To limit potential for resistance development do not apply more than 3 sequential applications or a total of 4 applications of all strobilurins per season. Do not apply more than 12 oz/acre per year.
 
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 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 Groupgroup numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action Groupgroup number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action Groupgroup number.
# Acceptable for use on organically grown produce.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438

Diseases

  • J. Nunez, UC Cooperative Extension, Kern County
  • R. M. Davis, Plant Pathology, UC Davis
  • T. A. Turini, UC Cooperative Extension, Fresno County
Acknowledgment for contributions to Diseases:
  • B. W. Falk, Plant Pathology, UC Davis
  • F. F. Laemmlen, UC Cooperative Extension, Santa Barbara County

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