How to Manage Pests

UC Pest Management Guidelines

Carrot

Powdery Mildew

Pathogen: Erisyphe polygoni

(Reviewed 1/09, updated 1/09)

In this Guideline:


SYMPTOMS AND SIGNS

Older leaves and leaf petioles of plants with powdery mildew are usually covered with a white, powdery growth. Young leaves develop small, circular, white, powdery spots that slowly enlarge to cover the leaflets. A slight chlorosis or yellowing may appear on leaves infected by the fungus.

COMMENTS ON THE DISEASE

The fungus survives on overwintered carrots and related weed hosts. The airborne spores may be carried great distances. Foliage symptoms usually do not appear until leaves are mature, and leaves can survive even with heavy infections. Fungal infection and disease development are favored by high humidity during evening and morning hours, and temperatures between 55° and 90°F.

MANAGEMENT

Cultural Control

Use tolerant cultivars and maintain good plant vigor.

Organically Acceptable Methods

Cultural control and sprays of sulfur and Bacillus subtilis are acceptable for use on organically grown produce.

Treatment Decisions

Chemical controls are not recommended, unless infections appear very early in the growing season.

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.
 
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. 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.
 
C. 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.
 
D. SULFUR# 3-10 lb 24 NA
  . . . or . . .
  SULFUR DUST# Label directions 24 NA
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Apply at first sign of disease and repeat at 14 days as needed to maintain control.
 
E. BACILLUS SUBTILIS
  (Serenade MAX)# 1–3 lb 4 0
  (Serenade ASO)# 2–4 qt 4 0
  MODE OF ACTION: A biological fungicide.
  COMMENTS: Serenade ASO has a special local needs registration for organically grown carrots. Apply Serenade MAX as a foliar spray; thorough coverage is essential. Serenade ASO is applied with chemigation equipment.
 
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.
NA Not applicable.

[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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