How to Manage Pests

UC Pest Management Guidelines

Carrot

Black Rot (Black Crown)

Pathogen: Alternaria radicina

(Reviewed 1/09, updated 9/12)

In this Guideline:


SYMPTOMS AND SIGNS

Black rot starts in the carrot crown at the point of leaf attachment, beginning with the outermost petioles; eventually all the leaves can be killed. The upper part of the carrot root will show a dark rot, which occasionally may continue into the lower part of the carrot root. Leaves weakened or rotted by black rot may break off when gripped by mechanical harvesters, resulting in roots being left in the ground. Black rot can also be a problem in storage.

Under certain conditions, the fungus also can cause a leaf blight, which is characterized by dark brown lesions along the leaf margins. The fungus can also cause a pre- and postemergence damping-off of carrot seedlings.

COMMENTS ON THE DISEASE

The fungus is favored by sprinkler irrigation or rainy weather and high temperatures, which may predispose tissue to infection. Alternaria radicina is seedborne and may be spread on carrot seed. It also survives in carrot debris and in the soil for several years. Black rot is important only in certain areas or regions in California. It is common in coastal mountain valleys, e.g., the Cuyama Valley.

MANAGEMENT

Cultural Control

Plant Alternaria-indexed seed or treat seed in a hot water bath; this is especially important in fields where black rot has not been identified or carrots have not been grown.

Furrow rather than sprinkler irrigation may reduce disease development. Crop rotation is highly recommended to prevent buildup of the fungus in the soil. Deep tillage may provide some control by burying inoculum of the fungus away from the carrot crown.

Organically Acceptable Methods

Cultural controls are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

Assay seed; if pathogen is found, treat seed. Foliar applications of fungicides to control black rot are marginally effective.

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.
 
SEED TREATMENT
 
A. IPRODIONE
  (Rovral 4F) 8 fl oz/6 gal water NA 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Soak seeds for 24 hours at 86°F (30°C) in a solution of iprodione and water. Use 6 gal of solution for 3 lb of carrot seed. Use of this material allowed under a 24(c) registration.
 
B. HOT WATER DIP#   NA NA
  COMMENTS: Soak seed in hot water (122°F; 50°C) for 25 minutes. Do not soak longer or seed could be damaged.
 
FOLIAR TREATMENT
 
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: Repeat applications at 7- to 10-day intervals if necessary to maintain control. After completion of the REI, follow safety precautions detailed on the label for 6.5 days.
 
C. IPRODIONE
  (Rovral 4F) 1–2 pt 24 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Do not make more than 4 applications of iprodione per season.
 
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.
 
G. 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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