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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Carrot

Black Rot (Black Crown)

Pathogen: Alternaria radicina

(Reviewed 1/09, updated 1/09)

In this Guideline:

SYMPTOMS

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/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider information relating to environmental impact.
 
SEED TREATMENT
A. IPRODIONE
  (Rovral) 4F 0.5 lb/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. IPRODIONE
  (Rovral) 4F Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Do not make more than 4 applications of iprodione/season.
 
+ 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 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.
# 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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