How to Manage Pests

UC Pest Management Guidelines

Carrot

Alternaria Leaf Blight

Pathogen: Alternaria dauci

(Reviewed 1/09, updated 9/12)

In this Guideline:


SYMPTOMS AND SIGNS

Alternaria leaf blight symptoms appear as dark brown to black irregularly shaped lesions on leaf blades and petioles. Spots are initially surrounded by a yellow margin and often begin on the older leaves. Leaves can be killed when spots grow together. Lesions that develop on petioles may kill entire leaves. Leaves weakened by blight may break off when gripped by mechanical harvesters, resulting in the roots being left in the ground. The pathogen also causes damping-off of carrot seedlings.

COMMENTS ON THE DISEASE

The disease is favored by rainy weather and/or overhead irrigation. The pathogen, Alternaria dauci, is seedborne, and survives on and is spread on carrot seed. It can also survive in carrot debris and on volunteer carrots. Spores are dispersed in air and splashing water. The optimum temperature for growth and infection is 82°F with some infection occurring at temperatures as low as 57°F and as high as 95°F. Although the fungus survives on carrot debris left in the field after harvest, once the crop residue decomposes, the fungus dies.

MANAGEMENT

Cultural Control

Planting Alternaria-indexed seed or treating seed in a hot water bath is very important. Turn under carrot residue by tillage or plowing to hasten decomposition of debris, because the pathogen only survives in soil in infected carrot residue. Practice 2-year rotations: avoid continuous carrot culture. Do not plant new fields near existing fields with blight symptoms. Differences in susceptibility exist among cultivars.

Most growers use sprinkler irrigation throughout the growing season. If possible, the use of furrow irrigation may aid in disease reduction.

Organically Acceptable Methods

Cultural control, hot water dips, and foliar sprays of Serenade ASO and MAX are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

Seed treatments may reduce Alternaria dauci. Assay the seed and if pathogen is found, treat. If seed is not treated, apply fungicides when the first blight symptoms appear, at biweekly intervals, and/or when conditions are favorable for disease development.

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 OFACTION 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#
  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.
 
F. 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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