How to Manage Pests

UC Pest Management Guidelines


Alternaria Leaf Blight

Pathogen: Alternaria dauci

(Reviewed 1/09, updated 9/12, pesticides updated 4/16)

In this Guideline:


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.


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.


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 REI‡ PHI‡
(Example trade name)   (hours) (days)

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  COMMENTS: Soak seed in hot water (122°F; 50°C) for 25 minutes. Do not soak longer or seed could be damaged.
  (Quadris) 9.0–15.5 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.
  (Bravo Weather Stik) 1.5–2 pt 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multisite 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.
  (Rovral 4F) 1–2 pt 24 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Do not make more than four applications of iprodione per season.
  (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.
  (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 three sequential applications or a total of four applications of all strobilurins per season. Do not apply more than 12 oz/acre per year.
  (Fontelis) 16–30 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
  COMMENTS: Begin application prior to disease development and continue on a 7- to 14-day interval. Use higher rate and a shorter interval when disease pressure is high.
  (Serenade MAX)# 1–3 lb 4 0
  (Serenade ASO)# 2–6 qt 4 0
  MODE OF ACTION: A biological fungicide. (44)
  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 (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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. 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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438


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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