How to Manage Pests

UC Pest Management Guidelines


Root Dieback (Forking and Stubbing)

Pathogen: Pythium ultimum and Pythium irregulare

(Reviewed 1/09 , updated 9/12 )

In this Guideline:


Root dieback of carrots produces excessively branched or stubbed roots. The fungus kills young taproots less than 2 weeks after seed germination, reducing root length and/or stimulating multiple root formation (forking). Pythium spp. can also cause damping-off of carrots. Forking and stubbing can also be caused by hardpans, nematodes, and excessive water, among other factors.


Only very young tissue is susceptible. The severity of the disease may be dependent on the density of thick-walled Pythium oospores (overwintering spores) or sporangia (a structure that encloses spores) in field soils. The pathogens are spread in water and soil. Very wet soil conditions favor the disease.


Cultural Control

Avoid overwatering and provide good drainage. Rotating to small grains might reduce soil populations of some Pythium spp.

Organically Acceptable Methods

Cultural controls are acceptable for use on organically grown produce.

Treatment Decisions

In fields with a history of Pythium-related problems, make a preplant treatment.

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.
  (Ridomil Gold SL) 1–2 pt 48 7
  COMMENTS: Preplant application. Do not apply more than 2.8 pt/acre per crop.
  (Ranman) 6.0 fl oz 12 14
  MODE-OF-ACTION GROUP NAME (NUMBER1): ubiquinone reductase, Qi site (21)
  (Presidio) 3-4 fl oz 12 14
  MODE-OF-ACTION GROUP NAME (NUMBER1): Mitosis and cell division (43)
  (Vapam, others) Label rates See label NA
  COMMENTS: Apply preplant by sprinkler, drip irrigation, or flood irrigation. Check label for rates and preplant timing. Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
  (Fosphite) Label rates 4 NA
  COMMENTS: Although proven effective in other areas, results from California studies have been mixed.
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.
* Permit required from county agricultural commissioner for purchase or use.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see 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.
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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