How to Manage Pests

UC Pest Management Guidelines

Carrot

Cottony Soft Rot

Pathogen: Sclerotinia sclerotiorum

(Reviewed 1/09, updated 1/09)

In this Guideline:


SYMPTOMS AND SIGNS

Sclerotinia infection may occur at any stage of growth, and extensive root decay may occur before symptoms of wilt and collapse appear on the upper part of the plant. Infection is always accompanied by a characteristic cottony, white mycelium that appears on the surface of the infected tissues. On or inside the white mycelium appear black, round-to-irregular-shaped structures (sclerotia), which are about 0.1 to 0.4 inch wide. Sclerotia are survival structures of the fungus. The fungi Pythium or Rhizoctonia may also produce cottony growth, but will not produce sclerotial bodies in the mycelium. Sclerotinia soft rot is usually soft and watery compared to Rhizoctonia rot, which tends to be firm and dry. Bacterial soft rot tends to be slimy and malodorous and is often secondary to cottony soft and other fungal rots.

COMMENTS ON THE DISEASE

Sclerotinia is most active when soil temperatures are 55° to 77°F. Moist soils are necessary for fungal activity. However, once infection is established, moisture from the carrot root tissue is sufficient to maintain fungal growth.

MANAGEMENT

Cultural Control

Deep plowing once per year to bury sclerotia 8 to 10 inches into the soil will reduce disease incidence, but not eliminate it completely because spores may be blown in from other fields. A 3-year rotation to cereals, corn, or cotton will also help reduce sclerotial populations in the soil. Trimming the sides of the foliage after the canopy closes may increase ventilation between rows and allow leaves to dry. Avoid planting into fields with a history of cottony soft rot.

Organically Acceptable Methods

Cultural controls and the use of the biological fungus, Coniothyrium minitans, are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

Begin applications when disease first appears and conditions favor 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.
 
A. BOSCALID
  (Endura) 4.5 oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxamide (7)
  COMMENTS: Do not make more than 2 applications before rotating to a fungicide with a different mode of action (i.e., group designation).
 
B. 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.
 
C. CONIOTHYRIUM MINITANS#
  (Contans) WG 1–2 lb 4 0
  MODE OF ACTION: A fungus that kills sclerotia of Sclerotinia sclerotorum and S. minor.
  COMMENTS: Apply before planting.
 
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.

[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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