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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Cavity spot-induced, irregularly shaped lesions across mature carrot tap roots.

Carrot

Cavity Spot

Pathogens: Pythium sulcatum and P. violae

(Reviewed 1/09, updated 1/09)

In this Guideline:

SYMPTOMS

Cavity spot is characterized by elliptical to irregularly shaped, depressed lesions oriented across the mature carrot tap roots. Individual lesions are usually less than 0.5 inch (1.3 cm) in diameter, but can be much larger, especially on processing varieties. Infections occur anywhere along the tap root, but lesions tend to be more abundant on the upper third of the root and are often found where lateral roots emerge from the taproot. Lesions begin as pinpoint, sunken spots and generally enlarge as roots mature.

COMMENTS ON THE DISEASE

The causal fungus is favored by cool soil temperatures; in culture it grows best at 58°F. The incidence of the disease is probably dependent on the number of thick-walled oospores (overwintering spores) in the soil. Pythium sulcatum and P. violae also cause lesions on alfalfa roots. Infections of other hosts such as celery, blackeye peas, wheat, cucumber, beets, and other plants, including weeds may not result in symptoms. Nonhosts include tomato, cotton, watermelon, corn, and potato.

MANAGEMENT

Cultural Control
Practice 3-year crop rotations with crops other than alfalfa or carrots. Do not over-irrigate. Harvest carrots soon after they mature because older carrots are more susceptible to infection. All carrot varieties are susceptible.

Organically Acceptable Methods
Cultural controls are acceptable for use on organically grown produce.

Treatment Decisions
Treat fields with a history of severe cavity spot.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider information relating to environmental impact.
 
A. MEFENOXAM
  (Ridomil Gold) SL See comments 48 7
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
  COMMENTS: Apply 1–2 pt/acre as a preplant incorporated treatment or as a soil surface spray immediately after planting. Postplant use is allowed under a Special Local Needs label. Beginning 28-50 days after planting, apply Ridomil Gold SL on a 14- to 21-day interval. Apply 0.25 to 1 pt/acre in water by chemigation or by ground equipment with a spray directed to the base of the plant or shanked in with liquid fertilizer. Follow all ground applications with an irrigation of one inch water to promote movement of material into the root zone. Refer to the Ridomil Gold SL label for use directions for chemigation. Do not exceed more than 2.8 pt/ acre/crop season or harvest within 7 days of treatment.
 
B. FENAMIDONE
  (Reason) 500SC 8.2 fl oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Alternate with a mefenoxam fungicide such as Ridomil Gold. Apply at a 14-21 day interval.
 
C. METAM SODIUM*
  (Vapam, others) Label rates 48 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.
 
+ 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.
* Permit required from county agricultural commissioner for purchase or use.
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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Statewide IPM Program, Agriculture and Natural Resources, University of California
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