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How to Manage Pests
UC Pest Management Guidelines
Carrot
Root Dieback (Forking and Stubbing)
Pathogen: Pythium ultimum and Pythium irregulare
(Reviewed 10/05,
updated 10/05)
In this Guideline:
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Root dieback of carrots produces excessively branched or stubbed
roots. The fungus kills young tap roots 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/Acre |
| (trade name) |
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| A. |
MEFENOXAM |
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(Ridomil Gold) EC |
1–2 pt |
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MODE OF ACTION: A PA
(Group 4) 1 acylalanine fungicide.
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COMMENTS: Preplant application. Do not
apply more than 2.8 pt/acre/crop.
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| B. |
METAM SODIUM* |
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(Vapam, others) |
Label rates |
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COMMENTS: Apply preplant by sprinkler,
drip irrigation, or flood irrigation. Check label for rates and preplant
timing. Fumigants, such as metam sodium and 1,3-dichloropropene, are a prime
source of volatile organic compounds (VOCs), which are a major air quality
issue. Fumigate only as a last resort when other management strategies have
not been successful or are not available.
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| C. |
PHOSPHOROUS ACID |
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(Fosphite) |
Label rates |
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MODE OF ACTION: A
phosphonate (Group 33) 1 fungicide.
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COMMENTS: Although proven effective in other areas, results
from California studies have been mixed.
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![[Precautions]](/IMAGES/btn-precautions_bul.gif)
![[UC Peer Reviewed]](/IMAGES/logo-ucpeerreviewed.jpg)
UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438
Diseases
R. M. Davis, Plant
Pathology, UC Davis
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara Co.
J. Nunez, UC Cooperative Extension, Kern Co.
T. A. Turini, UC Cooperative Extension, Imperial Co.
Acknowledgment for contributions to the diseases section:
B. W. Falk, Plant Pathology, UC Davis
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