How to Manage Pests

UC Pest Management Guidelines


Corky Root Rot

Pathogen: Pyrenochaeta lycopersici

(Reviewed 12/13, updated 12/13)

In this Guideline:


Infected roots of plants with corky root rot are distinctly corky. Extensive brown lesions, often arranged in bands with lengthwise cracking of the cortex, develop on the larger roots. The tips of infected older roots are pinched off. Small feeder roots may be completely decayed. Infected plants are stunted and slow-growing. Branches on mature plants may die back from the tips.

Comments on the Disease

The fungus survives for long periods as microsclerotia. Potential alternate hosts include cucurbits, peppers, safflower, and solanaceous weeds such as on nightshades. Corky root is generally a problem in early plantings under cool conditions. Disease development is optimal at 60° to 68°F (15.5–20°C). Corky root usually does not kill plants, but may reduce yields.


Cultural practices, or soil fumigation on fields with a history of corky root rot, will help to minimize problems from this disease.

Cultural Control
  • Plant when soils are warm in spring.
  • Avoid consecutive crops of tomatoes.
  • Although not extensively tested in California, soil solarization has been used to control corky root rot in other areas of the world.
  • Rotate with nonhost crops.
Organically Acceptable Methods

Cultural control is acceptable in an organically certified crop.

Treatment Decisions

In fields with a history of corky root rot, a preplant treatment with metam sodium may reduce disease in a subsequent tomato crop.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
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  (Vapam HL) 50–75 gal See label NA
  (Metam CLR 42%, etc.) 50–75 gal See label NA
  COMMENTS: Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone.
** See label for dilution rate.
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.
* 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 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.
NA Not applicable.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470


R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
K. V. Subbarao, USDA Agricultural Research Station, Salinas, CA
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier
B. J. Aegerter, UC Cooperative Extension, San Joaquin County (powdery mildew on field-grown tomatoes)
Acknowledgments for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis

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