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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Thielaviopsis Root Rot
Pathogen: Thielaviopsis basicola
(Reviewed 1/02,
updated 1/02)
In this Guideline:
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SYMPTOMS
Thielaviopsis root rot is also called black root rot after one of the major
symptoms. Plants are stunted and roots are badly rotted. Stems below ground may
enlarge and develop black, rough, longitudinal cracks. Characteristic dark
brown to black, thick-walled, barrel-shaped chlamydospores form in infected
tissues and may be visible under magnification.
COMMENTS ON THE DISEASE
The fungus has a wide host range: 120 species in 15 families are known to be
susceptible. Strains of the fungus are known that differ in pathogenicity and
virulence. Important ornamental hosts include begonia, cyclamen, geranium,
gerbera, kalanchoe, pansy, petunia, poinsettia, primula, snapdragon, sweet pea,
verbena, and viola. The disease is favored by wet, cool soil and any condition
that weakens plants; it is most severe from 55° to 61°F, while only a
trace of disease develops at 86°F. Alkaline soil favors the disease, which
can be prevented at pH 4.8 and greatly reduced at pH 5.5 or below. However,
many plants do not grow well under such acid conditions.
The fungus is soilborne and capable of prolonged survival in the absence of
susceptible plants. Two kinds of spores are formed: barrel-shaped
chlamydospores in short chains of 3 to 7, which are resting spores, and hyaline
endoconidia. The fungus can be spread in water, soil, by infected plants or
vectored by fungus gnats and shore flies.
MANAGEMENT
The use of pathogen-free plants, along with improved sanitation and cultural
practices, has gone a long way in reducing the importance of this disease,
which at one time was widespread, especially in poinsettias. The fungus can
still be troublesome in field-grown flowers. The benzimidazole fungicides such
as thiophanate-methyl are very active against the fungus and are used as soil
treatments to control it.
| Common name |
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Amount to Use |
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| A. |
THIOPHANATE-METHYL |
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(FungoFlo, Cleary's 3336F, etc.) |
20 fl oz/100 gal water |
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COMMENTS: Apply as a drench or heavy spray (1-2 pt/sq
ft). |
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| B. |
TRIFLUMIZOLE |
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(Terraguard) 50W |
4-8 oz/100 gal |
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COMMENTS: Apply as a soil drench at 3-4 week intervals
as needed on potted plants. For use in greenhouses only. |
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| C. |
IPRODIONE |
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(Chipco 26019) |
0.4 lb/100 gal water |
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COMMENTS: Apply as a drench (1-2 pt/sq ft) at seeding
or transplanting. |
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UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
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