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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Thielaviopsis Root Rot
Pathogen: Thielaviopsis basicola
(Reviewed 3/09,
updated 3/09)
In this Guideline:
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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.
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 acidic 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 (resting spores) in
short chains of 3 to 7 and rectangular-shaped endoconidia. The fungus can be
spread in water, soil, by infected plants or vectored by fungus gnats and shore
flies.
The use of pathogen-free plants, along with improved sanitation and
cultural practices, has reduced 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.
To treat container media,
steam (at 140°F for
30 minutes), or solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour). For flower
production in open fields, solarization in warmer climates has been successful
for control of most Thielaviopsis in many
crops. Solarization and steaming
are acceptable for organic production.
For more information, see MANAGEMENT
OF SOILBORNE PATHOGENS.
| Common name |
Amount to Use |
R.E.I.+ |
| (trade name) |
|
(hours) |
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| When
choosing a fungicide, consider the general
properties of the fungicide as
well as information relating to environmental impact. |
| |
| A. |
THIOPHANATE-METHYL |
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(FungoFlo, etc.) |
20 fl oz/100 gal water |
12 |
| |
MODE
OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1) |
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COMMENTS:
Apply as a drench or heavy spray (1–2 pt/sq ft). Generally applied
after sowing. Helps to control gray mold, Rhizoctonia diseases, cottony rot,
Thielaviopsis rots, and some Cylindrocladium diseases. Absorbed by plant
parts exposed to the chemical. Roots may absorb the fungicide (or its breakdown product carbendazim), which moves in the xylem to transpiring leaves. |
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| B. |
TRIFLUMIZOLE |
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(TerraGuard) 50W |
4–8 oz/100 gal |
12 |
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MODE
OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) |
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COMMENTS:
Apply as a soil drench at 3–4 week intervals as needed on potted
plants. A protectant fungicide; use is restricted to enclosed commercial structures such as greenhouses and shade houses. |
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UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
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