How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Pathogen: Sclerotium rolfsii
In this Guideline:
Southern blight, also called southern wilt, southern stem rot,
southern root rot and several other names, results from infection by the
soilborne fungus, Sclerotium rolfsii. The
fungus survives in the soil as small (0.04-0.08 inch), tan to brown, round
sclerotia. The sclerotia resemble mustard
seeds in size and color and the fungus is sometimes referred to as the
"mustard seed fungus." Plants are attacked at the soil line or below.
The fungus produces abundant white hyphae or mycelia around
infected parts and in and on the soil. Sclerotia are formed by the mycelia on
infected plant parts and in the soil; their presence is the main diagnostic
feature of the disease. The initial symptoms are similar to those caused by
other basal stem rots (cottony rot, Rhizoctonia stem rot, etc.): discoloration
of lower leaves, wilting, plant collapse, and death.
The disease is favored by warm moist soil,
hence it occurs in the summer months. The fungus has a wide host range and
includes many field, vegetable and ornamental crops.
Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for
30 minutes or 140°F for 1 hour), or chemically treat growing medium for
For open field production, soil fumigation or soil solarization
(in warmer climatic areas) is effective in killing soilborne sclerotia. Bulbs
and other planting stock may carry the fungus. The fungus is killed by exposure
to 122°F for 30 minutes and some plant materials such as caladium tubers, iris
rhizomes, and gladiolus corms can be treated successfully with hot water. Use of heat treatment (steam,
solarization, and/or hot water) is acceptable for organic production.
||Amount to Use
|When choosing a fungicide, consider information relating
to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
2007 Critical Use Exemption List allows fumigation with methyl bromide for
moderate-to-severe disease infestation.
||COMMENTS: Allowable for use under a Critical Use Exemption
only. Methyl bromide is a gas at temperatures over 40°F. It escapes rapidly
from soil if not applied under a gas‑proof cover. Polyethylene sheeting is commonly used
to confine methyl bromide, although the gas does slowly escape through
polyethylene. Diffuses rapidly through the soil and kills many kinds of
organisms, weeds, and many seeds. Soil generally can be planted a few days
after removal of plastic covers, although there are exceptions. A few plants
such as Allium spp., carnations, and snapdragons are sensitive to and
may be damaged by inorganic bromide that remains in the soil following
fumigation. Leaching the soil with water before planting is helpful in
reducing the amount of bromide in the rooting area. Fumigants such as methyl
bromide are a source of volatile organic compounds (VOCs) but are not
reactive with other air contaminants that form ozone: methyl bromide depletes
ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
||COMMENTS: Inject into soil and cover immediately with plastic
tarps. See Comments above regarding the use of methyl bromide, which is allowable
for use under a Critical Use Exemption only. Fumigants such as methyl bromide
are a source of volatile organic compounds (VOCs) but are not reactive with
other air contaminants that form ozone: methyl bromide depletes ozone.
Fumigate only as a last resort when other management strategies have not been successful or are not available.
||0.5–1 lb a.i./1000 sq ft
||MODE OF ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)
||COMMENTS: Helpful in preventing infection when incorporated
into top 2 inches of soil. Best available material for southern blight caused
by Sclerotium rolfsii. Insoluble in
water and must be thoroughly mixed with soil to reach its desired depth of
control. Works through vapor action and has good residual action. Germination
of some seeds may be inhibited and small plants may be stunted by this fungicide.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
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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