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UC Pest Management Guidelines


Foliage symptoms of Sclerotium root rot.

Floriculture and Ornamental Nurseries

Southern Blight

Pathogen: Sclerotium rolfsii

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

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.

COMMENTS ON THE DISEASE

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.

MANAGEMENT

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 container-grown plants.

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.

Common name Amount to Use R.E.I.+
(trade name)   (hours)

  Calculate impact of pesticide on air quality
When choosing a fungicide, consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
SOIL FUMIGATION
Note: The 2007 Critical Use Exemption List allows fumigation with methyl bromide for moderate-to-severe disease infestation.
A. METHYL BROMIDE* Label rates 48
  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.
 
B. METHYL BROMIDE*/CHLOROPICRIN* Label rates 48
  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.
 
SOIL FUNGICIDE
A. PCNB
  (Terraclor) 0.5–1 lb a.i./1000 sq ft 12
  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.
 
* 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 http://www.frac.info/). 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.
+ 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

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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