Symptoms and Signs
Southern blight, also called southern wilt, southern stem rot, southern root rot and 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 ground. 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.): discolored crown/stem lesion at the soil line, 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. This pathogen is a regulated pest in California nurseries and must be eradicated. Contact your County Agricultural Commissioner for details.
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 outdoor 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||REI‡|
|(Example trade name)||(hours)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|A.||CHLOROPICRIN*||Label rates||See label|
|COMMENTS: Inject into soil and cover immediately with plastic tarps. Fumigants are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.|
|(Torque)||4–10 fl oz/100 gal water||12|
|MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)|
|COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant.|
|(Palladium)||2–4 oz/100 gal water||12|
|MODE OF ACTION GROUP NAME (NUMBER1): Amino acids and protein synthesis (9) and signal transduction (12)|
|(Prostar 70 WG)||3–6 oz/100 gal water||12|
|MODE OF ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitor (7)|
|(Terraclor 400)||Label rates||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. 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.|
|§||Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.|