Symptoms and Signs
Southern blight is characterized by a soft watery decay of the taproot at or near the soil line. The disease develops rapidly, resulting in wilting and yellowing of the carrot top. White mats of mycelium develop on the carrot root and in the adjacent soil. Tan to brown round sclerotia (resting structures) about the size of a mustard seed (1.5 mm) develop on mycelial mats. The abundant sclerotia are a good diagnostic feature of southern blight.
Comments on the Disease
High temperatures (over 85° F) favor the disease. The fungus attacks a wide range of plants and survives for long periods in the soil as sclerotia. Southern blight is typically more prevalent in the Southern San Joaquin Valley in spring planted carrots. It also predisposes carrots to secondary infections such as soft rot.
Management
Cultural Control
Rotation to nonhosts such as corn or small grains for at least two years reduces numbers of sclerotia. Deep plowing to bury sclerotia and infested plant debris helps in reducing sclerotia levels in soil.
Organically Acceptable Methods
Use cultural controls in a certified organic crop.
Treatment Decisions
Common name | Amount per acre | REI‡ | PHI‡ | |
---|---|---|---|---|
(Example trade name) | (hours) | (days) | ||
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. | AZOXYSTROBIN/DIFENOCONAZOLE | |||
(Quadris Top) | 14 fl oz | 12 | 7 | |
MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/Demethylation inhibitors (3) | ||||
COMMENTS: Do not make more than one application before rotating to a fungicide with a different mode of action. Do not make more than four foliar applications of strobilurin fungicides per year. |
‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI The longer of two intervals is the minimum time that must elapse before harvest. |
1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with different group numbers are suitable to alternate in a resistance management program. In California, make no more than one application of a fungicide with a mode-of-action group number associated with high resistance risk before rotating to a fungicide with a different mode-of-action group number; for other fungicides, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number. |