Symptoms and Signs
Root dieback of carrots produces excessively branched or stubbed roots. The fungus kills young taproots less than 2 weeks after seed germination, which reduces root length or stimulates multiple root formation (forking), or both. Pythium spp. can also cause damping-off of carrots.
Comments on the Disease
Only very young tissue is susceptible. The severity of the disease may be dependent on the density of thick-walled Pythium oospores (overwintering spores) or sporangia (a structure that encloses spores) in field soils. The pathogens are spread in water and soil. Very wet soil conditions favor the disease. Forking and stubbing can also be caused by hardpans, nematodes, and excessive water, among other factors.
Management
Cultural Control
Avoid overwatering and provide good drainage. Rotating to small grains might reduce soil populations of some Pythium spp.
Organically Acceptable Methods
Use cultural controls in a certified organic crop.
Treatment Decisions
In fields with a history of Pythium-related problems, make a preplant treatment.
| 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. | MEFENOXAM | |||
| (Ridomil Gold SL) | 0.5–1 pt | 48 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) | ||||
| COMMENTS: Preplant application. Do not apply more than 2.8 pt/acre per crop. | ||||
| B. | CYAZOFAMID | |||
| (Ranman 400SC) | 6 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Ubiquinone reductase, Qi site (21) | ||||
| C. | FLUOPICOLIDE | |||
| (Presidio) | 3–4 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Mitosis and cell division (43) | ||||
| D. | METAM SODIUM* | |||
| (Vapam HL) | Label rates | See label | NA | |
| COMMENTS: Apply preplant by sprinkler, drip irrigation, or flood irrigation. Check label for rates and preplant timing. Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. | ||||
| E. | PHOSPHOROUS ACID | |||
| (Fosphite) | Label rates | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Although proven effective in other areas, results from California studies have been mixed. | ||||
| ‡ | 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. |
| * | Permit required from county agricultural commissioner for purchase or use. |
| NA | Not applicable |