Symptoms and Signs
Cavity spot is characterized by small, sunken, elliptical to irregularly shaped lesions on the surface of carrot taproots. Individual lesions are usually less than 0.5 inch (1.3 cm) in diameter, but can be much larger, especially on processing varieties. Infections occur anywhere along the taproot, but lesions tend to be more abundant on the upper third of the root. Lesions become dark and increase in size as carrot roots mature.
Comments on the Disease
The disease is favored by cool soil temperatures and excessive soil moisture due to poor drainage, low spots in the field, or rains. The incidence of the disease is probably dependent on the number of thick-walled oospores (overwintering spores) in the soil. Pythium sulcatum and P. violae also cause lesions on alfalfa roots. Infections of other hosts such as celery, black-eyed peas, wheat, cucumber, beets, and other plants, including weeds, may not result in symptoms.
Management
Cultural Control
Practice 3- to 4-year crop rotations with crops other than alfalfa. Do not over-irrigate. Harvest carrots soon after they mature because older carrots are more susceptible to infection. Some sources of genetic resistance have been identified but no commercially available cultivars are resistant.
Organically Acceptable Methods
Use cultural controls in a certified organic crop.
Treatment Decisions
Apply fungicides to fields with a history of cavity spot.
| 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) | See comments | 48 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) | ||||
| COMMENTS: For in season applications, apply Ridomil Gold SL on a 14- to 21-day interval, 0.25 to 0.5 pt/acre in water as side dress when furrow irrigation is used. Refer to the Ridomil Gold SL label for use directions for chemigation. Do not exceed 2.8 pt/acre per crop season. | ||||
| B. | FENAMIDONE | |||
| (Reason 500SC) | 8.2 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Alternate with a mefenoxam fungicide such as Ridomil Gold SL. Apply at a 14- to 21-day interval. When applying with ground equipment, direct spray to the base of the plant and follow immediately with 0.25–1 inch of irrigation to promote movement of material into the root zone. | ||||
| C. | CYAZOFAMID | |||
| (Ranman 400SC) | 6 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Ubiquinone reductase, Qi site (21) | ||||
| COMMENTS: Do not apply more than 5 sprays of Ranman per crop. Rotate with a fungicide with a different mode of action. | ||||
| D. | FLUOPICOLIDE | |||
| (Presidio) | 4 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Mitosis and cell division (43) | ||||
| COMMENTS: Do not make more than 3 total applications of presidio per year and do not make 2 sequential applications. For foliar applications, Presidio must be tank mixed with a labelled rate of another fungicide active against the target pest. | ||||
| E. | METAM SODIUM* | |||
| (Vapam HL) | Label rates | See label | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-Site Activity (M03) | ||||
| ‡ | 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 |
| NA | Not applicable |