Symptoms and Signs
Cercospora blight can occur on any above ground carrot tissue. Symptoms appear as small, circular, tan to brown lesions, and curled leaf margins. The tan to brown lesions initially surrounded by a chlorotic halo often enlarge into brown necrotic spots. As the disease progresses, the leaflets become yellowed and curl at the margins. Lesions on leaf petioles, stems, and flower parts are usually elliptical and dark brown. Under heavy disease pressure, severe loss of foliage may occur eventually leading to yield losses.
Comments on the Disease
Cercospora blight is usually of minor importance in California but often occurs in coastal carrot-growing regions. Cercospora carotae can be seedborne, but also overwinters on plant debris or in the soil. It can also survive on wild carrots. Infection occurs over a wide range of temperatures with an optimum at 82°F.
Management
Cultural Control
Use disease-free seed or apply fungicides to seed before planting. After harvest if the disease was present ensure crop debris is destroyed by plowing under crop residue and doing a 2- to 3-year crop rotation. Use commercially available resistant varieties.
Organically Acceptable Methods
Use cultural controls in a certified organic crop.
Monitoring and Treatment Decisions
In fields with a history of Cercospora leaf blight, start fungicide applications when conditions are conducive to disease development or when disease is observed.
| 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. | ||||
| AFTER PLANTING | ||||
| A. | AZOXYSTROBIN | |||
| (Quadris) | 9.0–15.5 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| 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 crop. | ||||
| B. | CHLOROTHALONIL | |||
| (Bravo Weather Stik) | 1.5–2 pt | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multisite contact (M5) | ||||
| COMMENTS: Repeat applications at 7- to 10-day intervals if necessary to maintain control. After completion of the REI, follow safety precautions detailed on the label for 6.5 days. | ||||
| C. | COPPER HYDROXIDE# | Label rates | 48 | — |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multisite contact (M1) | ||||
| COMMENTS: Repeat applications at 7- to 14-day intervals, depending on disease severity. | ||||
| D. | PYRACLOSTROBIN | |||
| (Cabrio EG) | 8–12 oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Do not make more than one application before rotating to a fungicide with a different mode of action. Do not make more than three applications of strobilurin fungicides per crop. | ||||
| E. | TRIFLOXYSTROBIN | |||
| (Flint Extra) | 2–2.9 oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: To limit potential for resistance development do not apply more than three sequential applications or a total of four applications of all strobilurins per season. Do not apply more than 11.5 oz/acre per year. | ||||
| F. | PENTHIOPYRAD | |||
| (Fontelis) | 16–30 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Begin application prior to disease development and continue on a 7- to 14-day interval. Use higher rate and shorter interval when disease pressure is high. | ||||
| G. | BOSCALID/PYRACLOSTROBIN | |||
| (Pristine) | 8–10.5 oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)/Quinone outside inhibitor (11) | ||||
| COMMENTS: Begin application prior to disease development and continue on a 7- to 14-day interval. Use higher rate and shorter interval when disease pressure is high. Do not use more than 63 oz/A/year. Rotate with a different mode of action fungicide after every application to limit the potential for resistance development. | ||||
| ‡ | 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. |
| # | Acceptable for use on certified organic crops. |