Symptoms and Signs
Older leaves and leaf petioles of plants with powdery mildew are usually covered with a white, powdery growth. Young leaves develop small, circular, white, powdery spots that slowly enlarge to cover the leaflets. A slight chlorosis or yellowing may appear on leaves infected by the fungus.
Comments on the Disease
The fungus survives on overwintered carrots and related weed hosts. The airborne spores may be carried long distances. Foliar symptoms usually do not appear until leaves are mature, and leaves can survive even with heavy infections. Fungal infection and disease development are favored by high humidity during evening and morning hours, and temperatures between 55° and 90°F. The disease is more severe on drought stressed plants or plants in shaded conditions.
Management
Cultural Control
Use tolerant cultivars if available. Maintain good plant vigor and avoid water/drought stress.
Organically Acceptable Methods
Use cultural control and sprays of sulfur and Bacillus subtilis in a certified organic crop.
Treatment Decisions
Chemical controls are not recommended unless infections appear very early in the growing season.
| 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 | |||
| (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. | 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. | ||||
| C. | 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. | ||||
| D. | 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. | ||||
| E. | PYRACLOSTROBIN/FLUXAPYROXAD | |||
| (Merivon) | 4–5.5 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/Succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: For optimal disease control, begin applications of Merivon prior to disease development and continue on a 7- to 14-day interval if conditions are conducive for disease development. Use the higher rate and shorter interval when disease pressure is high. Do not make more than two (2) sequential applications of Merivon before alternating to a labeled non-group 7 or non-group 11 fungicide. | ||||
| F. | SULFUR | 3–10 lb | 24 | — |
| … OR …. | ||||
| SULFUR DUST# | See label | 24 | — | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multisite contact (M2) | ||||
| COMMENTS: Apply at the first sign of disease and repeat at 14 days as needed to maintain control. | ||||
| ‡ | 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. |
| NA | Not applicable |