Symptoms and Signs
Alternaria leaf blight initial symptoms typically begin on older leaves as brown spots surrounded by a yellow margin. At later stages of disease development, dark brown to black irregularly shaped necrotic lesions develop on leaf blades and petioles. Leaves can be killed when foliar spots grow together or by lesions that develop on petioles. At advanced stages of disease development, leaves will have a scorched appearance. This disease weakens tops that then break during mechanical harvest leading to roots left in the ground and yield loss. The pathogen also causes damping-off of carrot seedlings.
Comments on the Disease
The pathogen, Alternaria dauci, is seedborne and survives on and is spread with carrot seed. It can also survive on volunteer carrots and crop debris, but once the crop residue decomposes, the fungus dies. Spores are dispersed in air and splashing water and disease is favored by leaf wetness either due to rain or overhead irrigation. The optimum temperature for growth and infection is 82°F with some infection occurring at temperatures as low as 57°F and as high as 95°F.
Management
Cultural Control
Planting Alternaria-indexed seed or treating seed in a hot water dip is very important. Turn under carrot residue by tillage or plowing to hasten decomposition of debris, because the pathogen only survives in soil, in infected carrot residue. Practice 2-year rotations and avoid continuous carrot culture. Do not plant new fields near existing fields with blight symptoms. Differences in susceptibility exist among commercial cultivars.
Most growers use sprinkler irrigation throughout the growing season. If possible, the use of furrow irrigation may aid in disease reduction.
Organically Acceptable Methods
Use cultural controls, hot water dips, and foliar sprays of Serenade ASO are acceptable in a certified organic crop.
Monitoring and Treatment Decisions
Seed treatments may reduce Alternaria dauci. Assay the seed and if pathogen is found, treat. If seed is not treated, apply fungicides when the first blight symptoms appear, at biweekly intervals, or when conditions are favorable for disease development.
| 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. | ||||
| SEED TREATMENT | ||||
| A. | HOT WATER DIP# | |||
| COMMENTS: Soak seed in hot water (122°F; 50°C) for 25 minutes. Do not soak longer or seed could be damaged. | ||||
| FOLIAR TREATMENT | ||||
| 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. | IPRODIONE | |||
| (Rovral 4F) | 1–2 pt | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2) | ||||
| COMMENTS: Do not make more than four applications of iprodione per season. | ||||
| 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. | 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: Begin applications 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 before alternating to a labeled non-group 7 or non-group 11 fungicide. | ||||
| F. | 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. | ||||
| G. | 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 a shorter interval when disease pressure is high. | ||||
| H. | BOSCALID | |||
| (Endura) | 4.5 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 a shorter interval when disease pressure is high. To limit the potential for resistance development do not apply more than two sequential applications before rotating to a labelled fungicide with a different mode of action. | ||||
| I. | POLYOXIN D ZINC SALT | |||
| (Ph-D) | 6.2 oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Cell wall synthesis (19) | ||||
| COMMENTS: Begin applications preventively when conditions are favorable for disease development. May be applied through overhead sprinkler irrigation. | ||||
| J. | BACILLUS SUBTILIS | |||
| (Serenade ASO)# | 2–4 qt | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Bacterial (BM02) | ||||
| COMMENTS: Serenade ASO is most effectively used in a preventive disease management program. Serenade ASO is applied with chemigation equipment. | ||||
| ** | Apply with enough water to provide complete coverage. |
| ‡ | 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. |
| # | Acceptable for use on certified organic crops. |
| 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. |