Symptoms and Signs
Alternaria leaf spot symptoms on cilantro initially appear as water-soaked, round to oval shaped lesions on leaf blades. The spots later are tan to brown with a white center sometimes developing. These lesions are visible from both top and bottom sides of the leaf. As disease severity increases, leaves can be killed when spots grow together. Fungal growth on leaf surfaces is usually sparse and appears as gray-green mycelium and spores when viewed with a hand lens. Alternaria leaf spot can be confused with bacterial leaf spot disease; contact your local UCCE advisor or specialist for help with diagnosing.
Comments on the Disease
The Alternaria species causing this disease on cilantro in California remains unidentified. Research indicates that this cilantro pathogen is different from the A. dauci that infects carrots and that has been found on cilantro in Florida.
This cilantro pathogen will likely share features with other foliar Alternaria pathogens. Disease spread can occur by rain and heavy dew, as well as overhead sprinkler irrigation. Spores are dispersed via winds and splashing water. The fungus will survive on cilantro debris, though once such crop residues are buried and decompose in the soil, the pathogen will not survive. Because Alternaria pathogens can be seedborne for carrots and other crops, it is possible that this cilantro Alternaria could also be seedborne; however, researchers have not proven this for cilantro.
Management
Cultural Control
To prevent disease development:
- Avoid planting next to infected cilantro in consecutive cropping patterns.
- Avoid planting a new cilantro crop into a field still containing visible cilantro crop residues.
- When possible, sprinkler irrigate in the morning instead of at night, so as to minimize the duration of leaf wetness.
- Consider not regrowing cilantro for multiple cuttings if the first harvest is infected with Alternaria.
Commonly grown cilantro cultivars such as Long Standing, Leisure, Santo, and Jaguar have similar susceptibility to the pathogen.
Organically Acceptable Methods
Use cultural control in an organically certified crop.
Treatment Decisions
| 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) |
15 fl oz/A | 4 | 0 |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) COMMENTS: Application prior to crop canopy closure may reduce the rate of infection. Due to the short production cycle of cilantro, one or two applications may be possible. |
||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the 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. |