Symptoms and Signs
Initial symptoms of bacterial leaf spot are small, water-soaked spots that are visible from both sides of the leaf. The lesions are usually limited by leaf veins, resulting in an angular, square, or rectangular appearance. These water-soaked lesions rapidly turn brown, dry out, and become papery and tan over time. Lesions tend to be relatively small (less than 0.25 inch [6 mm]) and restricted to leaves. On greenhouse transplants, bacterial leaf spot lesions may develop extensively on the foliage. However, in the field, the disease is usually found where sprinkler irrigation is used or only on the older leaves beneath the plant canopy, which experience longer periods of wetness and less sun exposure.
Under conditions favorable for the pathogen (free moisture), bacterial leaf spot lesions may coalesce and cause considerable necrosis of the foliage.
Comments on the Disease
Pseudomonas syringae pv. apii is a seedborne bacterium. Once introduced into transplant greenhouses, the pathogen can rapidly spread via splashing water. Disease development is favored by warm, moist conditions. Infected transplants carry the pathogen into production fields.
In the field, widespread or severe symptoms generally do not develop unless the crop is sprinkler irrigated or subjected to a light frost during the production cycle.
The pathogen survives in undecomposed celery residue.
Management
Cultural Control
- Use seed that has been tested and found not to have detectable levels of P. syringae pv. apii. Hot-water seed treatment (122°F for 25 minutes) will significantly reduce seedborne inoculum but may reduce seed germination. Using seed that is at least 2 years old can significantly reduce the incidence of this disease. Disinfect transplant trays because bacteria may survive on dirty trays.
- Lower the water pressure from overhead sprinklers in transplant greenhouses because high pressures favor entry of the pathogen into celery leaves. Avoid sprinkler irrigation in the production field.
- Avoid excessive application of nitrogen fertilizers as this favors disease development.
- Avoid planting consecutive celery crops because of the risk of infested celery residues left over from the first crop.
Organically Acceptable Methods
Cultural controls and copper sprays are acceptable for use in a certified organic crop.
Treatment Decisions
Only copper compounds are registered for use against this pathogen; however, copper has not been very effective.
| Common name | Amount to use | 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. | COPPER | |||
| (Cueva Fungicide Concentrate)# | Label rates | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Inorganic (M01) | ||||
| COMMENTS: Not very effective. Not all copper compounds are approved for use in organic production; check individual products. | ||||
| # | Acceptable for use on certified organic crops. |
| ‡ | 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 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. |