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 usually are limited by leaf veins and thus have an angular, square, or rectangular appearance. These water-soaked lesions rapidly turn brown and with aging may dry out and become papery and tan. Lesions tend to be relatively small (less than 0.25 inch in diameter) and restricted to leaves. On greenhouse transplants, bacterial blight lesions may develop extensively on the foliage. However, in the field the disease usually is found only on the older leaves that are protected by the plant canopy, except where sprinkler irrigation is used. Under favorable conditions (free moisture), bacterial blight lesions may coalesce and cause considerable blighting 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 indexed free of Pseudomonas 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. In the greenhouse, lower the water pressure from overhead sprinklers because high pressures favor entry of the pathogen into celery leaves. In the field avoid sprinkler irrigation. Excessive application of nitrogen fertilizers appears to favor disease development.
Organically Acceptable Methods
Cultural controls and copper sprays are acceptable for use on organically grown produce.
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# | Label rates | see label | see label |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Not very effective. Not all copper compounds are approved for use in organic production; check individual products. | ||||
| # | Acceptable for use on organically grown produce. |
| ‡ | 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. |