Bacterial leaf spot of coriander

Bacterial leaf spot of coriander (Pseudomonas syringae pv. coriandricola) is the most common bacterial disease of herbs and can severely reduce yield and seed quality. This species of bacteria does not affect any other member of the Apiaceae family. Bacterial leaf spot on coriander can occur in any herb production system as it is seed-borne, but may be more prevalent where crops are overhead watered.

Symptoms
The disease is seen on young coriander plants as brown leaf spots 2-5mm (⅛-¼in) in diameter, which may be surrounded by a water-soaked area. The leaf spots are often angular, being limited by the veins, and can be seen clearly on both leaf surfaces. Leaf spots may merge to cause a more extensive blight. On mature plants, there may be blackening of leaf veins, or black edges on leaves followed by leaf death. In severe infections, the bacterium can infect vein endings and spread down via the vascular system, resulting in dark longitudinal streaks on petioles and stems. If plants are left to bolt, then flowers and seed heads on infected plants shrivel, turn black and collapse.

Biological
There are no products currently approved in the UK for biological control of bacterial diseases.

Chemical
There are no products approved specifically for the control of bacterial diseases on protected herbs and it is unlikely that chemical control will be required under protection.

Prevention
Use of ‘clean' seed is important for healthy crop production, however, there are no clearly defined tolerance standards for this disease. If an outbreak of bacterial leaf spot occurs in a greenhouse, infected plants and debris should be removed and destroyed. If early symptoms are observed on a few plants, these should be bagged in-situ and disposed of outside the greenhouse in covered bins or skips. For soil-grown crops, avoid use of the same soil for at least two years. Disinfect glasshouse or polytunnel structures, benches and equipment, and re-circulating irrigation systems before re-introducing plants of the same species.

Similar conditions
The symptoms of bacterial leaf spot of coriander can be confused with:
 * Oedema
 * Blue spot
 * Tip burn