Bacterial Leaf Spot

On cilantro and parsley, initial symptoms of bacterial leaf spot are water-soaked lesions on foliage, which then develop into spots that are varying shades of tan or brown; advanced spots on cilantro can be black. The lesions are usually limited by leaf veins and thus have an angular, square, or rectangular appearance. With time, the spots may dry out and become papery. Lesions tend to be relatively small about 0.13 to 0.25 inch (3–6 mm) in diameter and are visible from both the top and bottom of leaves. Under favorable conditions (free moisture), bacterial leaf spot lesions may coalesce and cause considerable blighting of the foliage. Bacterial leaf spot can be confused with Stemphylium leaf spot of parsley and Alternaria leaf spot of cilantro.

Bacterial leaf spot is commonly found in cilantro plantings and can be a serious problem. The parsley disease has been more recently characterized and is of moderate concern. If either crop has significant amounts of this disease, those plantings will be unmarketable. The bacteria are likely seedborne in both crops. Water from rain, sprinkler irrigation, and heavy dews and fogs will splash bacteria from infected plants onto adjacent healthy foliage. The pathogens can survive in undecomposed cilantro or parsley residue. However, once crop residues decompose, the bacterial pathogens will not survive in the soil. Research has demonstrated that in the field bacterial leaf spot on cilantro and parsley are caused by either one of two pathogens: Pseudomonas syringae pv. apii (Psa) and P. syringae pv. coriandricola (Psc). Pseudomonas syringae pv. apii is also the causal agent of bacterial leaf spot diseases of celery and fennel.

• Use seed that has been indexed free of P. syringae pv. apii and P. syringae pv. coriandricola or that has been treated to reduce seedborne inoculum.
• Where the disease has recently occurred rotate to nonhost crops to reduce inoculum levels in soil or infected debris.
• A change from sprinkler to drip irrigation, or scheduling irrigations to enhance drying of the foliage may limit disease spread.
• Avoid excessive application of nitrogen fertilizers since over fertilization appears to favor disease development and severity.
• Be aware that these cilantro and parsley pathogens can infect other crops in the Apiaceae plant family; crop rotation decisions should consider the possibility of cross infection between cilantro, parsley, celery, and fennel.
• Among common cilantro cultivars tested in trials, the cultivar Leisure had greater bacterial leaf spot incidence compared to the cultivars Long Standing, Santo, or Jaguar.

Hot water seed treatment (122°F for 25 minutes) will significantly reduce seedborne inoculum but may reduce seed germination.

Use cultural controls in an organically certified crop.

Pesticides are not effective against bacterial leaf spot.