Symptoms and Signs
On both crops, 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 1/8 to 1/4 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.
Comments on the Disease
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. Research has demonstrated that in the field both the cilantro and parsley bacterial leaf spot problems are caused by 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 Pseudomonas 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 furrow or drip irrigation, or scheduling irrigations to enhance drying of the foliage may limit its spread. Cultivars vary in susceptibility.
- Avoid excessive application of nitrogen fertilizers since over fertilization appears to favor disease development.
- 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.
Hot water seed treatment (122°F for 25 minutes) will significantly reduce seedborne inoculum, but may reduce seed germination.
Organically Acceptable Methods
Use cultural controls and copper sprays in an organically certified crop.
Only copper compounds are registered for use against this pathogen; such copper sprays provide some protection but are generally not effective enough to achieve sufficient control.
|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.||COPPER#||Label rates||See label||See label|
|MODE-OF-ACTION GROUP NAME (NUMBER1): A multi-site contact (Group M1) inorganic fungicide.|
|COMMENTS: Not all copper compounds are approved for use in organic production; check individual products.|
|‡||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 two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.|