Beginning at least as early as 2006, a bacterial leaf spot disease of parsley was occasionally observed in California fields. In 2009 the disease is again occurring in growers fields.
Symptoms consist of small (less than a quarter inch in diameter), angular shaped, tan to dark brown leaf spots that are visible from both top and bottom sides of leaves. As disease progresses, the spots usually do not expand significantly and do not coalesce or merge together.
A key diagnostic feature is that these spots do not contain any signs of fungal fruiting bodies or structures. This absence of fungal structures differentiates this bacterial disease from the well known Septoria blight caused by Septoria petroselini. Septoria blight of parsley can cause a similarly shaped angular leaf spot; however, such spots almost always contain profuse numbers of tiny, black, spherical fruiting bodies called pycnidia.
A fluorescent bacterium in the genus Pseudomonas has been consistently isolated from diseased flat and curly leaf parsley cultivars. Inoculation experiments have demonstrated that these bacteria, tentatively identified as Pseudomonas syringae, are responsible for this problem.
Four parsley cultivars were evaluated (Evergreen, Moss, Forest Green, Dark Green Italian) and all appeared to be equally susceptible. A joint research project by UC Cooperative Extension (Monterey County) and the USDA-ARS in Salinas is in progress.
The development of the disease is clearly linked with splashing water that comes from rain and overhead sprinkler irrigation. Elimination of such water would significantly limit disease development.
Fungicides and bactericides are unlikely to provide satisfactory control for fresh market produce standards. The source of the bacterium is not determined, though seedborne inoculum is possible.
Additional samples of this disease would assist in the research effort. If possible bacterial leaf spot is observed on parsley, please contact Steve Koike (831-759-7350; 1432 Abbott Street, Salinas Calif., 93901).