Bacterial leaf spot of parsley has been confirmed in a number of fields in 2018. In the early part of the spring, disease development and severity were encouraged by rains. As the rains subsided, disease continued to spread because parsley is irrigated with overhead sprinklers. Early symptoms of bacterial leaf spot consist of small leaf spots that are usually less than ¼ inch in diameter; as disease develops, spots can significantly enlarge. Spots are noticeably and consistently angular in shape, with the margins of the spots restricted by leaf veins (Photos 1 and 2). The color of the leaf spots can vary and range from light tan to dark brown. These leaf spots penetrate the entire leaf, so that the spots will be visible from both the top and bottom sides of the infected tissue (Photos 1 and 2). Damage from chemicals or abrasion, in contrast, is usually only seen on the top side of the leaf. In severe cases the large number of spots merge together and result in blight-like symptoms (Photo 3). This parsley disease is caused by the bacterium Pseudomonas syringae.
Currently the most important foliar disease of parsley is Septoria blight caused by the fungus Septoria petroselini. In most cases Septoria blight and bacterial leaf spot will be readily differentiated from each other. While both diseases result in angular-shaped, tan to brown spots, bacterial leaf spot will not have any fungal fruiting bodies in the lesions. However, Septoria blight lesions will almost always contain distinctive, tiny, black fruiting bodies called pycnidia (Photo 4).
Controlling bacterial leaf spot of parsley will be similar to steps used to manage other Pseudomonas diseases that occur on leafy greens. (1) The initial source of inoculum is likely seedborne Pseudomonas on parsley seed; therefore, the use of pathogen-free seed or seed treatments may be appropriate. (2) Disease depends on splashing water to disperse the bacteria and create favorable conditions for infection and disease development; therefore, avoiding the use of overhead sprinkler irrigation or timing irrigations to enhance drying of foliage is advisable if possible. (3) It is likely that diseased crop residues may still harbor viable bacteria, so back-to-back plantings of parsley should be delayed until crop residues have dissipated; this pathogen is not a soil inhabitant, so once the host tissue has rotted away, the bacteria do not survive in the soil for long periods of time. (4) Highly effective pesticides are not available for this bacterial leaf spot problem. While copper-based fungicides may provide some protection, these products are generally not sufficient to provide the high-quality produce demanded by the market.
Photo 1. Top sides of parsley leaves infected with bacterial leaf spot.
Photo 2. Bottom sides of the same parsley leaves (from Photo 1) infected with bacterial leaf spot.
Photo 3. Severe cases of bacterial leaf spot of parsley can result in blight-like symptoms.
Photo 4. Septoria blight of parsley results in the occurrence of small black fruiting bodies of the fungus.