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Bacterial leaf spots are often angular with chlorotic halos, as seen on this high bush-cranberry. Photo by Janna Beckerman.

Purdue Landscape Report: Pseudomonas syringae pv. syringae is an opportunistic bacterial pathogen that attacks a diversity of woody ornamental plants. The bacteria cause flower blights, cankers, shoot blights, and diebacks.

Symptoms
Symptoms often begin as expanding leaf spots. On lilac and viburnum, small spots expand to irregularly shaped brown lesions with yellow halos (Fig. 1). For most other hosts (cherry, pear, basswood, dogwood, hydrangea, high bush-cranberry, mountain-ash), infected leaves turn reddish brown or black and usually remain on the branch after they die (Fig. 2). As the bacteria spread into woody tissue, dark, sunken sections of the stem (cankers) expand, working their way back toward the trunk from infected leaves and flowers. Leaves attached to a cankered branch will wilt while the tip of the affected branch curls and droops like a shepherd’s crook (Fig. 3). Cloudy droplets of sticky fluid (ooze) may accumulate on leaf tips, leaf surfaces, stems, and even infected fruit.

Image of infected flowers result in blossom blast.
Figure 2. Infected flowers result in blossom blast. The continued growth of the bacteria can result in cankering and blight. Photo by George Sundin.

Figure 2. Infected flowers result in blossom blast. The continued growth of the bacteria can result in cankering and blight. Photo by George Sundin.

Life Cycle
The bacteria overwinter and persist in cankers, along with asymptomatic bud and twig tissue. In presence of water and warming temperatures, bacteria multiply and may ooze from infected tissue. Wind-driven rain, insect, or mechanical pruning spread Pseudomonas. Bacteria enter the plant through flowers or injury.

Management
All bacterial pathogens, including Pseudomonas, invade flowers or wounded tissue. To prevent or minimize the risk of infection:

• Use disease-free plants and budwood.
• Prune in the late winter/early spring.
• When pruning, do not mix pruning to shape woody ornamentals with pruning to manage disease.
• When pruning to remove infected shoots, be sure to disinfect shears between plants.
• Minimize overhead irrigation when possible to minimize splashing and pathogen spread.
• Avoid over-fertilization practices that drive excess succulent growth.
  

  Figure 3. Bacterial blight is a common scourge to common lilac. Using resistant varieties is the best way to protect against the disease. Photo by Janna Beckerman.

• Avoid late season fertilization practices that prevent plants from achieving timely dormancy and promote freeze damage and/or winter injury (Fig. 4).
• Choose zone appropriate plants to avoid freeze damage and/or winter injury.
• Choose disease resistant plants, when available. The following lilac varieties were found to be resistant over a four-year trial: Donald Wyman, Royalty, Superba, Miss Kim, Edmund Boisier, Victor Lemoine, Dwarf Korean, Mme. Antoine Buchner Isabella, Sensation, Anna Amhof, Krasavitska Moskvy, Michael Buchner, and Alphonse Lavallée.

Foliar sprays of some copper-based bactericides (e.g., Camelot, Kocide, and Nu-Cop) were found to reduce disease incidence in trials on lilac (Vey and Palmer, 2018). Avoid using copper under cool, humid conditions to reduce the risk of phytotoxicity and damaging plants. Copper resistant populations of Pseudomonas syringae have been reported in other crops (vegetables, stone fruit). Products containing acibenzolar have provided inconsistent control in multiple trials but is labeled for use. Use of quaternary-ammonium disinfestants (KleenGrow) have been found to reduce bacterial populations and disease incidence and should be considered as part of any rotation with copper products. Due to the diversity of copper products, be sure to test for phytotoxicity issues prior to large scale treatment of crops.

To view this article and other Purdue Landscape Report articles, please visit Purdue Landscape Report.

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Resources:
The Purdue Landscape Report
Purdue Landscape Report Facebook Page
Find an Arborist website, Trees are Good, International Society of Arboriculture (ISA)
Phytophthora Diseases in Ornamentals, The Education Store, Purdue Extension’s resource center
Root Rot in Landscape Plants, The Education Store
Symptoms and Signs for Plant Problem Diagnosis, The Education Store
Tree Defect Identification, The Education Store
Tree Pruning Essentials, Publication & Video, The Education Store
Shrubs and Woody Vines of Indiana and the Midwest, The Education Store
Tree Risk Management, The Education Store
Equipment Damage to Trees, Got Nature? Blog
Tree wounds and healing, Got Nature? Blog
ID That Tree, Purdue Extension-Forestry & Natural Resources (FNR) YouTube playlist
Southwest Damage, Scalding, or Frost Cracking – Landscape Report
Subscribe to Purdue Extension-FNR YouTube Channel

Janna Beckerman, Professor of Plant Pathology
Purdue Department of Botany