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 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.
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.
All bacterial pathogens, including Pseudomonas, invade flowers or wounded tissue. To prevent or minimize the risk of infection:
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.
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Janna Beckerman, Professor of Plant Pathology
Purdue Department of Botany