Issue 11, July 2, 2010

Bacterial Blight of Ornamentals

Bacterial blight, caused by Pseudomonas syringae, has been common this past spring and early summer. In most plant pathology literature, Pseudomonas syringae is considered a weak pathogen. It requires a wound to enter the plant and does most damage to plants under stress. It can do major damage to a susceptible host in a stressed site or during a stressed season. It has been known to cause severe cankering on some cultivars of Callery Pear. There are strains of this bacterium that can aid in ice formation (ice nucleation) at temperatures just above freezing. Bacterial blight of ornamentals is more common with late spring frosts, cool, wet weather (spring or fall), plant injuries of any kind, and high nitrogen.

Hosts of this disease include (but are not limited to) lilac, magnolia, forsythia, mountain ash, flowering cherry, apricot, Callery pear, flowering dogwood, and viburnum. Most gardeners have seen it on lilac in early spring and probably thought it was frost injury. The image shows bacterial blight on lilac with an isolate of the causal bacterium.

Symptoms vary from flower blast or bud death to leaf spots to shoot tip dieback, and even cankers. I tend to see it most commonly as a shoot tip dieback that resembles frost or fire blight. Often the external tissues are darkened but inner tissue may still be green. Fire blight results in brown tissue inside and out. Microscopic observation of bacterial streaming and isolation of the bacterium may help rule out more serious pathogens.

The bacterium can overwinter on buds, in cankers, as an epiphyte on many plants, as latent infection in plants, and even in weeds and grasses. If you have had problems with bacterial blight in your landscape, there are a few management suggestions that may help prevent its recurrence.

  1. Avoid high nitrogen applications that produce great quantities of succulent growth in spring or fall. This tissue is most likely to be injured by frost, sudden weather changes, wind, etc; and injured tissue is most susceptible to bacterial infection. Fertilization is a good thing, but it should be balanced and not excessive.
  2. Some research in nurseries has shown that pruning trees in the fall and early winter actually increases their subsequent infection by Pseudomonas syringae. The suggestion is to prune in January or February.
  3. There is current research with the goal of developing plant cultivars resistant to this bacterial pathogen. Look for mention of such resistance when looking for plants for your landscape. As an example, most cultivars of common lilac are susceptible to infection by Pseudomonas syringae. Trials in Western Washington are attempting to identify resistant or tolerant lilac species and cultivars.
  4. Fixed copper fungicides have been tried with varying success in nurseries with production problems caused by Pseudomonas syringae. The compounds are used in the fall to kill the overwintering bacterium before winter injury occurs. Homeowner use of such compounds has not yet been advocated.

--Nancy Pataky

Nancy Pataky

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