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Bacterial Blight Caused by Pseudomonas syringae

May 8, 2002

This bacterial disease seems more prevalent in nurseries than in the landscape, but it is still a relatively regular spring inhabitant in gardens. You may be confusing it with frost injury, Botrytis blight, fire blight, or other factors that cause sudden blighting.

Hosts of this disease include (but are not limited to) lilac, magnolia, forsythia, mountain ash, flowering cherry, apricot, Callery pear, flowering dogwood, and even viburnum. 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 resembling frost or fire blight. Often the external tissues are darkened, but inner tissue may still be green. Confirmation in the lab consists of microscopic observation of bacterial streaming and sometimes isolation of the bacterium.

In most plant pathology literature, Pseudomonas syringae is considered a weak pathogen. It requires some sort of wound to enter the plant and does most damage to plants under stress. Nevertheless, 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. Strains of this bacterium can aid in ice formation (ice nucleation) at temperatures just above freezing.

Where does this bacterium come from, and what can we do about it in our landscapes? 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. Therefore, it seems it is just about anywhere we might grow plants. If you have had problems with bacterial blight in your landscape, there are a few management suggestions that may help prevent its recurrence.

Avoid high-nitrogen applications that produce great quantities of succulent growth in spring or fall. This tissue is most apt to be injured by frost, sudden weath-er changes, wind, etc; and injured tissue is most sus-ceptible to bacterial infection. Fertilization is good, but it should be balanced (N-P-K) and not excessive.

Some research in nurseries has shown that pruning trees in the fall and early winter increases their subsequent infection by Pseudomonas syringae. The suggestion is to prune in January or February.

There is current research with the goal of developing plant cultivars resistant to this bacterial pathogen. Look for mention of such resistance when selecting 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.

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.

Keep an eye out for this sudden blight of buds and shoot tips as our cool, wet weather continues. When in doubt, send a sample to the Plant Clinic for confirmation. Details on how to submit a sample can be found in issue no. 1 of this newsletter.

Author: Nancy Pataky


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