Bacterium sources can include old cankers, healthy buds, low populations within plants (with or without cankers), leaf surfaces, nearby weeds and grasses, even soil. Bacteria spread by wind, rain, insects, tools, and infected nursery stock. As in most bacterial diseases, mild, moist weather favors lilac blight.

Two common genetic traits increase the bacteria's ability to cause disease. Most produce a powerful plant toxin, syringomycin, which destroys plant tissues as bacteria multiply in a wound. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.

Some species of Syringa have shown resistance in western Washington including S. josikaea, S. Komarowii, S. microphylla, S. pekinensis, and S. reflexa. Most cultivars of S. vulgaris are susceptible, but some have been observed with less disease when planted in a garden; those include 'Edith Cavell', 'Glory', 'Ludwig Spaeth', and 'Pink Elizabeth'. Note that 'Ludwig Spaeth' is highly susceptible under intense nursery production systems.

On young stems, infection spreads around the stem and girdles it so the shoot bends over at the lesion and the parts above it wither and die. On mature stems, spots usually enlarge along the stem, causing leaf death only within the infected area.

Young, infected leaves blacken rapidly starting near the margin and continuing in a wedge-shaped pattern down to the petiole. Eventually the entire leaf dies. On older leaves, spots enlarge slowly. Sometimes, several spots will run together, and the leaf may crinkle at the edge or along the midvein. Flower clusters also may be infected and rapidly blighted and blackened.

Buds may fail to open or may turn black and die shortly after opening. Symptoms are similar to those of winter injury, and it would be difficult to tell the difference-if there is any.

1. Maintain adequate spacing between plants and prune to provide good air circulation within the canopy.
2. Prune out and burn all affected tissues immediately.
3. Plant resistant species or cultivars.
4. Do not fertilize late in the growing season. Do not overfertilize young plants.
5. In spring, protect from rain and frost with plastic hoop houses or similar structures. This treatment has been as good as the best chemical method.

1. Bordeaux (copper sulfate plus hydrated lime) at 4-4-100. An effective combination of products even when bacteria are resistant to copper.
2. Copper-Count-N at 1 quart/100 gal water. 12-hr reentry.
3. Junction at 1.5 to 3lb/A. Can be more effective than other copper based products when copper-resistant bacteria are present. Spray solution pH should be above 6.5. 24-hr reentry.
4. Kocide 2000 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1,000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. Effective against sensitive bacteria but ineffective against resistant bacteria. Wettable powder formulations have been better than dry flowable, and both are better than liquid formulations. 24-hr reentry.
5. Lilly Miller Microcop (90% basic copper sulfate) at 1.5 Tbsp/gal water.
6. Nu-Cop 50 DF at 1 lb/100 gal water. 24-hr reentry.
7. Phyton 27 at 1.3 to 2.5 fl oz/10 gal water. Effective against sensitive bacteria but ineffective against copper-resistant bacteria. 24-hr reentry.

Scheck, H.J., J.W. Pscheidt, and L.W. Moore, 1996. Copper and streptomycin resistance in strains of Pseudomonas syringae from Pacific Northwest nurseries. Plant Disease 80:1034-1039.