Cause Pseudomonas syringae pv. syringae, the bacterium that also causes bacterial blight of pear, blueberry, cherry, maple, and many other woody plants. Bacteria overwinter on diseased twigs or as epiphytes on healthy wood. Factors that weaken or injure plants predispose them to disease. Such factors include wounds, both accidental and from pruning or budding, frost damage, incorrect soil pH, poor or improper nutrition, and infection by other pathogens.
Bacterial 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. Mild, moist weather favors disease development.
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.
Symptoms The disease starts as brown spots on stems and leaves of young shoots as they develop in early spring. A yellow halo may also be around the spot. Spots become black and grow rapidly, especially during rainy periods. Further infectious development depends on the age of the part attacked.
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.
- Maintain adequate spacing between plants and prune to provide good air circulation within the canopy.
- Prune out and burn all affected tissues immediately.
- Plant resistant species or cultivars.
- Do not fertilize late in the growing season. Do not overfertilize young plants.
- 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.
Chemical control Integrate with cultural control tactics. Spray before fall rains and again before budbreak in spring. Bacteria resistant to both copper products and antibiotics have been detected in many nurseries. Copper products may be phytotoxic when used during the early growing season.
- Bordeaux (copper sulfate plus hydrated lime) at 4-4-100. An effective combination of products even when bacteria are resistant to copper. O
- Champ Dry Prill at 0.67 lb/100 gal water. 48-hr reentry.
- Copper-Count-N at 1 quart/100 gal water. 48-hr reentry general or 24-hr reentry for greenhouse.
- CuPRO 2005 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1000 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.
- Junction at 1.5 to 3 lb/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.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
- Nu-Cop 50 DF at 1 lb/100 gal water. 48-hr reentry.
- Phyton 27 at 1.3 to 2.5 oz/10 gal water. Effective against sensitive bacteria but ineffective against copper-resistant bacteria. Group M1 fungicide. 24-hr reentry.
References Gould, C.J. and W.E. Vassey. 1977. Lilacs resistant to bacterial blight. Proceedings of the International Lilac Society 6:48-57.
Scheck, H.J., Pscheidt, J.W., and Moore, L.W. 1996. Copper and streptomycin resistance in strains of Pseudomonas syringae from Pacific Northwest nurseries. Plant Disease 80:1034-1039.