Cause Pseudomonas syringae pv. syringae, a bacterium. A common problem in coastal British Columbia. The pathogen is ubiquitous and infects many other hosts; it is favored by cool, wet conditions. Poorly growing, stressed plants and spring frost can encourage disease development. Although the bacteria survive on the outside of the plant they must get inside and multiply in the space between plant cells (apoplast) to cause disease. These pathogenic bacteria inject several proteins and small-molecule toxins to get past host immune mechanisms. Once inside, the bacteria induce a watery, nutrient rich environment between the plant cells where they can multiply and continue colonization of the plant tissues. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.
Symptoms New cotoneaster growth wilts and turns black in April and May. Can be confused with fire blight.
Cultural control
- Prune out affected twigs and limbs with sterilized cutting tools.
Chemical control Spray at leaf drop in fall. Regular applications during wet spring weather may only encourage resistant bacteria. Do not overuse chemicals; bacteria resistant to them have been detected in many nurseries. Fixed-copper products may be phytotoxic and must be applied only under fast-drying conditions. Group M1 fungicides.
- Arbor-OTC is registered for trunk injection, see label for details. Group 41 fungicide (antibiotic). 12-hr reentry.
- Badge X2 at 1.5 to 2 lb/A. 24-hr reentry.
- CuPRO 5000 at 1.5 to 5 lb/A but only up to 2 lb/A when new growth is present. 48-hr reentry.
- Junction at 1.5 to 3.5 lb/A. May be useful when bacteria are resistant to copper products alone. Group M1 + M3 fungicide. 48-hr reentry.
- Nu-Cop 50 DF at 1 lb/100 gal water. 48-hr reentry.
- Phyton 27 at 1.5 to 3.5 fl oz/10 gal water. M1 fungicide. 48-hr reentry.
Reference Young, J.M. 1991. Pathogenicity and identification of the lilac pathogen, Pseudomonas syringae pv. syringae van Hall 1902. Annals of Applied Biology 118:283-298.