Lilac blight prevention
Thank you for contacting us with this question, we're always happy to help!
Before I get to the copper treatment, I do want to bring up a couple of options that may help. We always recommend starting with cultural controls first and only applying pesticides when absolutely necessary.
We do recommend using resistant varieties of lilacs, so if you are thinking of planting more of them, we recommend some of the species and cultivars found at this link: http://pep.wsu.edu/hortsense/scripts/query/displayProblem.asp?tableName=plant&problemID=433&...
It sounds like you might have some experience with this disease already, so you probably already know that you should prune and destroy any affected parts of the plant. Don't compost them at home, as many of our backyard compost piles don't reach the temperatures necessary to kill this bacteria.
You can also reduce the incidence of this disease by not injuring the plants in the late winter or early spring when the bacteria can spread. When possible, do your lilac pruning after they have finished blooming in the spring, or in the summer when the bacteria is less likely to spread.
Research shows that covering lilacs in the spring with hoop houses or something similar, protects them as well as any chemical application. This prevents injury to the plants from frost, and it keeps off excess rain.
There are actually a number of different fungicide mixtures that may prevent this disease from injuring your lilacs. I'm going to insert this link, so you can see the full list of those that are available, legal, and effective in Washington state.
http://pnwhandbooks.org/plantdisease/lilac-syringa-spp-bacterial-blight If you are a homeowner without a pesticide applicator's license, then look for the chemicals that have an H at the end of the entry. These are homeowner approved products, like the Monterey Liqui-Cop on this list.
If you have any further questions, please don't hesitate to contact us here, or you can call your local Extension office. If you aren't sure where your nearest Extension office is, follow this link to a map of the state and click on your county for contact information: http://mastergardener.wsu.edu/program/county/
Good luck with your lilacs, and I hope you have loads of (bacterial free) blossoms next spring! Than you again for contacting us.