Hello Paul,

Thank you for your patience as we worked to get answer for you.  It looks like the lilac bush has a fungal infection.  If you're interested in learning the exact pathogen to more accurately diagnose and manage the issue, that could be an option.  I'd be happy to connect with resources to do that.

https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac

https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight

Based on the photos you sent, the stems look like they are growing quite densely which can decrease air flow through the plant. 

I would recommend some different pruning options:

You can prune out all of the dying/infected stems and just leave the healthy stems, and see how it does for the rest of the season.

In addition, you can try some renewal pruning which is basically cutting out a third of the large stems and doing that each year until all the large stems are gone making space for the new growth.

Or you can also do a rejuvenation pruning and cut the whole plant to the ground in late winter. 

I recommend using a clean shrub saw for pruning the large tree-like stems.  For smaller stems you can use loppers.  It is good to use clean tools to reduce risk of introducing something new when the plant is weak, and then clean them afterwards so that the fungus doesn't spread to other plants.  It is best to prune a lilac soon after blooming or in late winter depending on what the goal and style of pruning you choose. Here is a resource for some pruning instructions but feel free to reach out if you have any questions.

https://extension.umn.edu/trees-and-shrubs/lilacs#pruning-1923160

https://www.chicagobotanic.org/plantinfo/lilac_rejuvenation

https://www.ag.ndsu.edu/news/columns/dakota-gardener/dakota-gardener-the-nuances-of-pruning-spring-flowering-shrubs This is an article so you can ignore the personal antics.  It has some helpful information especially in the bottom two thirds.  

Lilacs are not heavy feeders so it would maybe be appropriate to do a light feed in the early spring.  

I hope this information is helpful to you and please feel free to reach out again if you have further questions.

Best to you and hope all turns out well with the lilac!

You're very welcome!