It's not diseased......but it does have a massive infestation of oystershell scale!! Here is a link to a very good fact sheet published by the Kentucky extension service that explains the lifestyle of this insect and how best to control : https://entomology.ca.uky.edu/ef433

Thanks that's super helpful. It looks like that's much more easily treatable than any of the lilac diseases.

I'm not sure I'd agree with you :-) Scale is one of the most difficult of garden insect pests to control and armored scale even more difficult. Timing of any treatments is critical - if you miss the proper window, your success will be limited. And with an infestation that severe, it's not easy to be thorough enough to achieve full eradication in one go. Depending on the importance of the plant and how bad the problem is, many gardeners just opt to solve the problem by getting rid of the plant.

Oh dear I guess I misinterpreted what I was reading. It sounded like a systemic pesticide would get the job done. I'll have to do some more reading.

yes.. i would apply a proper systemic tree and shrub product.. and be done with them ...

use according to the label ... bayer used to have one ... but i am not current on such ... you can figure out if this is what you need

https://duckduckgo.com/?q=bayer+tree+and+shrub+for+scale&t=ffab&ia=web

keep in mind.. dead or alive.. scale looks just the same ... dont expect them to disappear or something.. just because you killed them ...

ken

Unfortunately, systemic insecticides are not 100% effective either. And almost all have imidacloprid as the primary active ingredient and that raises grave concerns about toxicity to bees, other pollinators and a host of beneficial insects. Everyone has their own gardening standards but I'd be much more inclined to rip out a very common garden plant than contribute - in even a very small way - to a very concerning ecological issue.