Hello,

The white, feathery substance you’re observing on your lilac bush is likely powdery mildew, a common fungal disease that affects many plants, including lilacs. More information at these links:

What should I do about the powdery mildew on my lilacs? | Extension (unh.edu)

The leaves on my lilac are covered with a white substance. Is this a serious problem? | Yard and Garden (iastate.edu)

This disease typically presents as powdery white spots that start on the lower leaves and can spread upward, eventually covering most of the foliage with a white or gray fungal growth.

Here are some steps you can take to manage powdery mildew on your lilacs:

Prune the affected branches to increase airflow.

Dispose of fallen leaves or plant debris, as the fungus can overwinter in this material. More information at this link:

What should I do about the powdery mildew on my lilacs? | Extension (unh.edu)

Plant resistant cultivars such as ‘Charles Joly’, ‘Sensation’, or ‘Old Glory’ if you’re considering new plants.

Ensure your lilacs are in a sunny location with good air circulation to limit disease issues. More information at this link:

What should I do about the powdery mildew on my lilacs? | Extension (unh.edu)

While powdery mildew is more of a nuisance than a serious health threat to the plant, it can be unsightly. Chemical control is generally unnecessary, but if you choose to use fungicides, they should be applied at the first sign of the disease, and you may need to follow up with repeated applications as per the product instructions. Always follow the guidelines for safe and effective use of any garden chemicals.

This also looks like Bankeraceae fungi too.

For a confirmed and precise identification of the problem, I recommend you send your image by e-mail to the MSU Plant & Pest Diagnostics Lab. The experts there can also provide you with the best treatment options. Their email is <personal data hidden>. Please include your name. The lab will let you know if they need a sample sent. FYI, the lab may also ask for additional images.

Their website is at this link:

Plant & Pest Diagnostics (msu.edu)

Their Contact Information is at this link:

Contact Us - Plant & Pest Diagnostics (msu.edu)

This link will explain the details of submitting a sample to MSU Plant & Pest Diagnostics Lab:

Submit Samples - Plant & Pest Diagnostics (msu.edu)