I need gardeners help again!!

It looks like powdery mildew on the stems and what you are seeing is the fungal spores. Also lilacs need air and circulation around it to help fight off diseases, so you might want to consider trimming back the surroundings.

Here's a link about what yo can do for the mildew and insects that are common to lilacs:

https://www.gardenguides.com/79916-problems-lilac-bushes.html

Hello. One suggestion might be to look at the substance on your branches with magnification a common household magnifying glass might be helpful. As master gardeners we are provided with an inexpensive jewelers loop to look at plant problems or deceased materials. Identifying if it is a fungus or an insect like scale might be helpful to address your situation.Here’s a link that could be helpful.

https://ask2.extension.org/kb/faq.php?id=337610

If none of the responses or suggestions here seem to identify your problem you may consider a trip to your local Cooperative extension. Master gardeners are volunteers manning the helpdesk there and are available to assess and examine your plant problems. If they cannot provide you with an answer or diagnosis the plant materials are shipped off to the local university for examination and diagnosis there.

You might have white aphids - check out this article: https://plantcaretoday.com/white-aphids.html

I would take a picture of it to a local nursery. They'll tell you exactly what it is and how to fix it. I always find that they know best when it comes to local plants and what attacks them.

Not so sure about the powdery mildew. That usually looks like just white lightly spray painted leaves. Super fine with no grain like this showing. It looks more like the leaves were dusted with flour. It is possibly mold. Powdery mildew is a fungus and is treated with a fungicide.

Like roses, tomatoes and other disease-prone plants, lilacs need morning sun to dry overnight dew and dampness off their leaves. If other plants are blocking that early sun, see if you can prune or remove them. Otherwise, do everything you can to increase airflow to the area.

This includes pruning the lilac immediately after flowering every Spring to open up the center and increase the internal airflow. Remove some entire older branches down at ground level first, then take out some of the younger branches that are crossing or otherwise restricting airflow. Remove all the spent flower heads promptly as well. Do these chores immediately after the flowers have faded and it will improve the following year's flowering as well as help keep the white stuff at bay.

here is info for you

https://homeguides.sfgate.com/rescue-diseased-lilac-bush-66015.html

Hi! Hope this helps:

https://extension.psu.edu/lilac-diseases

Powdery Mildew On Lilac Bushes: Tips For Treating Powdery Mildew On Lilacs

https://www.gardeningknowhow.com/ornamental/shrubs/lilac/powdery-mildew-on-lilac-bushes.htm#:~:text=Powdery%20mildew%20on%20lilac%20bushes%20isn%E2%80%99t%20caused%20by,leaves%2C%20stems%2C%20and%20flowers%20if%20conditions%20are%20right.

Hi Ms Marianne, hope this article helps you out,

https://homeguides.sfgate.com/rescue-diseased-lilac-bush-66015.html