White, powdery substance on my 2 year old lilac bush

could be a number of issues you need to post a photo

Ir sounds like powdery mildew to me. Would it be possible to post a photo?

Powdery mildew. Consult person in your local gardening center for advice.

Got this off google:The whitish appearance is caused by the powdery mildew fungus. The white"powder" is composed of fungal structures (mycelium and spores). ... These fungal structures are especially evident on lower leaf surfaces. Powdery mildew is seldom serious, causing more of an aesthetic problem than harm to lilac plants.A

Try this for powdery mildew: buy some raw, unpasteurized milk (if you can find it!). Dilute it with water, then spray it on your plants. I saw this work like magic on a friend's plants. Enzymes in the raw milk ate up the mildew. Good luck!

The white powder is definitely mildew. By now, you probably have all the advice you need but one more thing ... make sure there is good air circulation around the lilac bush. I planted a lilac too close to a lot line (new subdivision) with the result that when my (rear) neighbor put up a wooden fence, the lilac was too squeezed in with the fence on one side and shrubs on the other. Result ... mildew! I cut back some of the shrubs and removed one shrub resulting in better air circulation and no more mildew!

See if this answers your question and let me know