I have a small lilac in the corner of my garden and after replanting into a bigger pot and placing it a different place this summer, it has developed spots all over the leaves. It also has something white growing on the soil around the stem. I've attached three pictures. Do you know if there's anything I can do?
The most common diseases of lilac are powdery mildew and lilac blight - this doesn't look like either of those. It's hard to tell from the picture - I only received your third pic - but are the spots on the leaf undersides able to be scraped off? That would be scale insect, and easily treatable with neem oil or soap spray: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
Also, the fact that the problems appeared after you repotted suggest there might have been something wrong with the soil you used. You might want to access the local Master Gardener organization, who could tell you how to get a soil test done. They have loads of information and assistance available for home gardeners. This link will help you locate the nearest group: