Question
We are losing many lilac trees! They bud and then the buds wilt and die. The bark has grey round and oval shaped spots all over?
These spots vary in size. Sometimes the trees will bloom, or just grow leaves and then they wilt and start to die. The only thing we can think of is possibly a fungus?
We live in Nelson, British Columbia, Canada.
Any clues? and Thanks!

Answer
Hi Leah, I live in Sarasota, Florida and do not grow these trees.  So I checked the net and found this information, hope it helps you.

Pests
If properly located on an appropriate site, there are few problems, but borers can severely affect trees in certain areas.

Lilac borer larvae tunnel in the branches, causing wilting, particularly on drought-stressed trees. Severely infested branches may break off. Remove and destroy infested stems. Keep plants healthy with regular waterings during dry weather and by fertilizing.

Lilac leaf miner tunnels in the leaves in early summer. After mining the leaf, the caterpillars emerge and web leaves together and skeletonize the foliage. Light infestation can be controlled by hand picking.

Scales are most often found infesting the lower stems and often blend in with the bark. Inspect unhealthy-looking plants for scale infestations.

Diseases
Usually free of serious disease, but can be affected in certain regions by disease.

Bacterial blight is most serious on white flowered varieties. The young shoots develop black stripes or one side of the shoot turns black. Spots develop on the leaves, forming a water-soaked blotch. Young leaves turn black and die quickly. On older shoots, the spots enlarge more slowly. The flowers wilt and darken. The disease is worse when wet weather occurs as the new shoots are developing. Thin plants to increase air circulation. Remove and destroy diseased shoots and avoid excessive nitrogen fertilizer.

Phytophthora blight kills stems to the ground. The leaves turn black and shoots have brown lesions on them.

Leaf blotch causes zoned, brown spots. The infected area drops out, leaving a hole in the leaf.

Many fungi cause leaf spots.

Powdery mildew coats the leaves with white powder. During wet weather, Lilacs mildew easily. Mildew is especially severe on shade-grown plants. Ignore late season infections.

Verticillium wilt causes wilting and premature leaf drop. The disease may kill one, several or all the branches. Try fertilizing regularly to help prevent diseases.

Bacterial crown gall causes round, warty galls on the stems near the soil line. Remove infected plants and do not replant in the same spot.