My parents have had a lilac tree in their backyard for about 25 years. It came from saplings given by a neighbour. Over the years, some secondary shoots ("suckers", I think?) were removed as they broke under the weight of snow. Up until a year or two, it bloomed consistently every spring.
This year, it looks like this:
Large areas of the top have no flowers, only dried bits from last year, although all branches have leaves. There seems to be three main shoots that are devoid of any flowers.
On the older shoots, there seems to be moss or some white stuff on the bark. I don't know whether this is normal.
I don't think my parents have done any pruning of this tree over the years, except for removing obviously broken branches. They are now thinking about cutting the base of all the trunks that have no flowers. I asked them to wait until I could verify whether they could be salvaged. Is there anything we can do to make these trunks flower again?