I have a 13 year old Japanese silk tree planted in a clay and dirt. It gets sun mostly in the afternoon. I had no problems with it. It flowers on the third week of June. This year the leaves look limp and a bit yellow. Some have fallen on the ground. And some of the branches don’t have any leaves. The past winter seem to be a normal winter. I have done nothing out of the ordinary. I live in the west end of Montreal.
Thank you for contacting the Toronto Master Gardeners with your question.
While the Ivory Silk is a dependable tree with few pests or diseases, it is somewhat susceptible to blights, leaf spots, wilt, ring spot virus, lilac borer and lilac leaf minor. Of these the symptoms you described most closely resemble Verticillium wilt which causes wilting and premature leaf drop. The disease may kill one, several or all the branches. This fungus resides in the soil and hence you have to be careful to clear away fallen leaves making sure not to place them in your compost bin. Unfortunately, there is no know treatment for this disease except for proper sanitation and cultural practices.
One of our earlier archieved posts titled Syringa reticulata ivory silk lists a number of other possible causes and their treatment.
After reading the description of the disease, and you are not certain that this matches your tree, then please do not hesitate to write back and perhaps send a photo of diseased leaves. Good luck with that.