I have a few more questions to understand the site and tree conditions a bit more: 

• Is the tree in full sun? (6+ hours a day)
• What is the soil like?
• Where is it planted at your site? (ie. near drive way, road, etc.)
• Is there any sort of damage on the branches or trunk of the tree?
• When it was planted, was the burlap or any other material that was keeping the root ball together removed?

It is possible this is the result of frost injury. This species is susceptible to that in early spring and it could be affecting different parts of the tree in different years. Here is some more info about growing conditions for this tree from the Missouri Botanic Garden: Syringa reticulata 'Ivory Silk'

I wonder if it was freeze damage once again this year, and any sort of stress due to exposure to cold winter winds. 

One additional question - Are you using any sort of herbicide to control weeds in the surrounding lawn?



Okay, that could certainly be another factor that is leading to this decline. The active ingredient in most Roundup products - glyphosate - is a broad-spectrum herbicide, meaning it will injure or kill other plants contacted during application. It is important to avoid green plant material, exposed roots, and injured bark of desired plants. Depending on what herbicide was used in the lawn, it could also have a similar effect if there was accidental contact with the tree. 



Sounds like a good plan. One other thing to focus on is making sure the tree is getting enough water. This will bolster tree health and help it tolerate any stressors, such as drought. If there has not been any significant rain for ~2 weeks, watering the tree by running a hose near the base until the top 8 or so inches is moist can help. You may have to move the hose around to make sure you are watering the whole root system. More info on watering established trees from the Morton Arboretum: Watering Trees and Shrubs

Sure thing. Thank you for using Ask Extension!