Here are some possibilities to consider....
1. If the
plant transpires water faster than it can absorb it, the result is wilting. So what can trigger this? Not enough water, too much sun, hot weather and windy weather. Most susceptible
plants will be those recently planted/transplanted and those with big leaves (think hydrangeas).
In your case, one
plant may be planted in a more windy location or one
plant may not getting as much water as the other.
2.
lilacs require at least 1 inch of water per week but this amount increases if you have sandy soil; it increases the further south you go in the map; and it increases the closer you get to the summer months.
So, try to determine how much water they are receiving now. Next time you water both
plants, put some open/empty tuna cans besides each
plant, let the sprinkler system activate and observe how much water they get. Adjust the watering as deemed appropriate.
If you use drip irrigation, make sure that the rootball of both
plants is getting wet; otherwise, water could be falling nearby but the rootball would be as dry as sand.
Decrease watering times as temps go down in the Fall but continue watering through the winter provided your soil does not freeze.
3. Here in Texas, I have to mulch 2-3" for about 1-2 feet away from the main trunk; the
plant also cannot get sun after 12pm-ish.
3. Rootbound
plants have roots that tend to circle in a loop and refuse to grow outwards from the main trunk.
plants that have been sitting in the plastic pots for eons suffer from this. The roots get bigger as time passes but they get no more additional water. The
plant can also suffer from malnutrition as well since the minerals near the rootball get consumed first.
4. One of the
plants can be suffering from a fungus called Verticillium wilt. You may need assistance of your local ag. extension service to make this diagnosis. I am not aware of chemical/organic controls for this fungus.
5. Do a soil test looking for additional problems. You do not want the PH Level near the
lilacs to become very acid since
lilacs like alkaline soil (Ph of 6 or higher).
Good luck Annette,
Luis