I have a Dwarf Korean Lilac Standard that has looked fantastic for the last 11 years. Unfortunately, not so much this year! There are 3 main branches that all the leaves have turned brown and one branch with no leaves. I have scratched the bark on all of these branches and they are all green underneath. I don’t see any signs of insects or fungus on it, unless they are microscopic in size. I live in Toronto, so zone 6, and it gets full sun. I’m not watering it any more or less than I have in previous years. Is this a sign of old age …. the plant, not me! Is the plant about to die? So if I prune these branches back to the rootstock, will it rejuvenate? And if so, when should I do it — now, or later in the Fall? Any suggestions you can offer would be appreciated.
Hello: thanks for asking Toronto Master Gardeners for help with your Korean Lilac (Syringa meyeri). Although your lilac may look unpleasant this year, the good news is that the stems appear to be alive and that you don’t see any signs of pests. K. Lilac are not often affected by pests but it is possible that some as-yet-unseen pest is about. Please check carefully for scale which is know to effect Lilacs (but rarely) and is difficult to detect. K.Lilac pests
K. Lilac are often pruned into balls to give them a structural look, but yours looks relatively airy. However my advice would be to open up the plant further by pruning out a few of the older branches and some side branches. Do this now (in early July). You could trim or remove a few of your brown or leafless branches but no need to if their location is important to the shrub’s structure. However pruning parts of the browning stems will encourage new growth. Keep checking to confirm these branches are alive. If they die as the summer progresses please remove them. Here is a reference on how to prune. good pruning
As discussed in this Michigan extension services reference K. Lilac decline , it is likely that the environment has caused this problem. Although the winter was mild, if the section of the tree faces north-west, rapid changes in temperature or high winds might be the culprit. Also, planting too close to a neighbouring plant or fence, thus producing some friction or shade in this area may have been an issue. With good care over the summer (moist but not over watered soil, light topping of compost/manure now (if not done in spring) and later in the fall, proper pruning, and frequent observation by you for pests), these branches may respond by leafing out later this year or next. Note: it is hard to tell from the photo whether or not there are exposed tree roots: but the soil around the tree should cover all exposed roots. Leave a few inches free of new soil directly around the base of the trunk. Elevated roots might be a sign of poor drainage, or compaction. elevated roots
The links below contain information about similarly distressed K. Lilac and summarize nicely the possible environmental causes of your plants decline. I hope you find them interesting reading.