If the lilac (bot. Syringa vulgaris) loses its leaves in autumn, that is no cause for concern. Falling leaves in autumn is quite normal, as the shrub is only deciduous and gradually goes into hibernation from October.
If the lilac sheds its leaves prematurely, measures must be taken
These causes can be behind the loss of leaves
The situation is different, however, if the lilac sheds its leaves in the middle of summer or does not get any at all. In this case, there is a serious problem behind it, the cause of which should be found and eliminated.
lack of water
Especially in hot dry periods, a lack of water in lilacs quickly becomes noticeable: the shrub then lets its leaves hang and gradually dries out. Later, the leaves are shed to reduce evaporation. The ornamental tree has very large leaves that condense a lot of water. You can remedy a lack of water by watering vigorously, but you should not wet the leaves - otherwise mildew could attach to them.
waterlogging
But before you reach for the watering can, it is better to check again whether a lack of water is actually the correct reason for the leaf shedding. More often, too much moisture or even waterlogging is the cause, as a result of which the roots rot and can no longer adequately supply the foliage above ground. If the damage has not progressed too far, you may be able to save the shrub by pruning it back and moving it to another location, but in many cases clearing is unavoidable.
Diseases
If the leaves turn brown or get brown spots before they are dropped, then a fungus is the cause. In addition to a fungicidal infection, bacteria (e.g. wilt bacteria or the more common lilac disease) can also be behind it. To treat the lilac, you must cut back the affected parts of the plant down to the healthy wood and collect any leaves that have fallen to the ground. These are either to be disposed of with household waste or burned, but under no circumstances should they end up in the compost.
pest infestation
The larvae of the lilac leaf miner feed on the leaves of the lilac. You can recognize an infestation when the leaves develop brown spots, curl up, dry up and finally fall off. In the current year, no treatment other than collecting affected foliage is necessary. In the following year you can spray neem when the leaves shoot out.
tips
If lilacs are not pruned for many years, they gradually wither. This is not a sign of an illness, but a completely normal sign of aging. The shrub can be rejuvenated by a courageous pruning.