Actually, the common lilac (Syringa vulgaris) is a very robust, long-lived plant that is rarely attacked by diseases of all kinds. An infection is therefore often due to incorrect and inadequate care, an unsuitable location or unfavorable weather.
The lilac leaf miner moth can be responsible for brown spots
Leaf spot caused by pathogens
Brown spots on the leaves can be caused by various bacteria, fungi or viruses. The layperson can hardly diagnose the specific pathogen with the naked eye, but an expert gardener will be able to help you in this regard.
Pseudomonas syringae
The fungal pathogen causes a disease known as "lilac plague" or "bacterial shoot rot". It begins at the base of young shoots, which suddenly take on a striped, dark brown to black color from May. Later, irregular, dark brown spots also appear on the stems and leaves, and the inflorescences also turn brown and wither. The disease often occurs after very frosty or wet winters, and lilacs that are heavily fertilized with nitrogen are also at risk.
How to remedy the situation: Cut back the affected lilac deep into the healthy wood and burn the clippings. To prevent this, locations at risk of frost and fertilization with a high nitrogen content should be avoided.
Ascochyta syringae
This fungus causes leaf spot disease, which also causes the young shoots to wilt first, then turn brown and die. The leaves are also affected, they get irregularly shaped, blackish-brownish spots and the edges curl up.
This is how you can remedy the situation: The affected lilacs must be cut deep down into the healthy wood, the clippings must be burned or disposed of in some other way (but not on the compost!). You can also treat the plant with a preparation containing copper, which you can get in garden shops.
lilac leaf miner
The lilac moth is one of the most common pests and does not only occur on lilacs. They also find their feeding tracks on ash trees, forsythia, deutcia, snowberries and privet. The first damage occurs in early summer, when large, irregular, brown spots become visible on the leaves. Later the leaves wither and dry up. If you look closely (e.g. with the help of a magnifying glass) you can see caterpillars.
This is how you can remedy the situation: After an infestation in the previous year, you should spray neem several times in the following year when the leaves sprout. Further measures are not necessary.
tips
Old lilac varieties of the species Syringa vulgaris and the wild forms are usually more robust and less susceptible to diseases than new breeds or hybrids.