The lilac (Syringa) is considered to be quite robust and insensitive to diseases, but of course it can still get caught. The various pathogens - which include fungi as well as viruses and bacteria - are widespread in nature. All you can do is try to make the lilac strong and resistant to infection through a suitable location, sufficient planting distance and good, species-appropriate care.
What diseases can affect lilacs?
Lilac can be affected by various diseases such as fungal attack (Ascochyta, Gloeosporium, Heterosporium, Septoria), bacterial infection (Pseudomonas syringae), iron deficiency (chlorosis) or powdery mildew. For treatment, infected plant parts should be removed, the location checked or a suitable fertilizer used.
Flowers
If the lilac doesn't bloom, there isn't always a pathogen behind it. In most cases, the location is simply too dark or the plant does not receive enough nutrients. Possible waterlogging (caused, for example, by too frequent watering or too much rain) can also be behind it. In these cases, a change of location or a supply of fertilizer material helps.
leaves
In many cases, diseases become noticeable on the leaves, although in later stages other parts of the plant such as branches and trunks can also be affected.
Leaves turn yellow
If the lilac has yellow leaves, there are usually location problems behind it: either the plant is too dark, too wet (waterlogging) or it lacks nutrients. Replant them or provide them with suitable fertilizer.
Leaves are light green
Light green leaves with dark green veins are a clear indication of iron deficiency. You can treat so-called chlorosis by giving iron fertilizer. You should also check the location, as this phenomenon mainly occurs on soils that are unsuitable for lilacs. You should also remove any mulching (for example with bark mulch).
Leaves have brown spots
Brown spots on lilac leaves can look very different and therefore have different causes. Fungi are usually behind it: Ascochyta syringae (large, brown-rimmed spots), Gloeosporium syringae (very large, brown spots), Heterosporium syringae (gray-brown spots with a velvety surface) and Septoria syringae (yellow-brown spots). Furthermore, the bacterium Pseudomonas syringae initially causes light to dark brown leaf spots and ultimately shoot rot. Suitable measures in all cases: Remove or cut back affected plant parts, remove fallen leaves and spray lilacs with field horsetail broth. If there is a severe infestation, a copper spray (€16.00 on Amazon) from the garden supply store will help.
Leaves have whitish coating
Mildew fungi are quite common in lilacs. Special measures are basically not necessary, but you can prevent an infestation by spraying with field horsetail or garlic decoction in spring. Diseased plant parts are cut back and disposed of.
Shoots, branches and trunk
Wilting symptoms in lilacs can also have very different causes. For example, lilac disease caused by the bacterium Pseudomonas syringae and wilt disease caused by Verticillium fungi are common. In both cases, you should immediately cut back diseased plant parts deep into the he althy wood and dispose of the cuttings in household waste or burn them.
Root
You also have to be careful with honey fungus, which likes to attack old lilacs and cause root rot. If there is an infection, the only thing that helps is to clear the tree and eat the mushrooms.
Tip
Many of the symptoms described do not necessarily have to be caused by pathogens. Some pests also like to feed on lilacs.