Diseases That Affect Lilac Trees

Lilacs (‌Syringa‌ spp.) are a group of about 20 flowering shrubs and trees, most of which are native to Asia. They are grown for their fragrant, showy blooms that come in shades of purple and pink as well as white.

Even when not in bloom, lilac trees and shrubs are still attractive plants. However, leaf diseases caused by fungi and bacteria are common lilac problems that can affect the appearance of the plants.

Types of Lilac Trees

Lilac trees grow as large shrubs or bushes. The common lilac (‌Syringa vulgaris,‌ USDA plant hardiness zones 3 to 7) is a large shrub with a height up to 12 feet. Some cultivars of the common lilac are more resistant to diseases than others.

The Japanese tree lilac (‌Syringa reticulata‌, zones 3 to 7), on the other hand, grows in true tree form with a height of up to 30 feet. The crown of this tree has a rounded, oval shape.

Katrin Schneider, CC SA-4.0, via Wikimedia Commons

Diseases of Lilacs

Let's go over some of the most common lilac diseases, their causes and how they can be managed.

Powdery Mildew

One of the most common fungal diseases of lilac bushes and trees is powdery mildew, which manifests as a powdery white substance on lilac tree leaves. This disease tends to occur in humid conditions, so providing good air circulation is key to keeping plants dry.

This disease affects the appearance of the plants but does not cause significant damage.

Fungal Leaf Spot

Many lilac species, including the common lilac and the Japanese tree lilac, are susceptible to leaf spots caused by the fungus ‌Pseudocercospora‌. Like powdery mildew, this lilac disease tends to arise in humid conditions and can cause browning of the leaves and extensive leaf drop.

Fungicides containing copper sulfate, triticonazole or tebuconazole can help halt the spread of this lilac fungus.

Bacterial Blight of Lilac

Leaf spots on lilac tree leaves can also be caused by the bacterium ‌Pseudomonas syringae‌ pv. ‌syringae.‌ The disease begins as brown spots on leaves that result in leaf drop and eventually move to the stem, causing wilt.

The bactericide copper octanoate can help control this disease. However, it is important to note that without a lab test, it is often difficult to differentiate bacterial leaf spots and fungal leaf spots on lilacs.

Leaf Scorch

When the soil in which lilacs are growing becomes so waterlogged during rainy periods that some of the roots die, it can result in a condition known as leaf scorch. The leaves of the plant dry out but remain attached to the plant.

Lilac Tree Care

Lilacs that are stressed, meaning they are not receiving enough water or nutrients, are more vulnerable to diseases. Therefore, sound cultural practices can help keep the plant healthy.

Lilacs grow best in full sun in moderately moist, well-draining soil. Established lilac bushes should be fertilized every few years with a fertilizer formulated for shrubs. Avoid fertilizers rich in nitrogen, which can encourage leaf production at the expense of flowers.

The pathogens that cause ‌Pseudocercospora‌ leaf spot and other leaf spot diseases on lilac bushes and trees are known to overwinter on fallen leaves and other plant debris. Therefore, raking up and discarding all the debris around your lilac tree at the end of the growing season can help to control the spread of these diseases.