As one of the most fragrant flowering shrubs, gardeners throughout the United States enjoy lilacs, and they can choose from 28 species of the plant and over 100 different crossbreeds, according to Washington State University. Lilacs are durable and reliable plants that are able to withstand winter temperatures of up to 10 degrees C below zero. However, when affected by drought or poor nutrients, these shrubs can succumb to disease. Learning how to troubleshoot common lilac diseases will aid you in caring for your lilac bush.
Examine the leaves of the shrub. Damage with brown spots along the margins and central vein points to lilac blight, while discolouration along the veins and edges points to leaf scorch and holes or tatters points to leaf tatter. Yellowing of new foliage indicates micronutrient chlorosis, which is a lack of a needed nutrient.
Look at the bark on the bush. Splitting bark is rarely serious, although it can allow disease organisms into the plant, but diffuse or target-shaped cankers can be lethal.
Inspect the lilac bush for signs of powdery mildew, a fungus that appears like a dusty white or grey coating on the leaf and trunk. Powdery mildew comes off slightly if you rub the infected area, and it usually has a circular pattern.
Search for any sooty mould, which appears as a heavy coat of black mould on branches of the tree.
Diffuse cankers are caused by a fungus that grows through the bush's tissue. These cankers are shallow and cause discolouration. Because these cankers grow so quickly, healthy plants cannot keep up, and they can be lethal if not attended to. Target-shaped cankers are caused by fungi that grow through the bark in the dormant months. When the shrub enters the growing season, it may grow a callus around the infected tissue. Healthy plants can usually combat these cankers adequately. Lilac blight is treated through pruning the bush, preferably 20 to 25cm below the infection. If this is impossible because the infection is so widespread, consider using a fungicide with copper as an ingredient, as the Plant Clinic at Cornell University indicates this can be effective. Powdery mildew and sooty mould are both treated with fungicides. If cankers or bark splits are on the trunk of your lilac bush, damage can be reduced with tracing. This involves cutting outside the damaged or diseased area in an ellipse shape, which allows the tree to heal itself by growing a new layer of bark. Treat the micronutrient chlorosis by adding the nutrient to the soil around the plant.
If cankers or bark splitting cannot be removed without damaging structural integrity, the tree will need to be cut down and replaced. Cornell University warns against treating a micronutrient chlorosis without first testing the soil. Adding the wrong nutrient will make the condition worse. If tracing the trunk, disinfect your sheers with a mixture of 10 per cent bleach, 70 per cent alcohol and 20 per cent water for several minutes, as recommended by Cornell University. This protects the plant from disease.