- 1). Examine the leaves of the shrub. Damage with brown spots along the margins and central vein points to lilac blight, while discoloration 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.
- 2). 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.
- 3). Inspect the lilac bush for signs of powdery mildew, a fungus that appears like a dusty white or gray coating on the leaf and trunk. Powdery mildew comes off slightly if you rub the infected area, and it usually has a circular pattern.
- 4). Search for any sooty mold, which appears as a heavy coat of black mold on branches of the tree.
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