When you grow lilacs in your landscape, the lavender blooms are often one of the highlights of the spring season. If you discover a white powdery fungus marring the surface of your lilac leaves as you are admiring the blooming lilacs, you probably react with horror and dismay. Take heart--the white powder you see is mildew and, while it is unsightly, it will not harm your lilac bushes. You can take steps to prevent and remedy white fungus on lilac bushes.
Provide water for lilacs only during periods of extreme and prolonged drought. If rain has been absent for several weeks, water the lilac bushes by directing the garden hose at the base of the lilac bushes. Never spray the leaves and foliage of a lilac bush with water because the moisture on the leaves can lead to fungus. Water the lilac shrubs until the soil around the bushes is moist and then stop. Do not water lilacs any more than this because this can lead to powdery mildew.
Fertilize the lilac bushes with the all-purpose fertilizer in early spring. Mix the fertilizer with water according to package recommendations for the size of your lilac bushes. Apply the fertilizer to the base of the shrubs. Fertilize the lilac shrubs one more time after the flowers fade. Do not fertilize the lilac shrubs any more than this because over-fertilization will lead to an overabundance of new foliage. This new foliage would be especially susceptible to fungus.
Prune lilac bushes immediately after the flowers fade in late spring. Prune out any diseased or dead branches before they show signs of fungus. Remove branches on the interior of the shrubs to increase the air ventilation inside the lilac shrubs. This will help to reduce chances of powdery mildew infecting your lilacs. If you find foliage that appears to have mildew, prune it out only if there is another reason to remove it (the branch is dead or rubbing on other branches). The mildew alone will not harm the lilac plant.
Spray a fungicide onto the foliage of the lilac shrub at the first indication of powdery mildew if you desire. Coat all surfaces of the plant (tops and bottoms of leaves). Repeat the fungicide application once per week during the remainder of the growing season to control powdery mildew.