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Plant Diseases/Lilac Bush


I have a very old lilac bush that was at our house when we bought it. It has whitish mold on the leaves...which I guess from reading and researching that the mold is not a threat to the plant. But, the leaves are starting to fall off at a great rate of speed! It's located at the back of the house where there is not a great amount of sunlight. Should I cut it down to get rid of the old branches? And how do I stop the leaves from falling? Thanks!

Dear Diane, You have a couple of problems here.  The white mold is powdery mildew, which does indeed harm the plant, because the leaves are falling.  Powdery mildew is a fungus, but the best way to get rid of it is to spray the plant with a lightweight horticultural oil.  SunSpray is good if you can find it.  DO NOT use cooking oil.  It is not the same thing and will saute your plants.  The oil will coat the leaves, dry the mildew, and then flake off.  Spraying a few times about 5 days apart should do it.  Be SURE to clean up all the fallen leaves and discard them, as they will only spread the fungus.  When watering, try to avoid getting the leaves wet.  Often the humidity alone is enough to cause this mildew.  Now another issue is that the bush is in the back where there isn't much sun.  Is it blooming well?  Often lilacs won't bloom in the shade.  I would suggest if at all possible, cutting back limbs of other trees to make a bit more light for the lilac, or if that isn't possible, to move it carefully to a sunnier location.  Don't move it in the summer; that can be deadly.  Be sure to get it dug out well - you might want to hire a professional for this.  That's why if you can make it sunnier in the back yard it would be your best route.  I hope this information helps, but be sure to write back if you have more questions.  Good luck, Melissa

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Melissa Johnston


Plant diseases, landscaping, tropical plants, roses, herbs, plant care, grafting, horticulture, plant identification, anything about plants I can likely answer.


35 years experience in various plant businesses, 1984 Certified Texas Master Gardener.

Master Gardener Association of Texas charter member

none as of yet, but I have a plant q&a book I am in process of submitting.

Magna cum laude graduate of Texas A&M, 1978

Awards and Honors
Plant growing awards, highest grade for Texas Master Gardener graduates.

Past/Present Clients
Past member of and was very highly rated. Owned landscape company in the past, Almost Paradise, and was very successful despite little equipment, no help, and no advertising. Lived well for two years until 9/11.

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