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I have 2 mature lilac bushes and one dropped its leaves and what few flowers it had. This was in Spring so not normal. I think it is blighted and am unsure how to treat it. Do I cut it back and remove all old underbrush and plant debris? Do I need to start spraying them every Spring with fungal killer?
If the stems turned black from the tips back, then the shrubs do have blight. This disease is stimulated by stress; drought, damage, improper fertilization.
Did the entire bush do this?
The recommendation is to prune away all the diseased branches. Be SURE to sterilize the shears between cuts to avoid spreading the pathogen. Especially if the bush is near the second one. This pathogen spreads easily.
The diseased branches should be burned; do not add them to a compost pile.
Clean up all the surrounding area, rake away the dropped leaves, etc.
Chemicals are recommended only if the infection is very severe.
Hopefully, your bush will come back next spring.