Syringa vulgaris · Shrub
Lilac blight (bacterial): Prune out infected growth in dry weather and avoid overhead watering to reduce spread.
Powdery mildew: Ensure good air circulation by thinning the canopy and apply a suitable fungicide if severe.
Lilac leaf-mining moth: Remove heavily infested leaves and encourage natural predators like parasitic wasps.
Honey fungus: Improve drainage and remove infected plants completely to prevent spread to neighbours.
Willow scale insects: Scrape off heavy infestations by hand or use a horticultural oil spray in winter.