Acer campestre · Tree
Verticillium wilt: Improve drainage and avoid planting in soil known to be infected; remove severely affected trees to prevent spread.
Honey fungus: Ensure good soil drainage and avoid deep planting; remove infected trees and dispose of roots carefully.
Tar spot: Rake up fallen leaves in autumn to reduce spore load; generally no chemical control is needed.
Powdery mildew: Water at the base to keep foliage dry; apply a sulphur-based fungicide if severe on young trees.
Gall mites: Usually cosmetic; prune out severely affected shoots in winter to reduce populations.