Please check the water drainage system. if it is not proper and water is logging at roots it may cause root rot a fungal diseases like wilt or Honey Fungus.

No fungicide will eradicate this fungus. Try to plant varieties of privet that are known to resist the fungus. If your plants show symptoms, water them and use fertilizer that is low in nitrogen and high in potassium. Sterilize the tools you use for pruning. Try to remove infected plants with the root balls intact and do not plant more privet in the same hole.

To discourage honey fungus, you should make sure that your soil drains well and use a clean, woody mulch. If plant is infected, dig up the plant and its roots and get rid of them.

Spider MItes

To kill spider mites, you can introduce natural predators including pirate bugs, thrip, and the Geocons, species of big-eyed bugs. The Stethorus species of lady beetle destroys spider mites. These mites need humidity and mainly are used to attack spider mites indoors. One reason for severe spider mite infestations is that the use of insecticides have killed their natural enemies.