Brown patches and dieback on Leylandii’s can be caused by Honey Fungus (mushrooms), Kabatina blight, Phytophthora root rot (bad drainage), poor planting, drought, excessive clipping and Cypress Aphids.
So most of the above list is down to poor management apart form Cypress Aphids (Cinara cupressivora), which suck sap from the stems and even quite light infestations can have a significant effect and be a slow recovery process.
These sap-sucking insect that infests the stems and are hard to see, as the are the same colour as the bark, a blackish brown and only about 3mm long.
You’ll see yellowing shoots in the early summer which become brown and dead, you may find a sooty mould develop on the stems, the affected area is normally towards the bottom of the plant.
Spray in early summer to prevent damage occurring with an Aphid control product containing azadirachtin. Please look carefully at what you are buy or use, as there is increasing evidence that product containing thiacloprid or thiamethoxam, can harm Bees.