Honey fungus or Armillaria mellea can cause your hedges and trees to get brown patches of leaves/needles. On conifer's and cypress (leylandii etc.) this die-back can lead to you having to replace the plant.
What happens is the roots decay, with white fungus between bark and wood at ground level, then the sudden death of plant. Mycelium and rhizomorphs (spores that can show as white hair-like roots) may be present all year but the mushrooms only in late-summer to autumn.