Cypress aphid (Cinara cupressivora) is a relatively common cause of brown patches. A collaborative research project between the RHS and East Malling Research found cypress aphid to be associated with half of the cases of brown patches investigated.
Damage caused by cypress aphid develops in late spring and summer. It is found most often at the base of the hedge, but can develop at any height. Large greyish greenfly are sometimes found, but the browning often develops long after the aphids have left the foliage. Clues are sometimes left behind, including cast aphid skins and a black fungal growth (sooty mould) that grows on the sugary honeydew excreted by the pest.
Scale insects, usually juniper scale, are found quite commonly in affected areas. These, like the cypress aphid, feed by sucking the sap from the foliage. Heavy infestations of juniper scale are thought to cause some dieback, but this scale insect is common and is often incidental to the problem. If there are similar numbers of scale insects on parts of the hedge unaffected by the dieback then this pest is unlikely to be the primary cause of the problem.
A fungus called Pestalotiopsis is occasionally associated with brown patches, and can be seen as tiny, black fruiting bodies scattered over the affected foliage. This fungus usually attacks weakened or damaged plants, but once present it can sometimes cause quite severe die-back. Pestalotiopsis is more likely to be a problem in wet summers.
This disease is caused by the fungus Seiridium cardinale, and can cause branch die-back of leyland cypress and western red cedar. It is found most commonly on large trees, and is relatively rare in smaller hedges.
Honey fungus or Phytophthora root rot
If the browning affects most or all of the foliage of an individual tree in the hedge, or a tree dies completely, a root disease such as honey fungus or Phytophthora root rot could be responsible.
All conifers have little or no capacity to regrow from old wood. Over-enthusiastic hedge trimming can result in bare patches. The time of year when trimming is also important, even if it is just light trimming. A recent research project found that die-back appeared to be slightly more common after autumn trimming (mainly October). Trimming in the summer during times of plant stress, such as prolonged drought or hot, dry spells may also be a factor.
RHS Gardening Advice believes that many brown patches are likely to result from adverse growing conditions such as drought, frost, waterlogging or cold, drying winds, all of which could inhibit regeneration from the trimmed foliage.
If the browning affects most of the foliage of individual trees, check for root diseases, waterlogging or establishment failure.