Question
I Have a row of about ten leylandii trees 6ft tall they have turned a red brown colour from the bottom three quarters of the way up.The top part of the trees look healthy.How can I tell if they are dead or not.

Answer
On the limb that has brown needles scrape a small bit of bark off and if the color under the bark is green the branch is still alive at this point on the branch. If the color is brown the branch is dead. If it is dead it is best to prune these off and destroy or move them from the site. It sounds like a foliage disease and if the dead limbs are left the spore from the disease could spread to the healthy needles. Cut the limbs off  back to the trunk.
It is more than likely one of two diseases:
Seiridium or Botryosphaeria cankers

Seiridium canker, caused by Seiridium unicorne, is probably the most damaging disease on Leyland cypress. Plants of all sizes and ages are affected. Cankers may form on stems, branches and in branch axils causing twig, branch or, at least on smaller plants, stem dieback. Cankers appear as sunken, dark brown or purplish patches on the bark, often accompanied by extensive resin flow. It should be noted that resin exudation often occurs from the branches and stems of otherwise healthy plants of Leyland cypress thus resin flow by itself is not a diagnostic characteristic for Seiridium canker. Scattered twigs or branches killed by the fungus turn bright reddish brown, and are in striking contrast to the dark green healthy foliage. Fruiting bodies of the fungus appear on the bark surface of the cankers as small circular black dots barely visible to the naked eye. Spores of the fungus are spread to other parts of an infected tree, or from tree to tree by water splash from rain or irrigation. The fungus also can be spread from tree to tree on pruning tools. Long distance spread appears to be through the transport of infected cuttings or plants.

Currently there are no chemical control measures recommended for the disease in the landscape or nursery. Avoiding water stress and tree wounding may reduce infection. Infected branches or twigs should be pruned and destroyed as soon as symptoms are noted. Prune at least one inch below the canker, and sterilize the pruning tools between cuts by dipping them in rubbing alcohol or in a solution of 1 part chlorine bleach to 9 parts water. Tools should be cleaned and oiled after using bleach to prevent rusting. Severely affected plants should be removed and destroyed.

Botryosphaeria Canker

The outward symptoms of Botryosphaeria canker, caused by Botryosphaeria dothidea, are similar to those of Seiridium canker; that is, scattered bright reddish-brown dead twigs and branches occurring in otherwise healthy foliage. Cankers on small twigs and branches are also similar to those caused by S. unicorne, except that extensive resin exudation usually does not occur. Unlike S. unicorne, however, B. dothidea may produce long, narrow cankers on the trunk that might extend for a foot or more in length. These cankers rarely girdle the trunk, but will kill any branches that may be encompassed by the canker as it enlarges. Fruiting bodies of the fungus, perithecia and/or pycnidia, are produced just beneath the bark, but their presence can be discerned as tiny raised pimples scattered over the surface of the canker. Spread of the fungus is similar to that described above for S. unicorne, except that ascospores of B. dothidea may be spread by wind.

Botryosphaeria canker most often occurs on plants that are under considerable stress, and an effective control strategy should include keeping the plants growing as vigorously as possible. Mulch plants yearly and provide adequate water during extended dry periods. Avoid heavy fertilization and severe pruning of established plants. Prune out and destroy dead branches. Effective chemical control is not available.