Q: Three out of my twenty seven Leyland cypress’s are dead and most of the others have branches turning brown. Is there chemical that will stop this?
A: This is a common situation during and after a drought.
Leyland cypress has thin bark which splits easily in dry weather. The splits admit various damaging fungi. The most common is seridium canker. The canker encircles an infected twig and causes the needles beyond that point to die.
You can see the canker damage if you skin back the bark with a pocket knife.
There are no fungicides to control this disease. The best thing you can do is to prune out dead branches and spray rubbing alcohol on your pruning tool between each cut. You can also sterilize by filling a small bucket with 1:10 bleach:water and dipping the pruner head into the bucket after each cut.
In the future, remember that Leyland cypress is a shallowly rooted plant. Water it deeply once each week if the soil becomes dry.
canker damage on branch