"What's killing those?" My neighbor leaned over my back fence here in Zone 7 Virginia. He pointed at three trees in my 'Leighton Green' row going rust-brown while the rest stayed green. The trees in that brown patch sat crowded together with poor airflow. I scraped the bark and found sunken, oozing cankers. A Leyland cypress turning brown like that is rarely thirsty. It is sick.
Browning has four common causes. Seiridium canker and Botryosphaeria canker are the worst, since they spread and stay. Phytophthora root rot rots the roots from below. Bagworms chew the foliage bare. Winter burn dries out exposed needles in cold, windy weather. A Leyland cypress dying back from the inside or in scattered patches almost always points to one of these, not to a missed watering.
You diagnose by where the brown starts and what the bark does. Seiridium cankers often ooze sticky resin and show a clear sunken lesion where a branch dies. Botryosphaeria cankers show little or no resin, so the dieback can look like simple drought stress at first. This is the trickiest cypress canker browning to spot, because the tree just thins out branch by branch with no obvious wound to point at.
- Sticky resin often oozes from the sunken canker.
- A clear, dark lesion marks where the branch dies.
- Thrives on heat-stressed and drought-weakened trees.
- Little or no resin, so it mimics plain drought.
- Branches thin out one by one with no clear wound.
- Hits trees already stressed by heat or poor soil.
Root rot fools people the most. Phytophthora mimics drought from the top down, so you water more and the tree gets worse. Dig near the base and you find dark, mushy roots instead of firm white ones. The fungus lives in the soil for years, so it keeps killing. Heavy clay and a wet, low spot make it far worse. Bagworms are easier to catch. Each female lays 500 to 1,000 eggs inside one spindle-shaped bag, so a few bags this year mean a swarm next spring that strips whole branches. Pick the bags off by hand in winter before the eggs hatch and you skip most of the damage.
Winter burn is the gentlest cause and the easiest to rule out. It hits exposed sites where cold wind dries the needles faster than frozen roots can replace the water. You see it on the windward face of trees out in the open, not in your sheltered middle rows. The brown shows up after a hard, dry winter and often greens back at the tips by late spring. If your browning sits only on one exposed side and the bark stays smooth and clean, you are looking at burn, not canker. That tree usually recovers on its own.
Here is the hard truth about cankers. They have no effective chemical control. No spray cures them, so your only real tool is your pruning saw. You cut the disease out and stop it from spreading. Catch it early on a few branches and you can often save the tree. Wait too long and the canker rings the trunk, which means the whole tree comes down.
- Prune: Cut infected branches back to clean wood, well below any visible canker or resin.
- Disinfect: Wipe your tools with a 10% bleach solution between every cut so you don't spread spores.
- Open it up: Thin crowded trees to improve airflow and let wet foliage dry faster after rain.
- Fix drainage so roots never sit in soggy soil, the setup that invites root rot.
One last warning if root rot already killed a tree. Do not replant a Leyland cypress in that same spot. The Phytophthora is still in the soil and it will kill the new tree too. Plant something rot-resistant there instead, or move your screen a few feet over to fresh ground. Give the survivors room to breathe, keep your saw clean, and you stop most browning before it spreads down the row.
Read the full article: Cypress Trees: Types, Care, and Common Problems
