It starts with one branch. The rest of the Leyland cypress is green, deep green, and then a whole branch on the sunny side turns the color of a paper bag. Two months later there is a second one, lower down. By the next spring the tree has a hole in it that will never fill.
That is Seiridium canker. It is a fungal disease, and Leyland cypress is the tree it hits hardest in North Texas. The tight rows planted along back fences in Melissa, Celina, and Prosper were sold on fast growth. Nobody mentioned that the same crowded, sheared, sprinkler-soaked way we grow them is exactly what this fungus wants.
What the fungus does
Seiridium lives in the bark and the wood just under it. It enters through small wounds: a shear nick, a storm-bent crack, a bagworm-chewed patch, the fine stress cracks a drought-stressed tree opens on its own. Once inside, it kills a lens-shaped patch of living bark. That patch sinks as the healthy bark around it keeps growing. Girdle a branch and everything past the canker dies of thirst while the roots are fine. That is why the symptom is one whole branch at a time.
Spring is spread season. Rain splashes spores out of old cankers and onto wet bark down the row. A wet April can move it through a whole screen. Our tree diagnosis guide shows where a sign like this fits in a full read of a declining evergreen.
Reading the signs
Flagging is the first sign most people see: a single branch, brown from tip to base, hanging in a green tree. Do not stop at the color. Follow the dead branch back toward the trunk with your eyes and look at the bark where it meets live wood. The canker is a sunken, darker patch, often cracked, and Leylands answer it with resin: clear to amber beads and streaks weeping from the patch. Resin plus a sunken patch under a dead branch is about as sure as a field diagnosis gets.
Check the trunk too. Small cankers on twigs are a pruning job. A sunken, resinous canker on the main trunk, especially one that wraps a third or more of the trunk, is a different verdict: the fungus is in the wood the whole tree stands on, and no cut fixes that.
Lookalikes worth ruling out
Spider mites are the big one. Mites bronze a Leyland from the inside out, low and across the whole tree, worst in hot dry weather, and the paper test finds them crawling. Seiridium picks branches one at a time, in any weather, and leaves a canker you can point to. Bagworms strip a branch too, but the bags hang right there on the bare twigs as the receipt.
Drought browns the whole tree evenly and the soil probe comes up dry to the knuckle. Winter burn browns the windward tips after a cold snap and stops there; our winter burn guide covers reading that pattern and the watering that prevents it. If the browning walks branch to branch through spring rains while the soil holds moisture, think canker, not water.
Why screens get it and lone trees dodge it
A screen is a fungus hotel. Trees planted six feet apart grow into each other, air stops moving, and foliage stays wet for hours after rain or sprinklers. Shearing makes it worse: it forces a dense outer shell of fine growth, shades the interior dead, and opens hundreds of small cuts a season. Add the North Texas pattern of a wet spring and a brutal August, and the tree spends the year stressed at both ends.
Sprinkler habits matter more than most owners guess. A rotor that soaks the foliage every other morning keeps spores wet, moving, and landing on fresh wounds. The same tree on drip at the root zone, or deep water every couple of weeks, is a much harder target. Steady deep watering in a drought keeps bark from cracking in the first place.
What to do, in order
Cut the infected branch out. Go well below the canker, into clean wood with no dark streak in the cross-section, and cut in a dry stretch of weather, never in rain. Wipe the blade with alcohol or a bleach mix between trees. Haul the cut wood out of the yard. Do not chip it into mulch under the row, and do not leave it stacked by the fence.
Then change the conditions. Get sprinklers off the foliage and onto the roots. Open the row up where you can: a light thinning for airflow beats a tight shear. Keep mulch flat and off the trunks. There is no spray that cures wood the fungus already lives in. Fungicide talk is prevention talk for the clean trees left in the row, and it only pays on a high value screen.
What not to do: paint the canker, carve it out, or shear the whole row hard to even it up. Paint seals nothing, carving wounds fresh bark, and hard shearing a sick row spreads spores on the blades while it opens new entry wounds on every tree.
The replacement call
One flagged branch on an otherwise strong tree is a pruning job, and the tree can hold for years if the conditions change. A cankered main trunk, or a tree that flags new branches two springs in a row after cleanup, is a tree on the way out. Leylands do not refill a dead side. At that point the money is better spent on removal and a tougher screen: eastern red cedar, Nellie Stevens holly, or Little Gem magnolia, spaced for mature width. Our guide to privacy screen trees that survive North Texas lays out the tradeoffs before you replant the row.
\nCommon Questions
How can I tell if my Leyland cypress has Seiridium canker?
Look for one whole branch browned and dead while the branches around it stay green. Follow that branch back toward the trunk and look for a sunken, dark patch of bark with beads or streaks of resin on it. That sunken resinous canker under a flagged branch is the sign. Mites and drought brown the whole tree at once, not one branch at a time.
Can a Leyland cypress recover from Seiridium canker?
The infected branch cannot. The fungus lives in that wood for good. The tree can hold the line if you cut the branch out well below the canker in dry weather, stop wetting the foliage, and get the roots steady deep water. A tree that keeps flagging new branches two springs running is losing, and replacement is usually the better money.
Does Seiridium spread to the other trees in my screen?
Yes. Spores wash and splash from a canker to nearby branches and the next tree over during spring rains, and they enter through wounds and stress cracks. That is why one dead tree in a tight row is a warning for the whole row: cut infected wood out promptly, haul it out of the yard, and wipe blades between trees.
What should I plant when a Leyland cypress screen dies out?
Pick something that does not share the disease. Eastern red cedar, Nellie Stevens holly, and Little Gem magnolia all screen well in North Texas clay and do not get Seiridium. Space them for their mature width instead of crowding them, and water the root zone, not the foliage.