Symptoms and damage
this disease is caused most often by the fungus Seiridiumunicorne. Generally
speaking, cankers are dark, oval or elongated lesions that are usually dry, may
be sunken with a raised edge, and are surrounded by living tissue. More
specifically, Seiridium cankers form on stems, branches and in branch axils.
They are often thinly elongated, shallow, and the surrounding bark shows a dark
brown to purplish discoloration. A single Seiridium canker does not enlarge to
girdle a branch. Instead, multiple cankers form around a branch, reducing water
flow significantly. In addition, the cankers are often accompanied by an
extensive flow of resin from cracks in the bark. However, resin flow by itself
is not a defining characteristic of this disease as it often occurs in
otherwise healthy Leyland cypresses. A shallow cut at the site of the canker
typically reveals tissue beneath the bark that is reddish to brown in color and
sticky. Branches affected by Seiridium canker experience dieback. They are
often yellow to grayish tan to reddish brown in color and usually appear
randomly distributed on the tree. When a cankered branch has only recently
shown a change in color, needles will fall off easily when a hand is run along
the branch. Generally, the discolored branches are the symptom noticed first.
If the fungus gets into the main trunk, it can kill the entire tree.Environmental conditions that stress Leyland cypress (especially
drought, but also spring freeze damage) favor the development of infection. The
fungus survives winter in infected tissue. Spore-producing structures of the
fungus appear on the surface of the canker as small, black dots that are barely
visible without a magnifying lens. Spores can be spread (within the tree and to
new trees) via rain, overhead irrigation and pruning tools, typically entering
the trees through wounds and cracks in the bark. From the point of infection
until development of cankers and other symptoms often takes years.
When planting as a screen,
provide enough space (a minimum of 12 to 15 feet) between trees for good air
circulation and to minimize stress as they mature and enlarge. Avoid
over-fertilization. Place mulch under trees to at least the drip line (and
preferably further) to reduce water evaporation and competition for water as
well as potential damage to trees from lawn mowers and string trimmers.
Drought-stricken trees are significantly more susceptible to
infection. However, field trials have shown that trees inoculated with Seiridiumunicorne that developed cankers were able to
heal within 1-2 years once a regular irrigation schedule was implemented. As
such, make sure Leyland cypresses are irrigated during drought or semi-drought
conditions. Apply water at the base of trees to keep branches dry and reduce
disease spread. If overhead irrigation is necessary, it should be applied very
early in the morning.
Prune out and destroy diseased branches as soon as possible.
Make pruning cuts 3 to 4 inches below diseased tissue. Sterilize pruning tools
between each cut by dipping in a 10 percent bleach solution (1 part chlorine bleach to 9 parts water)*. Severely affected trees
should be removed and destroyed. *Note: Be sure to clean and oil pruning tools
after this procedure to prevent rust formation.
Fungicides are not considered an effective or practical means
for controlling canker diseases in general or Seiridium canker in particular.