Cytospora canker of spruce

About

Cytospora canker is a leading killer of Colorado blue spruce and is also common on Norway spruce, mostly on older, stressed trees. It kills branches from the bottom of the tree up, and trunk cankers can sometimes kill the whole tree.

Also called Cytospora canker, cytospora canker, spruce canker, Leucostoma canker, Cytospora

How to recognize it

Symptoms

Needles on a lower branch turn purplish, then brown, and either drop or hang on; the branch dies and the damage creeps upward over several years, though a single upper branch can also die. White to bluish-white or amber resin runs down the bark where cankers form. Shaving off a thin layer of bark shows tiny black pinhead-size dots.

Often confused with

Rhizosphaera needle cast also browns spruces from the bottom up, but it kills needles, not whole branches, and leaves rows of black dots on the needles. Resin alone does not prove Cytospora.

Worse when

Tree stress from drought, poor drainage, compaction, crowding, wounds and insect damage, mostly on trees about 15 years and older; infection happens in cool, wet weather.

How it spreads

Spores ooze from cankers in wet weather and spread in rain, wind, sprinkler water and on pruning tools, insects and birds, entering through bark wounds; the fungus lives on in infected branches.

When

Infection in cool, wet weather; dead branches are usually noticed in spring and summer.

Where

Mostly on spruces planted outside their native range, such as in the Midwest.

What to do, gentlest first

Prevention works best; most sprays only protect healthy leaves and can't cure infected ones. Always read and follow the product label.

1. Right spruce, right spacing Prevent

   Plant dwarf spruce varieties, or give full-size spruces enough room that branches do not overlap, and thin over time to let air through.

2. Reduce stress Prevent

   Water about an inch a week in dry spells at the drip line and beyond, mulch out to the drip line (kept a few inches off the trunk), make sure the soil drains, avoid bark wounds, and fertilize only by soil test.

3. Prune out dead branches in dry weather By hand

   Cut infected branches back to the trunk or a healthy side branch, only when the tree is dry.

   Disinfect tools between cuts with 70% alcohol (preferred) or 10% bleach for at least 30 seconds; rinse and oil them after bleach. No fungicide controls this disease.

Plants it affects

Colorado blue spruce