Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
3/8/99
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning March 12
Can Cankers Be Conquered On Blue Spruce?
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
Of all the nasty fungi molesting landscape trees, the canker fungi may be the worst. They slither stealthily into the bark of branches and trunks, then devour the live bark and wood underneath. Their assault shows up first as areas of dead bark, but they've only just begun. When the fungus weasels its way completely around a branch, it's curtains for the branch; it dies from the point of attack out to the tip.
You might tend to overlook the first few dead, cankered branches, but more appear every year. A cankered tree wastes away slowly. Long before it dies completely, its landscape value is minus-zilch. You can prune and prune, but finally the cankered tree has more gaps than foliage and it's time to call Dr. Tree-vorkian.
In Iowa and the Upper Midwest, the Colorado blue spruce may be the most common victim of this bleak scenario. Widely planted -- some say over-planted -- for its beautiful blue color and picture-perfect symmetry, blue spruce are often attacked by Cytospora canker. This disease, caused by the fungus Leucocytospora kunzei, seldom rears its ugly head until trees are at least 15 years old -- in other words, until after you've completely fallen in love with your blue spruce.
The first sign of trouble is subtle: one or more dead branches, usually near the bottom of the tree. Looking more closely at a dead branch, you notice a patch of sticky, whitish resin on the bark -- this is the scene of the crime. Attack by the canker fungus resulted in resin leakage from the canker. But the resin defense didn't impress the fungus, which chomped away under the bark until the branch was girdled.
Some blue spruce owners tell us they ignored these early symptoms, thinking that it was normal for a blue spruce to lose its lower limbs as the tree matured. On the contrary, if a blue spruce is in a full-sun location --which is where it needs to be --even the lowest branches should continue to thrive. So if you spot dead limbs on a blue spruce, it's usually a sign that the tree is in trouble.
It's a good idea to prune out these dead limbs as soon as they appear. In most cases, they can be pruned back to the next largest limb or the trunk. Make sure to prune out all the areas with sticky white resin, because these cankers are factories for making new spores. During rainy periods, spores splash from the cankers onto nearby limbs, spreading the infection. So if you amputate promptly, you can slow down the spread of the disease.
But there's a catch, and it's a big one -- you can't stop Cytospora canker once it's started. In most cases, pruning out the dead limbs will only delay the inevitable. The reason goes to the heart of the secret life of cankers: they prey on stressed trees. In other words, if a tree wasn't already very stressed out, it wouldn't fall victim to these fungal scavengers.
Stressed-out trees have no energy left for self-defense, so microbial jackals such as canker fungi can move in for the kill. Unfortunately, once a tree becomes severely stressed, it's often difficult or impossible to reverse the slide. Fungicides are ineffective against cankers, and genetically canker-resistant blue spruce varieties are a wish rather than a reality.
Can cankers be conquered? Can you say the preceding sentence out loud, rapidly, five times in a row? The answer to the first question is "Yes, but...." Prevention is a large part of the solution to the canker threat. The way to canker-proof a tree is to protect it from severe stress.
But what stresses a blue spruce? The most common culprit in the Midwest is drought stress. Research has shown that drought-stressed blue spruces are vulnerable to Cytospora canker, but trees that receive enough water don't develop cankers even when inoculated with the canker fungus.
We can't avoid drought in the Midwest. But we can shield blue spruces from severe drought stress.
The strategy goes like this. First, make sure the root system is blanketed by a ring of wood chip mulch, about 3-4 inches deep, extending to the outermost branch tips. The mulch allows the tree's roots to stay cool and moist, and to avoid competition from turfgrass, which can steal most of the water from nearby trees. As the tree grows, you'll need to extend the diameter of the mulched ring. And you will need to replenish the wood chips every few years as they decay.
Second, water the root zone thoroughly, once per week, during dry periods in the spring, summer and fall. Don't water more often, because over-watering is one of the fastest ways to kill a tree. If your weekly watering soaks the entire root zone under the tree's canopy to a depth of at least 8 inches (you can check this with a shovel), you've given the tree what it needs.
The time to start a "wellness" program for your blue spruce is when you transplant it into your landscape. If you wait until cankered branches have appeared, it's usually too late.
Another recommendation, but not always a popular one, is to plant hardier tree species. Colorado blue spruce, as the name implies, are a long way from home in an Iowa landscape. Many native species are much tougher and less trouble-prone. Red cedar, hackberry, oak (bur, white, black, red), white pine, black maple, basswood -- the list goes on and on.
Even some non-native conifer species, such as white (concolor) fir, Black Hills spruce and Norway spruce, are generally hardier than Colorado blue spruce. And if you plant hardy species, you won't need a visit from Dr. Tree-vorkian.
ml: isugarden
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