Here is an important point about trees, which bears repeating: Tree health is not the same thing as tree structural strength.

A healthy tree — one with a full canopy of perfect foliage, living branches and vigorous roots — can be rotten to the core.  Conversely, a sick-looking tree, with spotted, falling and discolored leaves can still be structurally sound.

Here are a couple of examples from recent incidents.  I stopped and took a close look at a huge eucalyptus tree that had fallen over. Driving by it on a regular basis, I had not noticed any signs of ill health. It had a normal-looking canopy of green leaves and living branches. After it fell over, it was easy to see that the wood inside the trunk was soft as a result of brown cubical rot. The decay-causing fungus must have been working inside the tree for years. If there were any outward signs, they were not readily visible to a casual observer.

On the other hand, I have seen at least three instances of eucalyptus trees in Napa Valley that dropped most, or all, of their leaves this summer. The shedding of foliage was almost certainly the result of a fungal infection that affects only leaves and small branches, not the inner wood that gives a tree structural strength.

That disease might have been caused by one or more species of a fungus called Mycosphaerella. Samples from trees with similar symptoms in San Francisco were submitted last month and are being analyzed by the California  Department of Food and Agriculture. Results will be published when they are available.

Such infections often result from environmental conditions that vary tremendously from year to year. This year, we had a spell of wet, mild weather from April 7-11, with high temperatures ranging from 57 to 64 degrees; conditions that are conducive to fungal infections.

Naturally, the distinction between health and structural stability is not simple. The two can be related and one can affect the other. Some diseases that harm tree health by attacking living tissue can also destroy the wood that keeps a tree standing. Oak root fungus is a good example. It’s a fungal disease that can begin attacking outer bark and living tissue, causing a loss of vitality, foliage yellowing and dieback. As it progresses inward, it breaks down the wood in the tree as well. So, it can result  in a tree that is both sick and structurally unstable. But some decay-causing organisms and structural defects  do not exhibit health effects in trees.

Don’t assume that a tree that looks healthy is structurally sound. Likewise, don’t assume that a tree that looks sick is a safety hazard.

Speaking of tree health, here is a letter I received about a  problem that is all too common. It’s about tree selection with respect to the local climate:

I live near Lake Berryessa and several years ago, my wife and I planted about a dozen Leyland cypress on our property and have enjoyed watching them grow and prosper until this winter.

I noticed that one of the trees, which are now over 14 feet high with trunk calipers of 4 to 5 inches, began to have dead branches. I noticed sap running down the trunks.  The trees are specimens, several feet apart, not a hedge, which we see elsewhere in the valley.

I finally removed the affected tree and am concerned that I will eventually lose the rest. I have tried to find causes and the only thing I can come up with is cankers. I added fertilizer and increased the water for the trees during the warm months. In late Spring I sprayed the trees with a fungicide. I have tried everything I can think of. The water and fertilizer has helped some in slowing the progress of the dead branches, which begin to wilt at the tips and then eventually affect the whole tree.

One suggestion was to use 20-20-20 fertilizer, which I have not been able to find.  I used a 10-10-10.

What could be causing the problem and what I can do to try and  save these magnificent trees?

A.C.

Dear A.C.

What you describe is exactly what I would expect:  cypress canker, a fungal disease caused by Seiridium cardinale.

It always infects, disfigures and kills Leyland cypress in and around Napa Valley. 

Sorry to have to say this, but you were ill-advised to plant them.

A fungicide spray program might slow the rate of new infections. One pest control applicator told me he has had success with a systemic fungicide (Banner Maxx). You would need to apply it continually for the life of the trees. 

The irrigation is a good step in favor of tree vitality. Mulch, properly applied, is helpful too. The fertilizer is not on my list of tools for this problem.

I suggest you start planning on replacing them, perhaps gradually as they die off, with something better suited to the local climate. Arizona cypress (Cupressus arizonica glabra) is a better alternative for the Knoxville/Berryessa climate.   Unfortunately, the available varieties are silvery grey, not dark green.

The cypress canker problem has been well known for many years. Still, not all nurseries and garden centers are on top of this, so let the buyer beware!  Sorry for the bad news, but I hope that helps.

Bill Pramuk is a registered consulting arborist. Visit his Web site www.bill

pramuk.com, e-mail questions to info@billpramuk.com, or call him at 226-2884.