New Threat to Walnut Trees

Another potential disease problem is over the horizon threatening our local trees.  This time it is the Black Walnuts that are at risk.

A fungal infection called Thousand Cankers has been killing Black Walnut trees in the western part of the United States for several years.  It has been confined to nine states in the Rocky Mountain area and westward until July of this year. Its range seemed to be associated with a different walnut called the Arizona Walnut.  Black Walnut is not native to that particular area but was brought in and planted by arborists, landscapers and others.

This summer it was confirmed that the disease had spread to at least one location in the Eastern USA,  Knoxville Tennessee.  Scientists now believe that it may have been present there for a number of years without anyone knowing about it.

At this time Thousand Canker disease is NOT present in Michigan.

The disease is caused by a fungus which is carried by a very tiny beetle called The Walnut Twig Beetle.  Despite its name the Twig Beetle  attacks larger branches or even the trunk of Black Walnut Trees by tunneling under the bark. They leave small “galleries” or tunnels in the wood caused by the beetle larvae feeding there.

When the larvae mature into adults they emerge from the branches out of small holes chewed through the bark. The fungus then infects the damaged area and causes a small lesion or “canker”.  These cankers spread very fast and merge together eventually moving from the outer bark into the cambium layer.  Each branch has a tremendous number of cankers which is how the disease got its name.

The cankers themselves are often difficult to see and identify.  A special lab test is needed for positive identification.

There is no cure or control for either the Twig Beetle or Thousand Cankers, plant pathologists are working on that though.

In the meanwhile we can help slow down the spread of this problem by not moving firewood just like we do to prevent Emerald Ash Borer from spreading.

There is no federal quarantine on moving wood products but the State of Michigan has issued its own quarantine against shipping articles made of wood from certain western states.

The USDA Forest Service has a good publication on this problem.

As I mentioned earlier, Thousand Cankers is not present in Michigan at this time. Keep in mind that there are a lot of other things that can cause a tree to show signs of  die-back  other than this disease.


Lichens on Trees

A few days ago a gardener proclaimed to me that an expert had told her that Lichens are found only on trees that are no longer growing. I don’t know what expert might have told her that but I’m sure she misinterpreted whatever may have been said.

Lichen growing on young crab apple tree.

The study of Lichens is a huge branch of Biology in and of itself. People who take an interest in that field of study often become quite rabid about the subject. I’ve been on expeditions where the Biologist talked as if the world existed for the sake of providing a place for Lichens to grow.

Lichens are actually a combination of two organisms, a fungus and an algae. They work together as one in order to survive in places where they couldn’t otherwise. The algae provides energy through photosynthesis while the fungus provides shelter and a place for the algae to live.

They can be found covering a wide variety of objects in addition to trees including rocks, roofs, bare soil and just about any other exposed surface.

On trees Lichens are harmless. They are actually quite attractive on trees adding an extra visual element to the landscape. Many gardeners will go to extreme lengths to get Lichens established on certain features in their garden. Lichens growing on the roof, on gravestones or other unwanted places is a whole different subject that we won’t cover here.

As for the gardener who thought that Lichens can be found only on non-growing trees, all she has to do is look in her own yard to see that Lichens are quite happy on actively growing trees.