By Tim Clancy.
My first arboricultural encounter with lethal disease killing a tree from the inside was about 25 years ago. The tree, a white alder (Alnus rhombifolia), was located in a small planter carved out next to an office building in La Jolla. The tree had been in decline for a few years; it was able to produce new foliage each spring, but that foliage would wilt and die prematurely.
Eventually, it was decided that the tree would never make a full recovery and it should be removed. This task was contracted out to a local tree care firm. They, too, were perplexed at the cause of decay. I decided to stick around for the removal to see if I could determine anything by looking at the inside of the tree. The contractor cut me a couple of "tree cookies" so I could examine the tissue of the vascular system.
The first thing I noticed was an odor that did not smell like freshly cut disease-free wood. (At least in my experience.) Besides the odor, there was some discoloration in tree rings representing the previous nine years’ growth. I was still unable to determine what the cause was, so I conferred with my friend and mentor. He put on his teaching hat and guided me through this mystery. One thing he confirmed was that in white alders, the odor could be diagnostic. He also confirmed that the discoloration of the tree’s xylem (water conducting vessels) was out of the ordinary. These clues and the assistance of a book (Diseases of Trees and Shrubs by Wayne Sinclair and Howard H. Lyon) eventually led me to learn about verticillium wilt.
Verticillium wilt is a systemic fungal disease that attacks many deciduous trees. The soil-borne fungal pathogen that causes it (either Verticillium albo-atrum or Verticillium dahliae) enters the tree through root wounds and causes a gradual wilting and death of branches, and can eventually kill highly susceptible trees. Affected trees will exhibit both wilt and dieback. Variable symptoms include rapid collapse of foliage and even large sections of the crown. Bark may crack and a brown fluid may ooze out of the trunk. Sometimes, leaf growth will be stunted, the leaves will cup and exhibit leaf scorch, and/or there is copious seed production, as well the eventual foliage decline.
It turns out that the tree I was dealing with was indeed subjected to root wounding nearly ten years prior to its removal. The building was taking on water at the base of the planter, so a waterproofing repair was made and the roots were compromised at this time. This is the year that the pathogen entered the tree and for the subsequent nine years, it slowly infected the tree until its untimely death.