Looking inside a swollen lower pecan trunk

I have visited many native pecan groves over the years and have always found at least one tree in each grove that has developed a swollen, disfigured lower-trunk. Trees with this condition are typically weak nut producers and often suffer from significant limb loss. As I work with growers to improve their native groves, I always recommend removing these unhealthy trees.

 The swollen lower trunk condition is most prevalent under two types of growing conditions:  Trees growing under extremely wet soil conditions (photo at right) or pecan trees growing in shallow soils along upland creek bottoms.  In both cases the trees are growing under water stress. I the first case, the tree and its root system is swamped by too much water while the tree growing on the upland site frequently suffers mid-summer drought. Less frequently, trees can develop a swollen lower trunk even when growing in close to ideal soil conditions.

While making plans to thin trees in our native groves this winter, I decided it was time to remove an unhealthy tree with a swollen lower trunk (photo at left).  But I’ve always wondered -- What is going on inside the trunk to make it swell like that? It was time for a closer look.  I started by looking at the bark all around the lower portion of the trunk. The affected tree had several obvious wounds that looked just like old pruning wounds (photo below). However, this tree has not had a limb removed from the lower portion of the trunk for more than 60 years. In that amount of time, an old pruning wound should have completely disappeared. Something else was causing these wounds in the bark. With further inspection of the lower trunk, I also found a nectria canker and a phomopsis gall (photo below).

Wounds visible in the bark of a swollen trunk

Nectria canker on the left and Phomopsis gall on the right

By looking at the bark of the tree, I could tell that several pathogens were infecting the lower portion of this tree’s trunk. However, I really wanted to find out what was going on inside this tree’s trunk to cause the trunk to swell.  I used my chainsaw to cut a section out of the lower trunk area so I could look at patterns of wood growth (photo at right). The piece of wood I cut from the tree extended from just above the point of swelling downwards towards the soil surface. Once I removed the trunk segment from the tree, I took the wood into my shop and sanded the vertical face perfectly smooth to reveal the wood grain (photo below).

Before I show you some of the wood deformities I found inside this tree, I think it is important to show you what normal wood growth looks like when cut longitudinally. Thephoto at right shows a section of pecan wood with a normal wood growth pattern. The annual growth rings, so easily recognized when a pecan tree is cut in cross-section, are a little harder to see this longitudinal slice through the wood. However, you should be able to see the alternating pattern of large-pore, spring wood and finer textured summer wood that provides a visual record of annual cycle of wood growth.

Towards the upper portion of the wood sample, I found what can generically be called a wood burl (photo at left). Growing outwards from a bark inclusion (the black crevasse in the wood) you can see deformed wood growth that almost appears like it is bubbling outwards towards the surface of the outer bark. Lower down on the trunk segment, I found multiple burls both near the surface and deep inside the wood (photo below). In every case, the deformed pattern of wood growth seems to have originated with a wound in the wood.     

After cutting into this tree, it is clear that trunk swelling at the base of pecan trees is due in large part to the development of burl wood. Burls can form in response to either fungal or bacterial infections but how these organisms penetrated the tree trunk is the real key to understanding lower trunk swelling. Each burl was formed in association with an injury in the wood. These injuries were most likely created sometime in the past by the feeding of wood boring insects. Trees that grow under stress (too much or too little water) seem to attract both dogwood borers and flat-headed apple tree borers. The movement of these insects in and out of the tree’s trunk most likely spread the pathogens responsible for burl formation and ultimately leads to swelling of the lower trunk.

Phomopsis trunk galls on pecan

   

     Every once in a while, I come across a native pecan tree with a baseball-sized, woody gall growing on the side of the trunk (photo at right). Look carefully and it doesn't even look like the gall is firmly attached to the tree. In fact, a sharp blow with your hand or hammer will usually pop the gall right off the tree.
    This gall is caused by a slow growing fungus in the genus, Phomopsis. The woody gall is formed by the tree's natural wound response to fungal attack. To prevent wood decay, pecan trees try to grow over and seal out invading fungi. However, in the case of a Phompsis gall, neither tree nor fungus seem to gain the upper hand in the wood decay battle. As a consequence the gall just grows larger.

   I knocked this gall off the tree with just the palm of my hand (photo at left). This was a fairly unique gall in that it represented the fusion of two infections into one elongated gall. Phomopsis infections usually start at a single point, most often at the site of a small broken off limb. In the photo, the yellow arrows point to the infection points of this double gall.
   Looking at the underside of the gall, (held in my hand) you can see that the woody gall is composed of a jumbled up mix of bark, wood, and fungus tissues. You can also see that this gall  actually started off as two round galls that later fused together when they began to touch.
    Phomopsis galls are not that hard to find in native pecan groves and they seem to have zero impact on nut production. 

Target canker starts with pruning wounds

   It looks almost like a platter attached to the trunk of a tree; but this is target canker caused by the fungus, Nectria galligena (photo at left).  The fungus doesn't attack healthy trees but requires a pruning wound or broken limb to enter the tree and attack the callus tissue that grows to seal over a wound.
   We usually don't make massive pruning cuts on pecan trees like the one in the photo, but following the 2007 ice storm, we removed limbs torn off by the weight of the ice. This wound became infected by the Nectria fungus. Each year, the tree forms new callus in an effort to seal over the wound but every year the slow growing fungus kills the callus. The result are concentric rings of dead callus tissue.

    Here's an example of a canker that demonstrates why they call this disease "target canker" (photo at right). This canker must have started on a small broken limb and the battle between tree and fungus continued over 15 years. In this case, the wound will never heal over correctly. The "target" will continue to grow in diameter right along with the tree.
    A single target canker on a tree seems to have little effect on nut production. However, trees with multiple cankers become obviously weakened and should be thinned from the grove.

    Normally a tree wound will become sealed over by a single large roll of callus tissue. Here's a photo (at left) of a tree wound that is healing properly. It will take just a few more years and this wound will be completely covered over.