Wednesday, June 5, 2013

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.