For
my ecology and management of invasive species class, we were required to give a
presentation on an invasive species in North America. For my presentation, I chose the butternut canker. The canker, Ophiognomonia clavigigenti-juglandacearum, is actually a fungus
that infects trees through any sort of scar or wound and quickly kills the
branches it infects before eventually killing the tree. The butternut canker is a prime example
of just how devastating and unstoppable an invasive species can be if given the
right environmental conditions. The fungus uses multiple vectors to travel from
individual to individual; these vectors included rain and wind dispersal,
insect dispersal, and seed dispersal.
The
butternut canker uses rain dispersal to move from their initial cankers on the
lower branches of the crown to infect the stem of the tree; once an
individual’s trunk has been infected, it usually dies within a matter of
years. Rain dispersal can also
lead to infectection between trees when coupled with wind. Butternut canker spores can be
transmitted via aerosols, or tiny droplets of rain, which can travel up to 40
meters away from the tree. Winds
can also disperse spores a similar distance on their own. What is truly terrifying is the
fungus’s capability to disperse under prime conditions. When temperatures are cool and there is
a high relative humidity, spores have been found to travel over 100 meters away
from the parent tree.
Another
common pathway that the canker uses to disperse its spores is via insects. Beetle species such as the butternut
circulio have bee found to carry as many as 8 million spores on one individual.
In addition to this, the circulio creates both feeding and ovipositor wounds in
butternut trees, providing the fungus with a direct route to infection of the host
tree.
The
last method that the butternut canker uses for dispersal is through infected
seeds. The canker can survive in
seeds at low temperatures for up to 18 months. This is dangerous for 2 distinct reasons: first, it kills
the young butternut while still a seedling and secondly, creates a new platform
for the fungus to further transmit the disease. The fungus can live on dead
trees for up to 20 months, meaning that it was plenty of time to disperse from
a dead host tree to a new butternut nearby.
Combating
such invasive species is an incredibly difficult, uphill battle; no matter how
much effort is put in to stop its spread, the fungus is already established in
the United States and has been found across almost the entirety of the
butternut’s range. At this point,
conservation of healthy butternuts is the only chance we have in preserving
pure butternut trees in the wild; there have been hybrids of butternut and
Japanese walnuts that are much more well adapted to resist the canker, but very
few butternut trees have similar resistance.