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.