TCD is a very interesting disease because it is caused by a fungus, but is carried from tree to tree by a beetle.
Without the walnut twig beetle, the fungus would have a very hard time infecting more trees. The walnut twig beetle’s life cycle includes four stages.
The adult lays eggs on a black walnut tree, and larvae hatch from the eggs.
The larvae burrow into the tree, and eat the xylem and phloem, leaving behind tunnels or “galleries”
where ever they go. The larvae will eventually pupate and turn into adults, who chew their way out of the tree and fly on to lay eggs somewhere else.
The fungus, Geosmithia morbida
, grows in the galleries created by the larvae.
The fungus also feeds on the tree, killing the plant tissues as it grows.
All of this happens on a really small scale, but eventually enough galleries are formed and become infected with Geosmithia morbida
that they kill the entire tree.
You won’t see the insect itself, but you will see the symptoms of yellowing leaves and top down dieback in black walnut.