Marin County 

When most of us think about the word "wilt," we don't picture trees. Instead, we think about green shrubs and flowering plants losing all their support and flopping over, usually from lack of water.

But many diseases of forest and urban trees are classified as wilts. The pathogens that cause these diseases grow into the water-conducting vessel elements and tracheids in the sapwood of the tree, clogging them up and stopping the flow of water. Some of the better-known tree diseases that do this include Dutch elm disease, oak wilt, blackstain root disease, and . . . Sudden Oak Death!

Douglas-fir killed by blackstain root disease in southern Humboldt County

Well, Sudden Oak Death is partly a wilt. Scientists haven't completely determined all the steps involved in pathogen infection and death of tanoak and true oak trees. For a long time, it was assumed that P. ramorum killed trees solely by consuming the cambium underneath the bark. When the pathogen had done this all the way around the circumference of the tree, the tree died because it could not produce new phloem to carry organic compounds or new xylem to carry water and mineral nutrients.

But within the past couple of years, a team led by Jennifer Parke at Oregon State University determined, with the aid of electron microscopy, that P. ramorum does grow into the vessel elements and tracheids in the sapwood of tanoak trees (read the abstract here). This invasion stimulates the tree to produce hard structures called tyloses, which are like little hardened embolisms that protrude into the vessels. These tyloses may help slow pathogen growth, but they also slow water transport.

Thus, P. ramorum's mode of killing trees appears to be a multifaceted one. The OSU team continues to investigate exactly how this works, while other scientists are investigating whether P. ramorum also produces toxic substances that help to kill plant tissue in its path. This variety of ways to survive and grow inside the tree is matched by a variety of movement and survival strategies on the part of the pathogen in the outside world, again demonstrating how tough and versatile P. ramorum is--truly a supremely difficult forest problem to tackle.

P. ramorum hyphae in tanoak xylem. Photo courtesy of Dr. Edwin Florance.