Sally Scalera

Brevard County Horticulture Extension Agent
3695 Lake Drive -- Cocoa FL 32926 -- 321-633-1702

Sudden Oak Death

August 12, 2006

The calls have been coming in for about the past month wondering why oak trees are dieing. One caller wondered if it might be Sudden Oak Death (SOD). When I was in Ft. Myers in June for the 2006 ISA Tree Conference I was able to get an update on SOD. I’m happy to report that the news was good so, read on.

For some background information, SOD was first reported in 1995 in the coastal area of central California. Since that time, tens of thousands of tan oak, coast live oaks and California black oaks have been killed by a newly identified fungus, Phytophthora ramorum. On these hosts, the fungus causes a bleeding canker on the trunk. This fungus also infects azaleas (Rhododendron spp.), huckleberry (Vaccinium ovatum), bay laurel (Umbellularia californica), bigleaf maple (Acer macrophyllum) and California buckeye (Aesculus californica) where it causes leaf spot and twig dieback.

As of January 2002 the disease was known to only occur in California and southwestern Oregon, but has since been found in western Canada. Symptoms of this disease vary from host to host however, roots of plants infected with P. ramorum typically appear healthy. Symptoms may progress rapidly after infection or my not be visible for significant periods of time. Symptom progression is favored by temperatures near 20 degrees C (68 degrees F). This is a point in Florida’s favor since this disease is favored by cool, wet climates.

The same pathogen, P. ramorum, has been isolated in containerized plants grown in California nurseries and its spread has been traced through shipments to nurseries in more then 20 states, including Florida. The infested plants, mostly camellias, were destroyed. Camellias are the most susceptible plants here in Florida but it can also infect rhodendrons and viburnums. It causes brown lesions on the leaves with dark brown centers and lighter edges. So far, there have been no samples taken from landscapes or natural areas that shows P. ramorum has been introduced into Florida soil.

Now for the good news, a map of the U.S. has been generated to show the risk around the county for contracting and sustaining SOD. The levels of risk are low, moderate and high. These risk levels were identified using the following three criteria: 1) distribution of known and suspected host species, 2) likely pathways of introduction to new areas and 3) climate conditions that favor or limit the development of the pathogen. So the good news is that Florida is at a low risk for SOD. The two main reasons for this are that in our natural areas we don’t have many understory plants growing under our oaks that are susceptible to the pathogen and the disease does best in cool, wet weather which we don’t have a lot of.

So, what are some of the reasons for oak trees to be dieing? The main one would probably be the drought that we are still experiencing. Even though we have begun to get rain again we are still under our normal rainfall totals for the year. To water trees thoroughly, even in the landscape, a tree needs an inch of water applied at one time to wet the entire root system. I have noticed many trees in natural areas, both oaks and pines, that have just browned out and died.

Another cause of tree death can be construction damage. Trees can be damaged during construction and yet can take as long as 8-10 years after the damage occurred to finally die. There are a number of ways a tree can be damaged such as; adding fill around the base of a tree, removing soil from around the base of a tree, driving and parking vehicles underneath the canopy of a tree and putting driveways and sidewalks within the drip line of a tree. To have any chance of protecting a tree during construction the tree should have a wooden fence constructed, at least out to the drip line if not further out, BEFORE the land is even cleared and kept in place until the job is completed.

Hurricane damage can act a lot like construction damage where the trees received damage (a lot of times to their root systems) and then that damage begins a downward spiral which can result in death of the tree from a combination of causes.

The last possible cause of tree death, and this one is for oaks and wax myrtle, is caused by the fungus Ionotus rickii. This fungus is easy to recognize on the trunks and limbs of oaks and wax myrtles because it creates a dark brown to black stain that runs down the bark of the infected tree. At times there is also a reddish-brown “beard” that is formed at the site of the stain. This is actually the fruiting body of the fungus and if you brush it with your fingers you will get dark orange spores on your fingers. This fungus is fatal, but the larger diameter the trunk is, the longer (I’m talking years) it will take to kill the tree.