Sudden oak Death syndrome
SODS or Sudden Oak Death Syndrome is caused by the pathogen, Phytopthera ramrum. Phytopthera is a type of fungal colony organism similar to a mushroom colony. Phytopthera is widely studied around the world because it's many variants infect a multitude of plant families. The Irish Potato Famine was caused by a Phytopthera infection on Ireland's potato crop.
Phytoptera ramrum infects many tree species. Discovered by observing the sudden onset of brown foliage that starts as a dead cluster of twigs, spreading into dead branches, then full sections of the tree die. Weaker trees tend to show signs of the infection before well tended trees. Wet patches may be seen on the bark of the tree within 4' of ground level.
A commonly prescribed treatment for trees found to have symptoms of Phytopthera, is to excise the bark exhibiting the characteristic wet patches on infected trees. Followed with an application of phosphorous blended with a penetrating solution.
I have other proven therapies to arrest the onset of Pytopthera. The most important being the application of wood ash to the soils around trees. This simple application seems to effect the Phytopthera colony by changing the soils ph range or possibly some other effect that retards the progression into the trees vascular system.
If you notice brown patches of foliage in your trees call an Arborist to get it checked out. Most tree services will give you a limited consultation for free.
If your tree is in a general state of decline, it often is not a Phytopthera problem, but it increases the likelihood of it becoming infected with Phytopthera.
If you are asking yourself if you should call an Arborist to check out your trees, you should call an Arborist to check out your trees. It is possible to help infected trees if caught early, ignoring symptoms will lead to the tree's death, the cost of removal and decline in property value.
11/14/12 : Returned from an arborist conference last week and the latest information is that it appears that there is a component of infection associated with California Bay Laurels (Umbellaria californica) that must be present before the Phytopthera pathogen can get established.There is no evidence to support the idea that infected tree mulch or firewood harbors and spreads the disease. There is no evidence that timing of pruning has any effect on the spread of the disease. It is now strongly recommended to remove all California Bay Laurel within 300' of an oak that is considered a landscape asset.
Furthermore, there are many other pathogens that can sicken and kill native oaks with symptoms that look simular to SODS (Phytoptera ramrum). Have an arborist inspect your oaks periodically to protect them.