No one has done more to promote investigation of the current epidemic of “sudden death” of oaks than the highly respected horticulturist, Peter Moerdyke. When the disease had annihilated several hundred coast live oaks and tanbark oaks on his rhododendron farm near Scotts Valley, Mr. Moerdyke was dismayed to find that not only was very little known about the disease, but that very little was being done to find the cause, or more importantly, a cure. He immediately began to publicize the importance of more extensive investigation as well as the need for treatment of infected trees. After many articles in nearly every local newspaper and news broadcast, entomologists, botanists and pathologists from all over the continent began to converge on the farm and eventually obtained federal and state funding for continued investigation.

The two main pathogens associated with the disease have been the western oak bark beetle and the oak ambrosia beetle. In a normal ecosystem, the western oak bark beetle attacks stressed trees. This additional stress attracts oak ambrosia beetle which introduces ambrosia fungus as it bores into main trunks near the ground. The fungus, which is the only sustenance for the beetles, quickly clogs the vascular system of the infected trees, causing sudden death.

Since the beginning of the epidemic however, both species of beetles have been attacking seemingly healthy trees. Because the beetles are considered to be secondary pathogens, it was necessary to isolate a primary pathogen, although any symptoms of infestation were only perceptible to the beetles.

Only recently has the primary pathogen been identified as a strain of Phytophthera. This discovery was made with a sample taken from one of Mr. Moerdyke’s trees in which an earlier infestation of beetles had been eradicated. Although this discovery is of major importance, there is a disturbing irony associated with it. A similar fungus was also isolated from a rhododendron.

Before I continue, I should explain that it is not yet known if the two samples of Pytophthera are of the same strain. It is also important to know that rhododendrons which have not been infected do not transmit the disease. The likelihood of infection of rhododendrons in the landscape relative to oaks is not yet known; but treatment of infected plants is much more manageable.

Infection of rhododendrons only occurs on foliage during rainy periods and only near the ground. Spores of the fungus are airborne, but seem to complete part of their life cycle in wet soil before being splashed onto low foliage. Infection causes leaves to turn black and rot. It easily spreads to nearby leaves, into petioles and eventually into small stems.

Rhododendrons should be periodically inspected during and after prolonged wet weather. Although leaves may discolor for a variety of reasons, it is best to remove any that appear to be damaged or infected, regardless of the symptoms, especially those close to the ground. Any stems which die should be pruned back to healthy stems. Leaf litter below plants should be removed. A loose mulch, such as pine needles, will help to minimize splash from surrounding soil. Soil fungicides may prevent the spores from developing.

Unfortunately, the effects of Phytophthera in oaks is not so obvious. Although both species of beetles may be controlled with insecticides, long term tolerance of the primary fungal infection is not yet known.