Answer: To begin with, let's clarify the causes of "sudden oak death syndrome". A species of Phytophthora, a fungus, is responsible for killing tens of thousands of coast live oaks, California black oaks, and tanbark oaks in coastal California. The responsible fungus can be distributed by beetles, contact with infected roots, or transfer of wood or infected soil into the immediate area. The fungus flourishes in cool, moist conditions and once it is established, the spores travel in splashing raindrops. Since the disease is caused by a specific strain of Phytophthora, it's unlikely the same disease causing fungi will be found on healthy, nursery grown rhododendrons.
Of more concern is the potential to stress the existing trees by changing the soil grade or natural drainage during construction of your landscape project. Very old trees are particular about change. To be on the safe side, don't add soil near their roots, compact the soil by driving heavy equipment near the trees, and don't change the grade to any great degree or you might change the drainage pattern in the area. Construction damage can be lethal to trees, but often the damage doesn't show up for several years. Tread cautiously and your existing trees should be just fine, regardless of what you plant in your new landscaping project.
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