Using a resistant variety is a very good idea.

Some varieties of rhododendrons are vulnerable (Chionoides, Catawbiense Album, Nova Zembla) and some are resistant ('Caroline,' 'D. A. Koster', 'English Roseum','Martha Isaacson,' 'Pink Trumpet,' 'PJM,' 'Prof. Hugo de Vries,' 'Red Head,' 'Scintillation,' 'Van der Hoop',R. davidsonianum, R. delavayi, R. glomerulatum, R. hyperythrum, R. lapponicum, R. occidentale, R. poukhanense, R. pseudochrysanthum, R. quinquefolium and R. websterianum).

But also keep in mind that culture is very important also.

Do not set new plants any deeper than the original soil level. Make sure you have good drainage. Do not plant in a low spot. Make sure the soil percolates. If it doesn't, then use a raised bed.

Use a soil mix that doesn't retain a lot of water. Nurserymen prefer a mix with a lot of bark dust.

Use a good mulch to keep the soil temperature down in summer. Phytophthora likes hot and wet.

Avoid stressing your plants in dry weather. Phytophthora also form cankers on plant in drought conditions. It

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Hi Rhodyman: Thanks for your reply. I should have added that these plants (and my house) are in native woodland, with virtually no interference from traffic, salts, urban pollution, etc. The soil is native leaf mold and dirt, very well drained and soft underfoot. No weed kiler or other artificial stuff. We like the "wild" look, and try to keep the (5 acre) yard as natural as possible.

The drainage is problematic or you wouldn't be having phytophthora problems. Phytophthora is everywhere, but it only raises its ugly head in areas that are moist. In a woodland situation, it can be leaf mold that is holding the moisture. Nurseries find they get more phytophthora if they use peat moss in their mix. It holds enough moisture to cause problems. Some still use shredded sphagnum peat, but also add elements such as bark dust, perlite and vermiculite to increase drainage. If you used raised beds with a well drained soil mix for the rhododendrons they would have less trouble. They have very shallow roots so it doesn't have to be raised much.

Your idea of using resistant varieties is the easiest way.