This is probably not phytophthora, although it would certainly do no harm to take a leaf to the nursery where you bought the rhododendron. The difference between phytophthora leaf spots and those caused by sunburn is not always clear. The margins of phytophthora spots are usually described as "fuzzy" or "diffuse" while those from sunburn are described as having straighter, undiffused margins. The trouble is, this is often a judgement call and is not 100% reliable in the first place. Given the pattern of sunlight coming through the fence in your photo, I'd say sunburn is the more likely explanation. With the very real problems of phytophthora in the PNW, there must be many places you can contact for positive identification, including your extension service. BTW, as far as I know, only laboratory tests can determine which species of phytophthora is involved in an infection

Send a representative sample (leaves and at least a length of branch with tip) to the WSU plant pathology lab in Puyallup. There is a fee involved but the link should explain what is required.

I'd tend to agree more with sun scald than phytophthora........the last two weeks or so of unseasonably hot, dry, sunny weather is probably not doing any newly planted rhodies any favors. And while it's not impossible that Phytophthora ramorum could be present, the degree of oversight currently active in the PNW nursery industry regarding this pathogen would tend to put this issue at the lower end of the potential problem spectrum. Most of the better retail nurseries in the area are extremely conscientious of bringing in susceptible plant stock from certified 'clean' sources. And are checked regularly by the state DoA.

I agree with mainegrower and gardengal. This is highly unlikely to be Phytophthora ramorum.

That's sunburn.

Looks like sunburn to me also, right through those cracks in the fence. It probably was being stored in a shady area prior to you planting it and the recent sunny weather got to it.
The damage is cosmetic and it should do fine if watered adequately for the rest of the summer.
Your rhododendron is planted too close to the fence. Way too close. I'd move it next Spring.
Mike