I may be misinterpreting the picture, but it looks like it is getting a LOT more sun than the others. Is the soil by its base dry? Is it mulched? Did there used to be something blocking the sun behind it?

(IME., deadheading rhododendrons is not vital, often just done for appearance's sake).

One to the right is showing indications of not being watered enough to grow and look its best - what these like is cool, moist organic soils that drain well, Whether or not the plant being asked about has been getting enough water it probably does have a soil conditions based problem of some kind. These can include soil based pathogens such as phytophthora root rot and honey fungus (Armillaria) - I suggest looking both of these up in order to see what to check for.

https://pnwhandbooks.org/plantdisease

It doesn’t get that much difference in sun. Maybe slight because the one on the left has another bush behind it. There did used to be an old fence behind them that I took out due to rot, 3 years ago. So it does have more sun than it used to have. There’s no mulch and I don’t water them much. Sounds like maybe they all need a drink. I read the link Embo posted and of all the rhododendron diseases listed, the lime induced chlorosis sounds most like this one’s symptoms. I don’t see evidence of root rot.

lets step back in time ... IT APPEARS .. but we dont know for sure ... that this has been there just about forever ... and to have grow to that size... it MUST have been pretty happy ... including sun.. light.. shade.. water and fert ...

so then we ask ... what had changed ... besides a change in a negligent owner ... [and to be clear.. often.. a negligent owner is the best.. because they dont screw things up with too much love]

and it was the removal of the fence ... which i think i understand to be on the side that blocked the sun from the plant and the root zone ...

i think you have a watering problem ...im thinking the soil is getting too much sun/heat .. comparatively speaking ... and its drying more or faster than it ever did before ...

you need to heavily mulch.. to keep the soil cooler ... and moister... in relation to the sun ...

i doubt a plant that grew to that size.. was lacking in any kind of nutrients.. else it would not have grown that big ... do not get caught up in the panacea... that fert fixes everything ... lack of fert is rarely the issue .. and is easily spotted within the first year of planting ...

https://duckduckgo.com/?q=panacea+define&t=ffcm&ia=definition

the first pic also looks.. from far away.. like someone was digging under the plant ... or disturbed the soil ... was that you ... or could you give a better pic ...

things happen in tree and shrub time... it could still be stressed from the fence removal ...

i would probably dig a few 3 to 6 inch holes.. around the plant.... by hand trowel... out halfway of the canopy ... AND FIND OUT.. about water down in the root zone ...

i suspect it is dry ... and i would put the hose on a trickle if that proved to be right ...

the trick is you learning how to water deeply on a hill ... it does no good to just let it all run down hill ...

all that said.. all kinds of thing can attack stressed plants ... but if you dont figure out the stress ... then fixing the secondary problems doesnt get you too far ...

all that said.. rhodies have a shallow fibrous root mass ... which would not appreciate heavy digging in.. and would be challenged by more heat/sun/watering problems ...

ken

Ken, when I moved here, there was a bunch of dying ground cover under this bush and the rhody to the right. I took that out and planted this new grou dcover you see, so I did dig holes to plant them, but there were small plants at the time, so not deep holes. I’m sure it’s pretty dry up there. I’ll try trickle watering and mulching. Here’s a more up close pic.

Here’s a picture of the week I bought the house and before the fence came down. Not sure it looked that much better then!

Soil alkalinity unlikely in Seattle. Can't say the same for drought stress, phytophthora or honey fungus.

I think it needs A good deep soak Hard to do on a hill) then mulched and then a good deep soak again. (easier with a deep mulch)You fertilized it, and that produces more growth, and that more growth needs more water. Rhododendrons, as a rule, don't need fertilizer in the Seattle area. Good soil health is more important.

The current flush of growth is quite stunted, so the fertilization may have needed to be repeated or more of it used during that first application. Like many rhododendrons in local plantings the two visible here have poor foliage color, showing in fact that fertilization of rhododendrons is often called for in this area - the one asked about having greener current season leaves than those of the previous year at this time is a hallmark of a starved plant that is concentrating nutrients into the younger leaves.

So is the consensus to water, fertilize and mulch, but don’t give up yet? And no cutting back?

As always effective remediation depends on examination and determination - nobody here can inspect your site and your plant, decide what it needs. You have to take care of that part at your end. We have suggested what to look for and think about, now the ball is in your court.