I would try to confirm the diagnosis with your local extension service before taking action. Follow the link below for more information. Your rhodies might have been affected by the lack of rain; some will withstand the low moisture conditions for a while but then surprise you and die off quickly.

As the article states, the fungicides only help prevent the infection before the plant is affected. If the plant is already infected, the fungicides only subdue growth of the spores but, once you stop treatment, the spores grow again.

So, consider if the other rhodies that are still alive may already be infected..... if this is truly a case of p. root rot and if they are close to the area where the dead ones were planted.

Good luck, Luis

Here is a link that might be useful: Oregon State University Extension Service

The prevention for phytophthora is good drainage. It is wishful thinking to think that fungicides will overcome a wet location's problems. Some varieties are more vulnerable such as Chionoides, Catawbiense Album, and Nova Zembla, while some are more resistant such as Roseum Elegans, Scintillation, and PJM.

What you are describing this time year is more likely drought damage or bark split.

Drought can cause entire branches or entire plants to die. We have had several years of drought here and we observe that if rhododendrons and azaleas are not watered during a drought some plants will die, but others will just have one section of the plant die. It seems to be the plants way to conserve what little moisture it has.

Bark split is most commonly caused by an early autumn frost while the sap is still high in the plant, or a late spring frost when the sap has already started to rise. For this reason it is dangerous to feed nitrogen to a rhododendron or azalea that could stimulate growth through to autumn. Don't use nitrogen after mid-June. When the bark is frozen, sap cannot get through and the bark splits. For this reason always keep mulch away from the trunk of plants. Bark split damage can be treated with grafting wax to prevent fungal and insect damage if caught early enough.

Other possibilities include:

Small cottony masses with long, waxy filaments extending from one end is a symptom of the long-tailed mealybug, Pseudococcus longispinus, which is a sucking insect that colonizes on the least accessible parts of plants. Since it is hard to see, systemic insecticides are preferred, but thorough treatment with a contact insecticide may provide control.

Plants wilt and die slowly when their roots become blocked. Root strangulation is best prevented by proper root pruning when planting. If the plant is not too far gone, it might be rescued by digging and removing the soil. Then cutting any circling roots that may be strangling other roots. The roots need to be opened up. On larger plants, some of the top must be removed to compensate for the weak state of the roots. Any time the roots are exposed, they must be kept moistened. Roots that dry out will die.

Wilt and slow death can also be caused by juglone poisoning from walnuts.

Plants wilt and die suddenly is usually caused by roots which are girdled by larvae of the Black Vine Weevil or Strawberry Root Weevil. Adult weevils feed on the leaves at night. Specimens may be collected at night for identification. The major damage is caused by weevil larvae which girdle the roots and kill the plant. Larvacidal drenches may be used to kill them but are of limited effectiveness. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays must be repeated until no adults emerge.

Entire portions of a plant die if a borer is in a branch. Borers only affect the portion of the plant away from the roots from the borer. If the borer is in the main trunk, then the entire plant will wilt and die. The plant can be save by cutting off the area with the borer and letting the plant regenerate from the roots.

Sooty mold growth on stems and petioles is a symptom of Azalea Bark Scale and Cottony Azalea Scale. These small sucking insects feed on the bark and exude a sticky substance that turns the stems black. To control, use repeated and thorough sprays of insecticide such as Orthene to the foliage and stems kill the vulnerable crawlers and nymphs.

Wow! Thank you both so much. These are fascinating. Well, depressing, but still, very interesting. I didn't want to wait ten days til I got back, to go to the arboretum nearby and have an expert look at the leaves, because I thought I might be able to stop the villain before it progressed, but it looks, sadly, as if that time period won't make that much difference. I already cut off the dead parts, but I'll go take another look at them armed with some of this extremely useful information that you've sent. And I'll get an expert opinion before taking next steps. Although it's been dry, I certainly thought it had rained enough for large very established (years and years for most of the shrubs around the house) shrubs to get by without supplemental water. I guess drainage could conceivably be a problem, but they've done so well for so long in that spot I guess I thought conditions must be right there. I have a lot of food for thought and observation here. Thank you again, so much. I'll keep studying. I really want to do something to stop the advance of whatever this is, it would be terribly sad if other of the big rhodies succumbed. Thanks again.

I live in Connecticut as well and have the exact same problem with a bunch of well-established 30 year old Rhodos. My tree service told me it was either drought or root rot but root rot is normally only found in the heat of summer. Since we both live in the same state, I am now thinking we are both victims of last year's drought. Please let me know how you do on this problem.

You've already received excellent advice and I'd add only these:
Root rot is generally associated with high temperatures and poor drainage. It's more of a problem in southern states than the Northeast. The fungicides used against it are pretty powerful and most require a licensed applicator as far as I know. By all means, don't use them unless you're sure root rot is the problem.
It's unlikely, but certainly not impossible, that root weevils (which are ever present) could do this much damage to large established plants.
So, I'm wondering if what you are seeing is the result of winter burn from unusually high winds coupled with low humidity - these conditions happened here 2 or 3 times this past winter. Do the rhododendrons look worse on the sides most exposed to sun and wind? If this is a case of winter dessication, it's possible that the plants will leaf out normally in time. Check the stems for small leaf buds. Check to see if the cambium layer just beneath the bark is green; if it is, the rhododendrons' vascular systems are still functioning and they should recover. They may not look great for a year or two, but they will not die. They will benefit from extra water (within reason) and good mulching this summer. Pine bark, especially if well aged, is an excellent mulch and has been shown to have some fungicidal action.