I called my local cooperative and, as a courtesy, a specialist in diseases of shrubs and bushes examined my plant. He found no evidence of phytophthora. Instead, he said it's infested with black vine weevil,
which he called a "formidable and severely destructive" pest.
The black vine weevil feeds on a number of bushes, including yew and hemlock, but its favorite is the rhododendron. Dead sections and yellow leaves in the spring can have a number of causes. If your bush exhibits these symptoms, examine the leaves thoroughly. They are food for the adult weevil, so check for bite marks along the edges, called notching, and small holes in the leaf bodies. The weevils are small, dark, and hard to spot, so this type of leaf damage is used as an indication of their presence. While they may look bad, those leaves don't pose a serious threat to the bush. Death generally comes by way of root-feeding larvae.
Adult weevils are slate gray to black, between 8.5 and 11.5 mm long. They have a short pronounced snout, a somewhat beaded appearance, asymmetrically arranged tufts of short gold/orange hairs along modified wings on the top of the abdomen, and elbowed antennae. They are flightless, and female only. They're also nocturnal.
Adults hide during the day in shady, moist locations like leaf litter, mulch, or on the stems of dense plants. If disturbed while on a stem, the adult will drop to the leaf litter where it becomes difficult to see due to effective camouflage. Source
After dark, they go about the business of eating and laying eggs. Adults require 21-28 days of foliar feeding in order to produce eggs.
Eggs are laid at night, either dropped to the ground while feeding, inserted into crevices on plants (Smith 1932), or deposited in the same hiding places where adults are found during the day, usually under the soil surface and up to a depth of 20 cm. (Nielsen et al. 1978). Once egg laying commences, the amount of feeding by the adults decreases. As long as moderate temperatures last, egg laying continues into autumn (Nielsen et al. 1978). Adults during their first year may lay 200-400 eggs apiece (Smith 1932). Source
The eggs hatch in waves, beginning in 10-14 days, into small larvae which are white with brown heads, legless and usually curled into a c-shape. These are the main source of damage, and, ultimately, death of the plant.
Injury caused by the larval stage feeding on the roots is highly destructive to plants. Feeding by larvae occurs from mid-summer through fall and in early spring. At first larvae feed on small tender roots, but in early spring, they move to the bark of large roots or the stem, sometimes completely girdling them. Source
When the weather turns cold in the fall, the larvae move deeper into the soil where they overwinter. They re-emerge in the spring, when they mature into adults, start eating leaves and the cycle begins again! Some adults also survive the winter. Those are particularly dangerous because in the second year, egg-laying increases to an average between 600 and 800.
Once a weevil colony is established, there's little hope for the plant, and aggressive insecticide treatment is generally required. There are a few management tools that work for some people, however, and can be worth trying. A popular method is to trap adults in burlap and eliminate as many as possible, thereby reducing the number of offspring.
Burlap sacks can be used as trunk wraps and are useful for single or multi-stem plants. Make 4-inch lengthwise accordion folds in the burlap bag, then hold one end against the base of the trunk, and loosely spiral the wrap. Do not tie the burlap to the trunk since adult weevils must climb into the openings in the vertical folds. To count captured weevils, unwind the burlap and shake it over a white sheet or concrete paving. Repeated and frequent removal of the adults could be sufficient for suppressing the weevil population in small residential plantings. Source
If you choose to use insecticide, in many states, it can only be applied by specially certified and licensed landscapers. Because eggs hatch in waves throughout the season, repeated applications are necessary. The first should be made in May, and continue at regular intervals through the fall. Soaking must include soil and foliage, and is best done at dusk, when the weevils are more active. Treatment is complicated by the fact that black vines are very resilient and develop immunity to individual chemicals, necessitating frequent changes in formulas. Also, even though they can't fly, they're fast walkers with a great sense of direction, so if a host rhododendron becomes unpalatable, they'll just move on over to the nearest one!
Some people prefer to remove the bush rather than using pesticides. We have a no-kill philosophy at our house, which includes all animals, but we love our plants too. I did speak with a certified landscaper, but he said he would charge $500 just to come discuss options. I'm waiting for a call from another landscaper, although we probably can't afford him either!
Here are some current pictures of my leaves. In case you're wondering, I was told that all the plants growing under my bush are not harming it, and there's no need to remove them.