Yellowing and dropping of leaves is normal toward the end of the second summer on the small-leaved lepidote rhododendrons. These should have dense enough habit that this doesn't matter. The larger-leaved elepidote rhododendrons keep their leaves for 3 or 4 seasons. Like all evergreen plants, rhododendrons and azaleas periodically lose some of their foliage, and the leaves may turn yellow, red, or purple before they fall. Often the only leaves that remain are those that surround the flower buds at the tips of the branches. This characteristic is linked to the genetics of the parents used to breed garden azaleas. The degree of leaf coloration or loss is a function of parentage and not the severity of the winter.

I think the reason this is such a problem on your rhododendron is that it isn't showing any vigor. Here are the conditions necessary for a rhododendron to show vigor:

Drainage: extremely important.

Mulching: they have shallow roots, do not cultivate around them.

Acid Soil: most varieties require an acidic soil (pH 4.5-6). Cunningham's White will tolerate a pH downto 6.4.

Fertilizing: they don't need much fertilizer, but an overall yellowing means a lack of nitrogen or magnesium.

Partial Shade: too much sun can cause leaf burn (browning) and some yellowing

In the fall about all you can do is make sure drainage is good, apply magnesium sulfate, and mulch. To improve drainage you may have to replant in a raised bed. To improve acidity use sulfur, never use aluminum sulfate. It kills rhododendrons.
Other cultural problems include: Cultivating rhododendrons and azaleas must be avoided. They have shallow roots and the roots will be severely damaged by cultivating. Weed killer from weed & feed products is a definite problem also. Salt from sidewalks in the winter is a killer to azaleas. Soil near masonry such as foundations and walks is usually alkaline (not acidic) and a problem. Lawn fertilizer in the fall can set an azalea way back. Another problem is the roots of walnut trees. They emit a chemical that is toxic to rhododendrons and many other kinds of plants.

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

How much do you water the rhodie and how often do you water? Be aware that you can go easy with watering, now that temperatures have moderated. I could not tell if you had mulch but it would help (a) maintain the soil moist for longer periods of days and (b) protect from winter. I hope the yellowing is just caused by the arrival of Fall. My azaleas will not do that for almost two more months, more or less.

I have s similar problem as Adam79, in fact, I came here to post a question about why my new PMJ Rhody leaves where turning yellow and dropping off and saw Adam's post and saw that he lived in the same state as me.

It's also happening to one of my larger leaf Rhodys beside them. I have 4 in a row out in front of my house but they're pretty far away from the foundation of the house. I bought them in the spring and the did beautifully over the summer.

My PMJ's are the smaller leaf variety, are they prone to dropping their leaves this soon, they are fairly young. I have another large leaf Rhody on the other side and that one's doing okay. I do not have the best soil, houses in my area where built over slate years ago some of my soil is very white clay like. I do amend my beds but they're not raised. I basically ammend around the area where I want the plant to grow.

When we purchased this house, there were Rhody's planted in this same area but were back closer to the house. They were about 8 feet tall and blocking our bay window. They hadn't been prunned or looked after in over 5 years but they were growing well. My husband pulled them out and I picked out a smaller variety for the area in front of the window. I'm having a hard time understanding what's going on with my new ones where the ones that were pulled out were growing okay in this same spot, probably for about 20 years of so.

Any advice is greatly appreciated.

Linda

PJMs are very tough plants. My MIL near Haverhill, MA says she has started to see indications of yellowing of many plants due to Fall Color Change so your PJMs could also be starting to do the same. Just to make sure, monitor the soil moisture so they are not in dry or almost dry soil moisture conditions. Check that they are also not getting too much water. Mulch, about 3-4" of any type of acidic mulch, will aid greatly so add mulch if you have not done so. And test the soil pH to make sure there is nothing wrong in that area either. There are other problems that can cause yellowing of leaves but first determine if the pH and soil moisture are adequate. Soil pH test kits can be found at many local nurseries. They may be difficult to find this late in the gardening season so call ahead to save some gas and driving time.

Bringing this back up to the top to get some help with my Del Valley Whites. I do not recall them turning yellow in the past. A few weeks ago, I laid a couple inches of mulch and the yellowing seems to have started after that. My garden helpers said they'd heard that azaleas will do that in response to mulching. Sounds odd to me, but hey....maybe they're right. Any thoughts on this?

What did you mulch with? Depth of a couple of inches of most mulches should be just fine and not cause any problems.

Where are the leaves that are yellowing - are they the older inner most leaves? If so, that's normal. If newest leaves at stem ends are turning yellow, come back and we'll rethink this.

I'm having the same issue w/ my azaleas and I think it could be due to incorrect pH. If the leaves are showing green color along their central vein and yellow on the rest of the leaf areas, this is a good indication pH problems. Azaleas prefer soil with a low pH~ more acidic~ So even if you have been fertilizing regularly, the plant cannot access the nutrients if the pH is too high. They need a pH that ranges between 4.5-5.5 in order to thrive.

They can survive with a slightly higher or lower pH.

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My question is how do I go about lowering the pH of my soil? The place where My azaleas are planted is a small shaded area against the side of the house that measures about 3'x 8'. ****************************************************

I just want to add that there are a handful of causes for azalea leaves to turn yellow. Mine always have a few yellow leaves and at certain times of the year seem to have more than at other times~ which I considered normal. But right now, the yellowing is on about half of the plant~ the lower part so there's definitely a problem.

If the soil is too alkaline, acidity may be increased by adding flowers of sulfur (powdered sulfur) or iron sulfate. I add 1 tablespoon of sulfur powder around the base of any plant showing signs of chlorosis. Around established rhododendrons and azaleas, do not disturb the shallow roots. Sprinkle the sulfur under the mulch and then replace the mulch. Do not use aluminum sulfate. Aluminum can build up in the soil to toxic levels eventually. One very common source of chlorosis is when lime leaches out of concrete, such as from a foundation or walkway, making the nearby soil more alkaline. This problem decreases each year as the concrete ages. An annual application of sulfur can compensate for this problem.

Pounds of elemental sulfur needed to lower soil pH of a silt loam soil to a depth of 6 inches are given in a table on the Clemson Extension site:

Note: For sandy soils, reduce amount by 1/3; for clayey soils, increase amount by 1/2; if aluminum sulfate is used, multiply by 6.9.

Here is a link that might be useful: Lowering pH by Clemson University