I'm embarrassed to say I've not been that observant to notice whether the yellowing leaves have been pre-leaf drop, but I've noticed that other rhododendrons in the neighborhood look much greener through the winter and spring. That said, they may be other species of rhododendrons. It also seems like a lot of the leaves on these catwabiense are pale.

Soil samples had shown alkalinity, which is why I was adding sulfur. I test at least once/year.

That fading green color may be normal. "Large, leathery, alternate, glossy dark green leaves; 3 to 6 in. long; may develop yellow-green winter color"

https://plants.ces.ncsu.edu/plants/all/rhododendron-catawbiense/

I'd suspect possibly drainage if you were in a wet winter area - or compacted soil - and not PH but in a PA winter I doubt that's the case. I think you are seeing normal coloration fluctuation for your shrubs if they are resuming a darker green color later in the season.

Alright then, I guess it may just normal seasonal leaf coloration for catawbiense. I thought they looked greener the first couple winters after planting, but I could be misremembering. Thanks all.

The green color comes from chlorophyl and in winter, the chlorophyl activity drops considerably due to less light and cold temperatures. A high pH problem (alkalinity) manifests itself with yellowing leaves with green veins. Your don't have the green veins.

Photo showing nitrogen deficiency.

Symptoms: Overall yellowing of leaves, generally more prevalent on
older or lower leaves. [http://cru.cahe.wsu.edu/CEPublications/eb1229/eb1229.pdf]

Causes:

1. Nitrogen deficiency. Lack of nitrogen, or available nitrogen, tied up in
   decomposing organic matter, such as sawdust in the soil around the plant.

2. Wet feet. An early symptom of poor drainage is yellowing and wilting
   of newly emerging growth. If the plant is wilted and the soil is very
   wet, there is probably a root problem or insufficient air space and/or
   drainage in the soil.

Wet feet is a very serious problem and can be fatal. It leads to root rot and eventually death. The easiest way to avoid wet roots is a raised bed. This means digging up the plant, moving in good acidic soil to a height of about 8 to 12 inches out to the drip-line of the plant and replanting. Then mulch with a good mulch line pine straw or pine bark mulch, but not sawdust.

The chief cause of uniform yellowing leaves is insufficient nitrogen. This is not too common, but could be your problem. I never recommend Miracid, but in your case I would make an exception. It is only a temporary quick fix and not recommended for continued use. I would also use Holly-tone which is a more permanent solution and a much better product. When using fertilizers on rhododendrons, only use half the recommended amount and only apply in the spring.

Rhodyman, thanks for the additional info.

I doubt the problem is wet feet since newly emerging spring and summer growth hasn't been yellow or wilting. They're planted in a sheet-mulched border on a mild slope. Drainage isn't a problem. Nitrogen deficiency is possible.

I just fed them Holly-tone.