Answer: There are several possible explanations, ranging from pest or disease to fertility to pH to weather, meaning drought.
An annual application of a granular slow release fertilizer for acid loving plants (such as HollyTone) according to the label instructions should be fine. Do be sure not to overfertilize, however, since this can be just as harmful -- or worse than underfeeding. It is also not a good idea to give a heavy feeding to plants that are already showing stress.
A soil test would be a good idea in any case and allow you to check the pH as well as fetility levels.
A careful examination of the foliage, perhaps in consultation with knowledgeable nursery personnel, could also possibly be instructive.
However, knowing that we have had extremely dry weather for the past year would lead me to suspect that the problem is possibly drought/winter stress beginning to show on the more susceptible plants, more susceptible meaning a slight difference in rooting success, a slight difference in exposure to light or wind, or a slight difference in soil moisture such as that caused by surface runoff or a slight irregularity in the irrigation system.
Take care to keep the soil evenly and slightly moist (but not sopping wet) whenever it is not frozen. The occasioanl deeper water is preferred over a daily light watering. Use several inches of organic mulch such as shredded bark to help moderate the soil temperature, keep it more evenly moist and hold down weeds at the same time. It also helps feed the soil as it breaks down, and this is also important for these plants because they prefer a more humusy organic soil.
Since it is difficult to diagnose this type of problem long distance, I would also suggest a consultation as mentioned above, either on site or using several foliage samples. Good luck with your shrubs!
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