Answer: Sarcococca is generally one of the most reliable bloomers in shade, and typically adjusts to varying light levels as the season progresses. Wish I could positively diagnose the problem with your plants, but there are at least three possibilities, based on the information you've supplied:
What you're witnessing may simply be the normal senescence of older leaves (first turning yellow, then straw colored, and finally brown before dropping), which is often mistaken for a disease or pest problem. The oldest leaves are on the innermost part of the plant, closest to the stem.
On the other hand, poor soil drainage can cause root rot, which shows the same symptoms as you describe.
Although the area gets more moisture that the one with healthy plants, it's possible that the tree roots are competing for moisture and nutrients and your sarcoccocca is suffering.
I wouldn't give up on the plants just yet, but I would do a little investigative work. First, check the soil moisture. If it remains soggy for extended periods of time, suspect root rot. If it dries up almost immediately after watering, suspect competition for moisture and nutrients from your birch tree.
If the soil moisture seems adequate, the problem may not be a problem but the natural shedding of older leaves. To check soil moisture, dig down 3-4 days after watering normally; if the soil is still moist 3 inches beneath the surface, you won't need to water for a few days; if it is soggy wet, the soil has poor drainage; if it's dry, dry, the tree's roots are sucking up the moisture.
The only other suggestion I can make is that you take a sample of the affected leaves to your local Cooperative Extension office for hands-on diagnosis.
Best wishes with your garden!
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