Answer: Based on your description, I am not certain what is happening to your shrub. There are several possible reasons for yellowing and foliage loss. The most common would include over or underwatering, and too much sun.
Aucuba needs an organic, acid soil that is evenly moist yet well drained meaning damp like a wrung out sponge and not saturated/sopping wet or dried out. Yellowing and dropping foliage can be a sign of overwatering or poorly drained soil, especially if it happens near the bottom of the plant first. If it is happening at the top or tips of the branches, it could be due to underwatering.
It can also be a sign of excess sun or sun burn. Aucuba tolerates deep shade and does well in bright dappled light or some morning only sun. Depending on what time of day it is receiving direct sun, it may be too sunny for it. If it is in direct sun at noon or in the afternoon, you might want to transplant it.
If none of these sound plausible, you might want to consult with your local county extension and/or your retailer. I hope this helps.
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