Aucuba prefers to be in the shade. If provided too much sun the plants can develop black spots or blotches on the leaves. If entire sections of leaves are branches are turning black suddenly in early spring it could be due to cold damage, especially if they are also subjected to more direct sun at the time, such as they would be under a deciduous tree canopy. A fungal organism called Botryosphaeria often attacks plants that are stressed and or damaged. The solution to all these issues is to prevent exposure to direct sun and if any black spots appear, to remove the affected foliage. Avoid fertilizing during late summer and fall so as not to stimulate tender late season growth. When an unseasonably cold snap is forecast in late winter or early spring, consider covering the plants.
Aucuba japonica (Gold Dust Plant) can have problems with fungus and bacteria diseases, especially when they are kept on the wet side and the foliage and stems are wet frequently. The symptoms are similar to what you described.
I suggest you trim away all the black leaves and stems. Then back off on the watering, allowing them to dry out before watering again. It is better to water them by allowing a hose to run on the soil beneath the shrubs rather than sprinkling the foliage.
If the problem does not respond to these cultural measures, you may want to try a copper-based spray (available at most garden centers). Copper products can help shut down bacterial and some fungal problems. Avoid getting the spray on sidewalks, bricks and other masonry as it can stain it blue temporarily.
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