|My Acububa has a problem of the leaves wilting and then turn jet black. I would like to know why this happens and what I can do to prevent it.|
Aucuba 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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