I have a hedge, composed of about 6 holly bushes (I believe it is holly, not totally sure) that always looked great- leaves were dark green and the bush was always thick and full. Today I noticed that the bottom half of these bushes are bare, as if all the leaves fell off. The remaining leaves are now turning yellow, and the tips of many of them are dark brown. I have included a photo. Does anyone know what the culprit may be, and if there is anything I can do to mitigate the problem? Thank you in advance.

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