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Is This Powdery Mildew, and How Should I Treat?

I have nursed a hedge of Photinia back to a better state of health, but now the new growth leaves are all covered in a white, somewhat sticky substance. Is this what they call "powdery mildew"? The new blossoms are not white. They appear to also be covered in the same material. How do I confirm a diagnosis, and how should I treat this?

How effective is neem oil compared to other treatments? I guess I would need to really cover the plant heavily because this material is everywhere.

How did this happen? We have moderate humidity in the San Francisco Bay Area, and we had a dry Winter. I associate a fungal disease like this with high humidity.

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