My very young Hydrangea seems to be growing well enough, but the leaves are turning purple. I've located information that indicates this is a fungus, but can't find anything that tells me what to do. I live in South Alabama, so the weather is mild, seldom getting to freezing and usually sunny and in the mid 40's-50's in the afternoons of winter.
Hydrangea With Fungus
Certified GKH Gardening Expert
Have the soil tested. It may be a phosphorous deficiency. Purple leaves are a sign for this deficiency. Regardless, adding a little phosphorous will not hurt the plant and will help with blooming. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm
I think the fungus you are referring to typically shows itself as purple spots, rather than whole leaves turning purple. But, if you are still concerned that it is a fungus, treating the plant with a fungicide will not harm it. You can buy fungicides at your local plant nursery.