British Columbia: Magnolia sargentiana 'Blood Moon' brown patches on leaves

My Magnolia 'Blood Moon' has developed dry, brown patches on some leaves. See pictures. The tree is about 8 feet tall, located in a sunny spot on a slight slope with good drainage. The soil is clay but has been well amended with organic material. I've examined the leaves for signs of pests but can't see any. The unaffected leaves look normal and healthy.

I hope someone can tell me what is affecting the tree and how to fix it. I'd hate to lose it after babying it through a couple of bad winters. I live in North Saanich.

Thank you.

 

Possibly fungal, although I see no fruiting bodies. So then maybe physiological, resulting from recent hot weather.