Hi,
I planted this Rainier cherry tree past winter in a sunny slope in Scotts Valley, which is located between San Jose and Santa Cruz, CA. It has been growing OK since spring but I have noticed that it has been developing brown patches on some of its leaves since last month. Please see pictures.

I suspect fungi infection so I have been actively removing browned leaves and spraying Serenade fungicide (every other week), but the disease is still spreading slowly. I also tried Agri-Fos fungicide once last month but am seeing no difference so far.

It is pretty hot recently here, at around 80 to lower 90s in the hottest hours. But I don't think the browning is caused by heat, because the browned leaves are clustered, and they are slowly spreading to adjacent leaves. Leaves farther away are totally fine even though they could be getting more sun.

I have other cherries like Bing and Lapins, and luckily that they haven't had this disease yet.

Any recommendations are welcome! Thank you!