I planted this Rainier cherry tree past winter in a sunny slope in Scotts Valley, California, which is in zone 9 and located between San Jose and Santa Cruz on the central coast. 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 (end of May or early June). Please see pictures.
The brown patches usually start from the tip or edge:
It could also develop in other parts of the leaves (see the lower leaf, which was also damaged by deer, probably).
There have been about two dozens of sick leaves so far. Ultimately, a sick leaf would become brown and shriveled. Sorry that I didn't take pictures of those very sick leaves before I disposed of them.
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 sprayed Garden Phos fungicide (phosphorous acid) once last month but am seeing no change.
It is pretty hot recently here, at around 80F to lower 90Fs (about 32C) 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 that are farther away are totally fine even though they could be getting more sun.
I have a regular fertilizing program using a balanced 18-5-12 fertilizer and garden compost/chicken manure.
I have other cherries like Bing and Lapins, and luckily that they haven't had this disease yet.
Any comments are welcome!