Mandevilla Disease

Mandevilla Disease

I'm hoping you guys can help me figure out what's bothering my mandevilla and what to do about it. There are always 3-4 leaves that are yellow and blotchy and then fall off. Mandevilla with Diseased Leaf This is the front of the leaf. Diseased Mandevilla Leaf This is the back. The black stuff looks like some sort of fungus to me. Diseased Mandevilla Leaf It shares a pot with a couple of pelargoniums. As you can see, except for a couple of yellow leaves both the mandevilla and the pelargonium look pretty happy, if a bit crowded. Mandevilla and Pelargonium I'm in a hot and sunny part of the San Francisco Bay Area in California. The balcony is facing south.
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