Could be fungal, if it's just on the leaves. Also, the underside of the leaves are looking silver, which could be spider mites. Wipe the leaves off with a damp, white paper towel and if you see a rusty residue, you may have red spider mites. I would treat with a fungicide and also something for mites. Fungal problems usually don't kill the plants, but mites will. They also spread disease and are usually a problem in the fall and winter when the humidity drops. Good air movement decreases both problems.
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