Hi everybody!

Today I've found some strange black spots on my adeniums' leaves. I'm not sure what's happening, so I'd love to hear what somebody else thinks.

I have many adeniums but only 3 have been affected. What they have in common is the placing (light, warmth), watering and that the ill leaves are of "lower" quality - they have grown very fast in the extreme heats the had struck my country about two months ago. All their other leaves are fine, all my other plants are fine. I also have a feeling that some sap had crystallized on the bottom side of the leaves.

At first I thought it might be a fungus, then that they are going dormant, (but my oldest one, "Root", actually is going dormant now and it looks different; 'cause, acting like a normal adenium is too mainstream). The last thing that comes to my mind is, that they might have been frostbitten from being on the window while it's open but a) it's not yet that cold outside in the evening, b) all the others are fine.