I have seen brown spots on a few plants in this family and concluded they were disease-related (virus, fungus, etc.) without necessarily knowing what the exact problem might be. My solution typically involves culling the affected plant if it's not the only one I have, assuming that the problem may be contagious. When I have left plants alone, the problem seemed progressive (slowly taking over all the leaves) and did not spontaneously resolve. When I have subtracted affected plants from the patio, the remaining individuals of the same cultivar or whatever avoided any apparent infection. So I guess culling the herd works to keep it strong.
I did recently have a powdery mildew problem with 2 Euphorbias, and I treated it with a fungicide spray (Ortho product), but I suspect the solution will require repeat visits. Fungal issues like that white mildew and a similar black version tend to pop up at this time of year when we have lots of fog accompanying our usual high (80%) humidity.
