Answer: I can find no reference to Diablo being prone to diseases of any kind, so powdery mildew is quite a rarity. When plants are labeled disease resistant, they are resistant, but not totally immune. So if the weather has been wet and humid, if your diablo is not getting good air circulation, if it is growing in a shady or partially shady area, and if it is otherwise under stress, it can develop powdery mildew. You can spray with a fungicide and/or you can prune away the infected plant parts. Pruning should open the plant up to better air circulation. Best wishes with your diablo!
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