Pretty plant! Looks like PM. Did the nights go cold? Air flow is important to prevent this and the site looks well protected, how is the air flow? If that was the only leaf I hope you picked it. The outdoor organic farmer in me says - You might take a wait and see but be ready to pounce on it with a fungicide on hand because if conditions go right for the PM it is highly contagious. The cannabis grower in me says spray now ask questions later. Just use a cannabis approved product with no heavy metals. Just sayin
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Oh yeah keep the plant leaves dry.