Clone question.... Powdery mildew

Once a clone gets powdery mildew, does it always have it even after treatment and no visible signs of it?
I had sold six clones, 10 days ago, all clean and healthy. Just got a very angry email about them having PM. But, that was 10 days ago they left my care. Where does my responsibility lie? A lot can happen in 10 days… Unless this is like herpes.

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Seriously? Anyone have a thought?