Yeah this is one of the unknowns (so far) that we need to learn about, with an eye on viral infections......
Unless it's something else. If it was due to cold weather like brief frosts at night, I'd believe that too. He said he's in VA, Spotsy are you up in elevation/the mountains?
My only other guess would be something with the soil or water......contamination or way too much or little of something?
It does look very much like that "virus" link though......
It doesn't look like HLV to me, aside from that, HLV is very hard to diagnose and they say the only way to confirm its presence is with a PCR test.
This does look like the "Dark Purple Disease" that MIGrampaUSA linked, though. Which is a whole 'nother mystery yet to be uncovered in the cannabis industry.
To the OP, what kind of RH are you running in your grow and does it ever get fresh air? A lot of molds (if this is in fact a mold) can be held at bay simply by starving said mold of moisture. I.E. - avoid spraying your plants and keep the RH as low as possible for a week or two, and ensure fresh air is taken in at least once per day. A good air filter on the intake is very beneficial, as well. However, I am not certain this is a mold at all. It does look more like a virus of sorts.
One other piece of advice I can give, is try to quarantine that and any other affected plants separate from your main grow.
Edit to add - sorry I just woke up and I just noticed you said it's an outdoor grow so obviously there's not much you can do about the RH, quarantining and such.
If moving the plants is not possible, maybe at least try not to handle the infected plants, and if you have to, wear gloves and make sure you don't touch the clean plants after touching the infected ones.
