I'll place my bets on Stinky and Ganja and say a virus as well......be aware I'm only parroting here but I think they both are the shits with knowledge. I DID look in my Garden Problem Solver and it DID talk about viruses but only in a very, very vague way (plants get viruses) with no info except destroy the infected parts OR the infected plant as the virus is spread by insects. Perhaps that is part of the neem oil treatment GG is talking about.

I'll check back. Good luck.