Only on one leaf of one plant. Is this some fungus or virus?
It looks a bit like coffee rust, but I couldn't find anything else like it.
How should I proceed?
Thanks in advance!
Pretty certain this is Leaf Spot Fungus otherwise known as Rust Fungus... and you'll need to jump on this quickly as it spreads fast... it spreads by spores that you can't even see so don't think you've got any time to waste here... I won't go into all the details of what you need to do as this article is very thorough.. Do give it a read and get going so you don't lose your entire grow...
https://www.urbanorganicyield.com/rust-spots-on-leaves-during-flowering/
Good luck!
I would just keep an eye on it. If it spreads then you probably have a deficiency. Also pay attention to where it start. New leave is usually a mobile nutrients, the old bottom one it not.
Hi there
I will just remove the leaf keep a close eye on it and make sure no more pop up… seeing as your at week 9 flower the end os so close i would just cruise on
Best of luck 🌱