4 week old cutting...

this just started showing up yesterday and it's starting to get worse on the single leaf. It hasn't shown up anywhere else and the rest of the plant looks perfect. What could this be?

thanks

need alot more info on the specifics of your grow....things like temps/ph/watering schedule/etc

temp: 73
ph: 6
soil: happy frog

I mist the soil twice daily with R/O 0 ppm water

is misting the soil the only watering youve done? cannabis likes wet and dry cycles.......

sorta, last night I misted them a lot till the soil was very damp deep down. so now I'll be starting the water when its dry 2~ inches down

anyone have any ideas? This is still occurring I'm starting to think its Mg deficiency ? I'm still just doing the RO water and I've only given a single 1/8 dose of nutes...