NL auto

Did you do anything different in the days before day 16? Any humidity or light adjustments? Did you water on day 15 or 16? Are you feeding the plants yet or still just letting them eat whatever is in the soil? Septoria cannabis can cause similar looking spotting in the early stage, but so can early nute deficiency - or nute burn for that matter, if its slight. Strains, soils, and additives can provide protection from fungus as the high humidity they like in week 1 is also very friendly to mold/fungus, and spores could be just now becoming visible if it is leaf septoria. Your humidity shouldn't be too high as your plant no longer needs more than 55%, at most. Any higher and it has trouble "sweating" so it needlessly creates a fungus friendly environment to go above that. If your humidity is above 55%, step 1 is dump the excess. Next is to collect some runoff to check the ph of your soil. If its outside the range of about 6.2-6.5 in soil, you may have a nutrient deficiency regardless of whether they're getting fed yet. So step 2 is to correct the ph since that plays a role in protecting from fungus too. It looks more like a nute deficiency (or very light nute burn), but there is no harm - and some benefit - to adding 2 teaspoons per gallon of hydrogen peroxide 3% (which you can buy at a drug store for cheap) to your next watering and also lightly spraying the leaves with the same mixture as a foliar spray. If its fungus, that will kill it. If its not fungus, then your plant is hardened against fungus and as a bonus it improves aeration of the roots and keeps them free of fungal attack too. Used as a weak solution like this - when needed - its not going to hurt your plant as it breaks down to plain water with an extra hydrogen atom. Just be careful you don't get it in your eyes, as even 3% hydrogen peroxide is strong enough to bleach your hair (keep it away from your eyes). If its a nutrient problem, where you are growing in soil, this early I would suspect ph unless you're feeding them, then what and how much becomes as suspect as the ph until you know what is going on. So you really need to check your soil ph as that can also narrow down the list of possible nute deficiencies and you can't correct that until you correct the ph, if its off (which might be itself also solve any nute deficiency). Of the two, fungus begins doing the most damage and is the one I'd be more concerned about so I'd give it the hydrogen peroxide treatment anyway as there is no downside or harm to doing this once.