These plants are 50 days old and had many problems...

Hey there, the roots got contaminated with water mold from not enough drainage, take a pair of sizors and just cut off most of the problem roots (max 20% of the total root mass), then use something like enzymes, humic acid or a diluted flushing agent to try and fix the rest. Basically it's a humidity and temperature issue if you're at 27C + high humidity and just a bit of overwatering can cause issues. Next time use a chelation agent every watering or add beneficial bacteria like bacilus subtilis in the soil from the start and sometimes this will help prevent the problem you're seeing. Hope this helps ! 🚀

@Jankery Hi mate it's mold fungus And it is caused thru the environmental conditions you kept the plant in. Basically if you do not allow the plants soil to dry between waterings the soil stays damp and the perfect conditions for fungal growth begin to take hold. You can prevent it next time by Only watering when soil is nearly Dry allowing air to get to the roots And I also like to use a little Hydrogen peroxide in with the watering Once a week. The hydrogen peroxide will kill the pathogens and also add oxygen to the soil. I hope I have been able to provide the solution to your Question Good luck next grow and feel free to message me for any advice you may need I will do my best to answer If I can

don 't give them to much water. learn from the signals the plant give u. You can see it by the weight of the soil.

Would help to have some more information about the plants and pictures id bet this is root rot because of the browning which can be caused by over water and a number of other reasons .if you really want some good information go to growweedeasy.com and look up root problems that site has a vast amount of great information ...hope that helps