That doesn't look.iv only ever seen Pythium,root rot on seedlings,if that's what it is and not for over 10yrs as I use microbes with my seedlings.The good bateria eat the bad. I'm not sure though.Your soil is very wet and needs dry cycles.The top layer "half inch" needs to dry out completely so if you've kept it wet the whole time this will be the causes.The plant looks healthy though.I'd get some microbes/bacteria from the shop if poss if mot out of the garden and brew some compost tea.Ecothrive do instant tea called Biosys that doesn't have to be brewed if you haven't got an air pump.