These were planted in 12 inch pots with around 2 parts old self-made potting soil (vermicompost+perlite+vermiculite+cocopeat) and 1 part garden soil. I fell sick and was unable to walk up and water the plants for 3~4 days. Don't know how dry they got. Two of them died then, but afterwards one was alive. However, it died too even after watering. Was there a problem in the soil/sunlight or did these die because of the lack of optimal watering?
Here is how the plants dried up.
The damage on the mint plant looks like it could be a fungal, bacterial, or viral infection to me--although a quick look at common ones doesn't turn up anything that looks similar to what you have. I don't think the mint could have gotten too dry in a couple of days, even in a pot! (Source: lived through a 3-month drought this summer, didn't water my outdoor mint once, and it survived although definitely took a beating. The leaves kind of curled up, went limp, and started to yellow, but didn't look like what you've got there!)
Another idea could be excess nitrogen, although the soil mix you used sounds ok to me. Or even overwatering--mint doesn't really need constantly wet soil.
Good luck with your mint! It's very hard to kill--I'm sure the next batch will be successful!
'Sorry for your loss, but the mint in my herb garden is almost impossible to kill. So try again. My soil is very sandy and it has been over a decade since I supplemented it. But pots are different: they dry out so easily. You could try adding seed-starter potting soil to your mix: it seems to hold moisture and have root-promoting additives. If you have a place to plant it outdoors, be careful: mint in a garden needs to be controlled! Good luck!
