Of course I can't be certain but this usually happens from over moisture which results in the roots to drown. Make sure the pot has good drainage and the soil(?) that you are using drains properly so your roots have a chance to airate. If you don't want to remove the tinfoil, perferate it with many small holes. I know this might sound stupid but I use disposable pie plates at the bases of my moms. There's a reason for this but this is not the thread(just added for reference).