I’ve recently planted a mango tree sprouted from a store bought mango seed, which was just over a month ago. From the beginning, it was a little weird as it only grew one leaf. It seemed like there was another small leaf shooting out from the side, but it never grew into a full one. A couple days ago, I noticed that its leaf was beginning to wrinkle and dry from the sides, and this condition has gotten worse now. The affected part had a darker color and was not as smooth. It seems that the tree will die. 
I’ve only fertilized it with those Miracle Grow Plant Food Spikes, and I’ve since repotted the plant but to no avail, so I don’t think it’s due to fertilizer burn or the type of dirt used. However, when I was replanting it, I noticed that it does not have much root, but I didn’t really notice any root rot.
I’m not really sure what is going on here. I suspect that there might just be something wrong with the seed to start. Any thoughts on what’s happening?
