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I started apple seeds in a ziplock bag in early Spring. As soon as roots sprouted I planted the seeds in a large pot outside filled with potting soil that had extra perlite for good drainage. All the seeds sprouted and looked healthy, but when the soil got too dry the seedlings started to wilt very easily. So I kept them watered and moved them to a shadier location with less direct sunlight so the soil will not dry out as fast. They seemed to do well, but suddenly all started to wilt, and the soil was moist but not soaking wet, and within 2 days I lost all my seedlings. :(
I've never had any plant from seed die so easily on me, so I'm confused what I did wrong?
I do not know why your apple seedlings died, but if you are successful with your next attempt, you will not know for 3 or 4 years if the tree from seed produces edible apples. So, if you do get some seedlings going, you might think about bud grafting with buds from a tree that produces apples that you like. When the seedling is about 15 months old, in July or August would probably be a good time to bud graft and grafting is not difficult.
Detailed instructions are available on Google, but three important tips for you. If the bark on the seedling slips or lifts from the wood easily, a "Tee" graft will give you the highest rate of success. If the bark does not slip, then use a "Chip" graft. The most important things are, Use a very sharp knife, a Utility knife with a new blade is okay. Do not let the bud dry out.
Use Budding tape, and wrap it securely to keep the air out. The edge of the chip bud MUST touch at least one side of the cut on the seedling. Practice a few on a branch before putting on the permanent ones.
I'm going to guess that they didn't like being stressed by drying out. This may have damaged the root hairs, which limit's the seedlings ability to taking in moisture and nutrients. They were then being stressed more by not having enough sun light. Little seedling have limit energy reserves and if they run out they die.