Answer: Apple trees always perform best when another compatible variety is planted nearby. Generally spring and fall are the best time to plant trees; though if the apple tree at the nursery is growing in a container and looks healthy, it's probably fine to plant it now. Just be sure it gets plenty of water throughout the rest of the summer.
There are so many different varieties of apples--including some that are resistant to common apple diseases. You might want to do some research and take your time choosing your tree (and whether you want a full size tree or semidwarf.) Then, you can order one for fall planting. Just a thought.
Visit this site of a midwest fruit grower's association; it also has links to other sites:
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