Clearly, the verb "is" refers to the apple, regardless of what it is adjacent to. One would say, for example, "At least one of the kids is...", because "is" refers to the "at least one", and is not affected by the fact that it is adjacent to a plural noun ("kids"). Sometimes the ear is fooled by the words that get between elements that need to agree, so that one sometimes hears people say "She gave the books to John and I.", but if you take out "John and", and say it with just the "I" element, you can immediately hear that "me" is correct.