if your apples developed a bitter rot condition (brown spots inside and on the fruit) then most likely cause is lack of calcium. Putting calcium in the soil is usually ineffective and the only and best way of dealing with it is spraying the trees with calcium supplement.
Problem - you can't buy this stuff in any stores or nurseries and the Orchard supplies sell this stuff in large quantities. For an ordinary home owner who has 2-3 trees, you need only about 0.5 liter per season.
I can sell you 0.5 liter for $15 or 1 liter for $30 - to see if this helps your trees.
