Question
We have apple tree leaves that have yellow spots with a dark spot in middle. On
the underside of the leaf it appears to be a collection of objects behind the dark
spot. Is this a disease or attack of some type of bug?

Answer
What you are describing sounds like cedar apple rust.  This is a fungal disease which can cause defoliation of leaves and over the years will cut back on the number of fruit the tree makes.  You will probably need to treat this tree each year as there are cedar trees within a mile or so of your apple tree that also carry a phase of the disease.  Maneb fungicide is probably a good choice or you can ask your garden center guy for a fruit tree disease and insect spray that you can use.  Apply the fungicide early in the growing season to prevent infection by the fungus.  Several applications may be necessary.  I hope this helps fix the problem for next year.  Good Luck.