Today I noticed some yellow spots starting to form on my apple tree. Can anyone help me determine what it is and how to remedy the problem?
Those look like cedar apple rust. I sprayed my apple trees with copper during dormant and at buds break with excellent result. No rust this year. your trees will be OK, just remember to spray during dormant season.
Ok, so you're saying that I don't need to take any action right now? Just wait for the dormant season, then spray it?
The smaller, yellow spots on the bottom picture and certainly CAR. Nothing you can do about it now, just be prepared next year. The brown spots on the top pic are not CAR. I'm not sure exactly what those are, maybe scab or leafhopper damage.