Some diseases and pests are easy to spot, many are not. Apple scab is a good example of a disease that is kind of subtle, especially in the early stages of development. Plus it has diffuse spot margins, making it blend into host tissue more easily. Try holding branches and leaves up to the sky to detect pest problems more easily. Or, walk under branches and look up towards the sky. Look for shadows and outlines. It will make monitoring and detection much more effective and time efficient.
Apple scab (Venturia innaequalis) is a lot more noticeable in the canopy of apple (Malus spp.) these days. It was fairly dry for a lot of the leaf emergence period, when infection takes place. But some areas that would have received some rain or significant dew events during leaf emergence would see some level of apple scab. Also, some Malus cultivars are a lot more susceptible to this disease than others.
It is too late to manage this disease and symptoms may worsen, with premature leaf drop later this month on some cultivars. Make a note of it and protect susceptible Malus during leaf emergency period next spring. Especially before rain events.
Oak leaf blister (fungus: Taphrina caerulescens) is visible in some areas on red oak. Oak leaf blister is not usually a tree health issue but it can be in the landscape, especially where there are irrigation sprinklers. The blisters often become orange-red in early summer.
Although it looks unsightly, it does not significantly impact tree health as you might have guessed, its too late to do anything about it now. Make note of this issue and protect emerging foliage next spring, especially before precipitation events.