Ok here's a gallery of the leaves on three apples and one pear: http://imgur.com/a/0SPXv (click on the pics to zoom in closer) The trees seem to be doing overall much better since I sprayed them (before they had many black leaves that have now fallen off and most of the new and remaining leaves are showing less and less sign of a problem). But anyways, any idea what's actually causing the problem?

olympia: A spray for peach leaf curl in autumn is often recommended. I believe this would be near the time of last leaf fall. It's probably not as important as later sprays but in your climate it's probably what I'd do.

Horticultural oils are insecticides. The ultra-fine types are just more highly refined, therefore lighter weight and so can be used year round rather than just in the dormant season. Often, they are combined with a fungicide for dormant season spraying as they have little effect on disease control. They do not prevent insects. Their mode of action is to smother soft bodied insects and in some cases, disrupt feeding patterns and alter digestive processes. They must make contact with the insect in some form to accomplish this. To my knowledge, they have no effectiveness against beetles although could have a negative impact on beetle larvae should they come into contact with that.

OSHA has no role, nor does it establish the requirements for appropriate/mandated personal protective equipment for the pesticide end user. That is established during the registration and legal labeling process. MSDS are not sources for appropriate PPE for end users of a product. MSDS have nothing to do with risk evaluation for the end user, typically they have value for those in the manufacturing process of the active ingredient, spills of concentrated material in transportation, and so on. End users should consult the label of the product they are using to find out the mandated protective gear. For example, "Malathion Flowable 8" mandates an applicator or handler to use:  Long-sleeved shirt and long pants  Chemical-resistant gloves, such as barrier laminate, butyl rubber>14mils, nitrile>14mils, or Viton >14mils  Shoes plus socks