What to spray depends on the type of tree and the pest you are trying to control. The tree in the photo you sent has some wood decay and moss/lichen growing on it. Wood decay is usually a disease of older trees, or trees that have been wounded or weakened, allowing an entry point for the fungi. The main concern is that wood decay can weaken a tree, which may lead to limb breakage and/or uprooted trees during wind or ice storms. For more information on wood decay, including management practices, refer to the following: https://ipm.ucanr.edu/PMG/PESTNOTES/pn74109.html.
The moss and lichen will not harm the three, however, lichen growth can hinder sunlight penetration to the tree bark which may affect photosynthesis. No treatment is necessary, however, pruning to open up the canopy to sunlight and air will help, maintaining proper nutrition, and monitoring soil moisture can reduce the growth.
For information on how to manage the most common pests of fruit trees, refer to this OSU Extension publication on Managing Diseases and Insects in Home Orchards. If you need help diagnosing pest problems, you can email photos of symptoms to the Lane County Master Gardener Plant Clinic (<personal data hidden>).