A likely cause is Cedar Apple Rust, a fungal disease that begins its life cycle on junipers before spreading to crab apple and other susceptibility trees. Symptoms include bright yellow or orange spots on leaves, swollen corky gall on twigs, and large yellow or orange spots on the fruit. Contacting your local arborist for plant health care and diagnosis will help you determine what is causing your trees to look ill and offer a treatment plan if needed.