It is important to remember your trees are living things which will be joining the ecosystem of your landscape. Aphid infestations, and/or fungal leaf spots, mildews, etc. are common and natural, particularly in fruit trees and in deciduous trees in general towards the end of the summer. A certain amount of tolerance will allow your trees to play their natural role in promoting biodiversity. If, however, the problem is spoiling your enjoyment of the trees, or threatening permanent disfigurement, you may need to apply a pesticide/fungicide. Many are available from good garden centres, or contact a tree professional for assistance.
Unfortunately the pesticides, fungicides and herbicides that we use commercially are often not available at the garden centre, but an alternative is usually available. Of course we do offer our AfterCare service which will include this service, but you must remember that many problems will need repeated and regular spraying and when the weather is ideal to be successful, and sometimes more often than every 2 weeks.