This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Your questions embrace very wide subjects. The cause of canker is generally considered to be a cold wet subsoil. Reasoning from these premises, thorough drainage and planting on slightly-raised mounds of earth to increase the temperature of the soil is the remedy. Some sorts of fruit-trees are much more subject to canker than others, and such varieties as thrive best in any such soil as referred to above, should predominate in forming a collection. Crown and whip grafting are the best methods of grafting Pears and Apples.