Most tree diseases are transmitted using a soil/root avenue or through a wind/insect avenue. There are a few diseases that can travel using either of these methods.
Insects and wind can carry many tree diseases from tree to tree (air-borne) if the condition of the tree provides a comfortable home for the pathogen. Soil-borne organisms can move diseases (sometimes called pathogens) through the soil via water, are then taken up by tree roots and can be spread by root grafts to other trees.
To prevent and control these diseases, you must first identify the disease, then take measures to limit or remove the elements that provide disease transmission. Treating a tree directly for an embedded pathogen should be used as your last effective effort.
American Chestnut Blight, USFS/FIDL Photo