Bark diseases prevent the movement of the food supplies.
Sapwood ailments cut off the water supply that rises from the roots.

Seed and flower diseases prevent the trees from producing more of their kind.
Most of the tree parasites can gain entrance to the trees only through knots and wounds.

Infection usually occurs through wounds in the tree trunk or branches caused by lightning, fire, or by men or animals.

The cone-bearing trees give off pitch to cover such wounds.

In this way they protect the injuries against disease infection.