SUMMARYInspection of naturally or artificially infectedHevearoots showed thatForms lignosuscan penetrate undamaged roots directly, but does so more readily through wounds or natural openings like lenticels, or through the bases of lateral roots and bark scales. Therefore, Pomes‐infected trees should be identified by leaf symptoms rather than by uncovering and inspecting roots, as this generally leads to root injury, which facilitates fungal penetration.Initial fungal entry into host tissue appears to be by mechanical pressure alone, but deeper penetration is through the action of extracellular enzymes. The fungus remains intercellular in the cortex but is intracellular in the woody tissue. Ray cell walls are penetrated mechanically, but the xylem through pits. The time taken for various stages of infection to occur is assessed.The amount of damage done by the fungus to roots and the blocking of xylem vessels by tyloses suggest that yellowing, curling and buckling of leaves on infected trees are drought symptoms and not a reaction to fungal toxins. The host reacts to the invasion of the cortex by forming a cork cambium and to the invasion of the woody tissue by blocking individual cells with phenols and resins, which could be important when breeding disease resistantHevearoot stock