During the course of pathogenesis, normal activities of the infected host plant undergo malfunction. Consequently, morphological and physiological changes occur.
A. Morphological or structural changes:
Physiological malfunctioning of the host cells causes disturbances in chemical reaction which ultimately lead to some structural changes viz., overgrowth, phyllody, sterile flowers, hairy roots, witches broom, bunchy top, crown gall, root knot, leaf curling, rolling, puckering etc.
B. Physiological changes:
i. Disintegration of the tissues by the enzymes of the pathogen.
ii. Effect on the growth of the host plant due to growth regulators produced by the pathogen or by the host under the influence of the pathogen.
iii. Effect on uptake and translocation of water and nutrients.
iv. Abnormality in respiration of the host tissues due to disturbed permeability of cell membrane and enzyme system associated with respiration.
v. Impairing the phenomenon of photosynthesis due to loss of chlorophyll and destruction of leaf tissue.
vi. Effect on the process of translation and transcription.
vii. Overall reproduction system of the host.