RESISTANCE MECHANISMS OF PLANTS TO ROT AND WILT DISEASES CAUSED BY FUSARIUM OXYSPORUM
In bulb rot diseases, penetration is followed by intercellular, later also intracellular, colonization of the cortex and degradation of the colonized tissue. In case roots are initially penetrated, colonization and rot proceed up through the cortex into the bulb. In wilt diseases, the xylem is infected via wounds or, at the root tip, by fungal growth through the cortex towards the area of xylem differentiation and through the vessels up into the stem.
The suberized exodermis and endodermis of roots, and the cutinized epidermis of subsoil stems and leaves (scales) constitute structural barriers to penetration. Preformed fungitoxic compounds or their precursors in the outer scales of bulbs also protect against penetration. Cultivar resistance is mostly due, however, to active retardation or localization of the fungus after penetration. Physical resistance to rot-causing forms is achieved by strengthening of cortical cell walls. In wilt diseases, resistance is expressed by local occlusion responses (tyloses, gums) which block or retard the vertical spread of the fungus, and by cell wall-strengthening responses. Chemical resistance is achieved by among others production of phytoalexins, formed locally at the site of the defence response. In retarding fungal growth, phytoalexins prolong the period that the host is able to form structural barriers to colonization.