Histology of root rot of flax seedlings (Linum usitatissimum) infected by Fusarium oxysporum f.sp. lini

The histology of root rot of flax seedlings infected by Fusarium oxysporum f.sp. lini was studied using semi-thin sections of plastic-embedded roots. Within two days, the fungus colonised the root cap cell layers by intercellular and intracellular growth. Attempted intercellular penetration of root cap cells via the middle lamella induced the formation of appositions next to penetrating hyphae. Other cells next to invading hyphae collapsed, which was accompanied by swelling of the cells neighbouring the collapsing cells. Invasion of the root cap and growth towards the protodermis seemed retarded to some extent by the natural sloughing off of root cap cell layers. The protodermis and cortex were reached and penetrated in four days, which was followed by rapid and massive colonisation of the entire root tip. The protoxylem was reached in eight days. From eight to sixteen days after inoculation, the lower parts of the roots were colonised throughout and the cortical region was degraded. Colonised tissues were severely plasmolyzed. Heavily colonised roots were hollowed out, the only remaining tissues being the epidermis and exodermis outside, and remainders of the colonised xylem inside. Upward spread of root rot was restricted in the period studied to the first 10 mm from the root tip, the upper parts of the root and the hypocotyl being unaffected except for invasion through lateral roots infected at their respective tips. Mature roots with a well-developed epidermis and exodermis were not invaded from outside. Disease development was similar in partially resistant 'Hermes' and susceptible 'Regina', except for rot development that was consistently slightly more extensive in the susceptible cultivar. Distinct extravascular resistance factors were not detected in 'Hermes', suggesting that extravascular resistance in flax to F. oxysporum f.sp. lini is of a quantitative nature.