COLONIZATION AND HISTOPATHOLOGY OF SUSCEPTIBLE AND RESISTANT CARNATION CULTIVARS INFECTED WITH FUSARIUM OXYSPORUM F-SP DIANTHI

Stems of the susceptible ''Early Sam'' and resistant ''Novada'' carnations were inoculated with a conidial suspension of F. oxysporum f. sp. dianthi. Stem segments of either cultivar were sampled regularly and used for determination of fungal growth and for microscopical investigation. ''Early Sam'' showed typical Fusarium wilt symptoms and its stems were colonized intensively. The observed vascular browning appeared to be caused by discoloration of primary walls of infected vessels and surrounding cells. Vessels were rarely occluded with gel. Cell wall degradation led to the formation of stem cavities. Hyperplasia of xylem parenchyma was not seen. In ''Novada'', fungal colonization remained low throughout the experiment. Macroscopic symptoms were absent except for longitudinal bursts in the stem, which appeared to be caused by hyperplasia of xylem parenchyma bordering infection. Vascular gelation occurred in the infected tissues, causing some vascular browning also. Xylem vessel regeneration was observed in the hyperplastic layer. Cavities were not formed, and wall discoloration was rare. Vascular gelation is considered part of the Fusarium wilt resistance mechanism. It is followed by xylem vessel regeneration, which expresses a general plant response to vascular dysfunction rahter than being part of the resistance mechanism. Although of different origin, vascular browning as such occurs in both susceptible and resistant interactions. In breeding for resistance, care should hence be taken with the current use of browning as an indication of disease.