Colonization and histopathology of susceptible and resistant carnation cultivars infected with Fusarium oxysporum f. sp. dianthi

Abstract

Stems of the susceptible 'Early Sam' and resistant 'Novada' carnations were inoculated with a conidial suspension of Fusarium 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 discolouration 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 discolouration 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 rather than being part of the resistance mechanism.Although of different origin, vascular browning as such occurs in both susceptible and resistant inr.eractions. In breeding for resistance, care should hence be taken with the current use of browning as an indication of disease.

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“…Development of wilt symptoms in inoculated carnations was accompanied by a quadratic increase in PG activity. This coincides with (roughly) quadratic increase in the amount of mycelium present in the stem (Baayen and Elgersma, 1985). As previously suggested for PME and pectin lyase in tomato plants infected with F. oxysporum f.sp.…”

“…A complex of host reactions including phytoalexin accumulation, gum formation, lignification and suberization are involved in containment of the fungus inside the infected xylem vessels (Baayen, 1988;Baayen and Elgersma, 1985;Niemann and Baayen, 1988;Niemann et al, 1990). Phytoalexins can be quantified by HPLC and have been found to be correlated to resistance (Baayen and Niemann, 1989).…”

“…Fungal action in the host is characterized by severe degradation of the vascular tissues, eventually resulting in hollowing-out of stems and wilting of leaves (Baayen and Elgersma, 1985;Baayen et al, 1988). Degradation of host tissues generally begins with production of polygalacturonase (PG) enzymes that hydrolyze pectin, a constituent of the middle lamella and primary cell walls (Cervone et al, 1986;Cooper and Wood, 1975;Jones et al, 1972).…”

“…The marked degradation of xylem in carnations infected with race 2 of F. oxysporum f.sp. dianthi (Baayen and Elgersma, 1985;Baayen et al, 1988) prompted us to investigate the involvement of polygalacturonase and related cellwall degrading enzymes such as pectin methyl esterase (PME) in this process and to evaluate their prospects for the development of novel methods to assess the level of partial resistance in carnation to fusarium wilt. This study presents an evaluation of the prospects of such novel methods compared with those of existing ones.…”

Changes in chemical composition related to fungal infection and induced resistance in carnation and radish investigated by pyrolysis mass spectrometry

“…Possibly, this points to a selective removal of syringyl-rich secondary cell wall material and retention of guaiacyl-rich middle lamella [cp 47] in radish. In carnation, probably both secondary wall and middle lamella are being removed, leading to cavity formation in the susceptible interaction [4 ].…”