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Fusarium Wilt of Susceptible and Resistant Tomato Isolines: Host Colonization. M. E. Mace, Plant Pathologist, Disease Resistance Investigations, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; J. A. Veech, Plant Physiologist, Disease Resistance Investigations, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 61:834-840. Accepted for publication 16 February 1971. DOI: 10.1094/Phyto-61-834.

The effect of the site of inoculation and inoculum concentration on disease development (as expressed by killing, stunting, vascular browning, and colonization) was investigated in susceptible and resistant host isolines. No killing, a nearly constant amount of vascular browning, and no stunting were observed in the resistant host inoculated with 5 × 106, 5 × 104, 5 × 103, and 5 × 102 spores/ml. The susceptible host, however, showed progressive sensitivity as the inoculum concentration increased. In both host cultivars, the amount of mycelial colonization of the hosts decreased as the distance from the point of inoculation increased, even though there was nearly systemic distribution of fungus spores through the axis xylary system. Furthermore, the site of inoculation greatly affected the extent to which the host was colonized by the pathogen. Both the susceptible and resistant hosts showed increased resistance to colonization as the site of inoculation moved up the plant axis. It was concluded that two morphologically distinct zones, tissue above the cotyledonary node and tissue below the cotyledonary node, should be considered separately in comparative physiological or biochemical studies of susceptibility and resistance to Fusarium wilt of tomato.

Additional keywords: primary xylem; Fusarium oxysporum f. sp. lycopersici, race.