THE RATE OF VASCULAR COLONIZATION AS A MEASURE OF THE GENOTYPIC INTERACTION BETWEEN VARIOUS CULTIVARS OF TOMATO AND VARIOUS FORMAE OR RACES OF FUSARIUM-OXYSPORUM

The extent of vascular colonization, as determined by hyphal growth from vascular elements, and the degree of foliar symptoms were compared in eight cultivars of tomato at 4 weeks after inoculation with each of three races of Fusarium oxysporum fsp. lycopersici or race I of F. oxysporum f.sp. pisi or cubense. These interactions included hosts having polygenic or single-dominant I, I-2 or I-3 type resistance. In all cases the degree of symptom expression was closely related to the extent of vascular colonization of the host (R = 0.8). A colonization quotient was calculated for each host-parasite combination by which the genetic interactions were compared. In no instance was there any indication of a physiological tolerance to extensive colonization on the part of the host. The genetic interactions appear, rather, to modulate the rate and degree to which the processes of localization are activated. The colonization quotient can provide a significant numerical measure for comparing various genotypic interactions.