Abstract
Tobacco root and stem systems were studied to determine their relative importance in resistance to Verticillium dahliae Kleb. Three cultivars, the most susceptible and resistant of which differed widely in resistance to the disease, were grafted so that the stems and roots were united in all possible combinations. The act of grafting lowered resistance in the more susceptible cultivars. The rootstocks of the most resistant cultivar markedly reduced symptoms in the susceptible scions grafted to them, and the stems of the resistant cultivar had more resistance than the stems of the susceptible ones. A method of estimating the amount of susceptibility of the different graft combinations at different periods of growth is described. The data most closely fitted a hypothetical model in which 74 percent of the resistance in all varieties at all stages of growth was contributed by the root system, the remainder corning from the stem.
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