EVALUATION OF INOCULATION METHODS FOR SCREENING HORSERADISH CULTIVARSFOR RESISTANCE TO VERTICILLIUM-DAHLIAE

Four inoculation methods-colonized oatseed, root dip, infested soil, a nd set dip-were tested in the greenhouse for their effectiveness in id entifying horseradish cultivars that are resistant to Verticillium wil t of horseradish. Examination of the inoculum density-disease relation ships derived with each of these methods on susceptible (647A) and res istant (769A) cultivars showed that all were effective, though at vary ing degrees, in differentiating between susceptible and resistant reac tions. Results were more consistent with the root dip method as it pro duced the largest least-squares means difference in wilt index between the two cultivars, the highest r(2), the lowest coefficient of variat ion, the shortest incubation periods, and the highest incidence of fol iar symptoms. Overall, inoculum concentrations accounted for only a sm all amount of the total variation in wilt index (0.14 less than or equ al to r(2) less than or equal to 0.73). This observation, in accord wi th previous reports on other hosts of Verticillium dahliae, would sugg est that inoculum densities may not be a good predictor of the severit y of Verticillium wilt of horseradish.