INFLUENCE OF PLANT-AGE, DISEASE INCUBATION-TIME, AND HOST-PLANT RESISTANCE ON VERTICILLIUM WILT SYMPTOM EXPRESSION IN LUCERNE (MEDICAGO-SATIVA)

The most economical means to control Verticillium wilt in lucerne, cau sed by Verticillium albo-atrum, is through resistant cultivars. Most V erticillium wilt resistance evaluations use some form of root-cut-soak inoculation procedure. In Trial 1, three lucerne cultivars were inocu lated at eight plant ages and plant symptoms were scored at ten succes sive weekly intervals. Trial 2 consisted of two cultivars inoculated a t ten plant ages and symptoms were evaluated at eight weekly intervals . In an experiment to study the genetics of resistance to this pathoge n, four plants showing no visual symptoms were selected from each of f our germplasms. The selected populations were crossed in diallel fashi on. The population crosses plus the parents were evaluated for resista nce to V. albo-atrum. Results from Trial 1 and Trial 2 showed disease severity increased with increasing inoculation age and with increasing scoring age. Cultivar symptom expression interacted with inoculation age, but not with scoring age. The diallel analysis supported resistan ce to Verticillium wilt being conferred by additive gene action, as he terosis was not detected and general combining ability mean square was 3.75 times larger than the specific combining ability mean square.