Abstract
Resistance to Pythium aphanidermatum (which causes die-back and collar and root rot) in seven diploid and four induced autotetraploid lines of Catharanthus roseus was studied in the field during 1983 to 1985. The tetraploid lines were, depending on the year of evaluation, about 13 to 155, 18 to 218, and 17 to 366 times more resistant than diploid lines, when resistance was measured in terms of plant mortality, infection rate, and area under disease curve, respectively. On an average over three years, the tetraploid lines yielded about four and five times more leaf and root total alkaloids, respectively, than the diploid lines. The relative levels of resistance of diploid and tetraploid lines to Pythium aphanidermatum determined in the laboratory were similar to those observed in the field.
