Evaluation location: Wisconsin, United States
The study included between one and seven accessions of each of 31 ingroup (sect. Petota) and three outgroup (sect. Etuberosum) species. Half of the seedlings of an accession were placed in one flat and half in another flat to construct two replications. Seedlings were inoculated 2 to 3 wk after transplanting. Flats were not disturbed until the fourth day after inoculation. Four days after inoculation, the dome was reremoved from each flat and each plant was scored for its response to the fungus. A plant was given a score of one if it was alive and zero if the fungus had caused the stem to collapse and the plant to die. This process was repeated every day for 14 d. A survival score was determined for each plant on the basis of the number of days it remained alive. The mean score is the average of all plants evaluated. High scores are more resistant.