Purple spot, caused by Stemphylium vesicarium (Wallr.) E. Simmons, is one of the most important diseases facing asparagus producers in Ontario. Four cultivars, Jersey Giant, Guelph Millennium, Guelph Eclipse, and Gijnlim were examined in the field and in detached-spear controlled environment assays for reaction to S. vesicarium infection. Consistent results were achieved in the controlled environment assays and cultivar, location on the spear, isolate, wounding, and lighting were each found to affect lesion formation. In the field, an interaction was identified between cultivar resistance and acute weather, especially heavy rainfall, before and during a critical infection period. In controlled environment studies there was an interaction between cultivar and location of inoculation on the spear, whether near the top or base. The bioassay was useful for assessing some mechanisms of resistance, but inoculating attached spears in the field, while accounting for weather, is recommended for determining realistic field results.