Alternaria leaf spot of Brassicaceae can be a devastating disease caused by several species of Alternaria. Differences in virulence (disease incidence and severity) and consequent defoliation were evaluated for 101 isolates across 10 Alternaria spp. (Alternaria alternata, A. brassicae, A. conjuncta, A. ethzedia, A. eureka, A. hordeicola, A. infectoria, A. japonica, A. metachromatica, and A. tenuissima) on seedlings of Brassica napus ‘Thunder TT’. In terms of disease incidence, severity, and defoliation, all isolates tested were virulent, but levels of isolate virulence varied within and between Alternaria spp. tested. Virulence also varied in relation to the disease assessment parameter used. A. japonica showed the highest virulence and overall mean values of percentage leaf disease incidence (%LDI 97.5), percentage leaf area diseased (%LAD 90.0) and percentage leaf senescence index (%LSI 70.0), followed by A. brassicae with 65.0, 55.0, and 35.0, A. ethzedia with 50.9, 41.8, and 15.9, and A. alternata with 48.4, 41.1, and 14.8, for %LDI, %LAD, and %LSI, respectively. The remaining Alternaria spp. also expressed varying levels of virulence in relation to the particular disease parameter used. These findings highlight the variation in relative levels of virulence, in terms of disease incidence, severity, and defoliation, within and between Alternaria spp. associated with leaf spot on rapeseed and reflect the relative importance of different pathogen populations of Alternaria spp. In particular, the potential of A. japonica and A. ethzedia to be significant pathogens of rapeseed in Australia, along with A. brassicae and A. alternata, is highlighted.